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Satellite Event: Tutorial Lecture by Laleh Memarzadeh
Satellite Event: Tutorial Lecture "Structure of Quantum Channels"2h
One of the most promising tasks of quantum information is establishment of secure and reliable quantum communication channels over distant nodes. Beside theoretical success in formalizing the role of quantum features of systems for communication, current technological progress supports practical implementation of communication protocols. However, the role of quantum channels in quantum information science and technology is not restricted to communication protocols. The most general form of evolution of quantum systems is described by a quantum channel or a completely positive trace preserving (CPTP) map. Analyzing the mathematical structure of the space of CPTP maps, not only deepens our understanding on the nature of quantum evolution, but also plays an important role in implementation of quantum information tasks. In this tutorial, after a short review on the role of quantum channels in communication, we review the convex structure of quantum channels as well as divisibility properties of quantum channels. We will discuss how such an abstract knowledge leads to practical applications such as quantum simulation.
(Sharif Universiy of Technology, Tehran)
Invited: Quantum Optics
Structured glass for low power actuation of thermal phase shifters15m
Femtosecond laser micromachining (FLM) is a versatile technique that allows cost-effective and rapid fabrication of 3D photonic integrated circuits providing devices for various applications, ranging from lab-on-a-chip to quantum interferometry. Up to date, the possibility to reconfigure the operations performed by these circuits mainly relies on thermal phase shifters. However, the actuation of an integrated microheater requires several hundreds of milliwatts (around 600 mW) to induce a 2π phase shift in FLM devices operating at telecom wavelength, thus preventing the integration of more than a few microheaters on the same chip. Therefore, we devised a new FLM fabrication process, based on water-assisted-laser-ablation, able to reduce the power dissipation for a given phase shift of more than one order of magnitude, with no compromise either on the compactness or on the passive optical performance of the circuit. We realized Mach-Zehnder interferometers encompassing high-quality optical waveguides in aluminum borosilicate glass (0.29 dB/cm propagation losses and 0.27 dB/facet coupling losses at 1550 nm) and two different types of thermally insulating microstructures. In particular, isolation trenches on the sides of the heated photon path and bridge waveguides, structures in which the glass is ablated also under the optical path. As a result, interferometers featuring trenches show a reconfiguration period of 57 mW, whilst bridge waveguide ensures a reduction of the power dissipation required to induce a 2π phase shift down to 37 mW. In the end, we performed the same experimental measurements in a vacuum environment, demonstrating a further reduction in the required power dissipation when air is removed from the ablated regions. The advantages of structured devices are also underlined by performing thermal crosstalk measurements. These results will lead to an increase of the devices complexity attainable with FLM technology, opening new scenarios both in classical and quantum information applications.
Entanglement-based quantum key distribution using a deterministic quantum dot photon source15m
Entanglement-based protocols for quantum key distribution (QKD) provide additional layers of security compared to single-photon prepare-and-measure approaches, despite presenting the challenge of a less immediate hardware implementation. As remarkable technical achievements have been used to demonstrate entanglement-based QKD over longer and longer distances , the main opportunity for further development is related to multiphoton emission. This is a fundamental limitation for state-of-the-art photon sources based on spontaneous parametric down-conversion, which can be solved using deterministic quantum emitters. Here we focus on semiconductor quantum dots, which can generate nearly on-demand photon pairs with record-low multiphoton emission  and Bell state fidelity currently up to 98% . We experimentally demonstrate the viability of this technology in a realistic urban communication scenario . We employ a modified asymmetric Ekert protocol and perform QKD comparing two choices of quantum channel: over a 250 m single-mode fiber and in free-space between two buildings across the campus of the Sapienza University of Rome. The key exchange is successfully performed with error rates of 3–4%, well below the protocol threshold, and with substantial violations of the Bell inequality. The results are discussed in relation to the technical solutions employed for transferring the signal and to the current state of development of the source. In this regard, an outlook is presented based on the latest and foreseen advances in source design that can lead to unprecedented pair emission rates and boost secure key exchange over long distances.
 Yin J., et al., Nature 582, 501–505 (2020).
 Schweickert L., et al., Applied Physics Letters 112, 093106 (2018).
 Huber D., et al., Physical Review Letters 121, 033902 (2018).
 Basso Basset F., Valeri M., et al., arXiv:2007.12727 (2020).
Francesco Basso Basset
(Sapienza University of Rome)
Photon-number resolving detectors have experienced a wide spread throughout the last decades and proved to be versatile for a large number of applications. In particular, Multi-Pixel Photon Counters (MPPC) have been shown to be promising for Quantum Optics applications [1,2,3]. Unfortunately, these detectors are typically affected by correlated noise, which is especially detrimental for the detection of quantum correlations. The most important source of correlated noise is the Optical Cross-Talk (OCT), i.e. a photon emitted by a decelerating photoelectron fires a neighboring pixel, thus providing a spurious count. We have recently shown [4;5;6;7] that a commercial class of MPPC, i.e. the silicon photomultipliers, allows to detect the nonclassicality of a conditional state even in the presence of the OCT. In particular, we generated a multimode twin-beam state and used the silicon photomultipliers to perform conditional measurements. We successfully revealed the sub-Poissonianity of the conditional state. However, as far as we know, a theoretical description of a conditional measurement in the presence of the OCT is still lacking. Here, we extend the model for the conditional measurements with photon counting introduced in  by including the effect of the OCT. We provide the statistics of the number of detected photons for the conditional state and retrieve the analytic expression of the related Fano factor.
 I. Afek et al., “Quantum state measurements using multipixel photon detectors,” Phys. Rev. A 79, 043830(1-6) (2009).
 D. A. Kalashnikov et al., “Measurement of two-mode squeezing with photon number resolving multipixel detectors,” Opt. Lett. 27, 14(2829-2831) (2012).
 G. Chesi et al., “Optimizing Silicon photomultipliers for Quantum Optics,” Sci. Rep. 9, 7433(1-12) (2019).
 G. Chesi et al., “Measuring nonclassicality with silicon photomultipliers,” Opt. Lett. 44, 6(1371-1374) (2019).
 G. Chesi et al., “Effects of nonideal features of silicon photomultipliers on the measurements of quantum correlations,” Int. J. Quantum Inf. 17, 1941012 (2019).
 G. Chesi et al., “Autocorrelation functions: a useful tool for both state and detector characterisation,” Quantum Meas. Quantum Metrol. 6, 1(1-6) (2019).
 M. Bondani et al., “Measuring nonclassicality of mesoscopic twin-beam states with Silicon Photomultipliers,” Proceedings 12, 48-52 (2019).
 A .Allevi et al., “Conditional measurements on multimode pairwise entangled states from spontaneous parametric downconversion”, EPL 92,1-6 (2010).
(Università degli Studi di Pavia)
Two-membrane cavity optomechanics15m
The optomechanical behaviour of a driven high finesse Fabry-Pérot cavity containing two vibrating dielectric Si3N4 membranes will be presented. The presence of the second membrane inside the optical cavity enhances the optomechanical coupling making this system interesting to reach the strong-coupling regime [1-2]. Moreover, the presence of two optical resonators provides the opportunity to couple mechanical resonators with very similar frequencies. This pave the way to the realization of an efficient state transfer and even entanglement between the mechanical oscillators. Multi-element systems of micro/nano-mechanical resonators offer promising prospects for exploring multi-oscillators synchronization [3-5]. The first experimental characterization of the optical, mechanical, and especially optomechanical properties of a sandwich constituted of two parallel membranes within an optical cavity will be reported. We find that the optomechanical coupling strength is enhanced by constructive interference when the two membranes are positioned to form an inner cavity which is resonant with the driving field. Specifically, we determine a gain of ∼2.47 in the coupling strength of the relative mechanical motion with respect to the single membrane configuration . Finally, the behaviour of the non-linear dynamics of such a system in a pre-synchronization regime where both large and small amplitude resonator motions are transduced in a nontrivial way by the non-linear response of the optical probe beam, will be discussed .
 J. Li, A. Xuereb, N. Malossi, and D. Vitali, J. Opt., 18, 084001, 2016
 J. Li, G. Li, S. Zippilli, D. Vitali, and T. Zhang, Phys. Rev. A., 95, 043819, 2017
 P. Piergentili, L. Catalini, M. Bawaj, S. Zippilli, N. Malossi, R. Natali, D. Vitali, and G. Di Giuseppe, New J. Phys., 20, 101001, 2018
 W. Li, P. Piergentili, J. Li, S. Zippilli, R. Natali, N. Malossi, G. Di Giuseppe, and D. Vitali, Phys. Rev. A, 101, 013802, 2020
 P. Piergentili, W. Li, R. Natali, N. Malossi, D. Vitali, and G. Di Giuseppe, in preparation
(University of Camerino)
Noisy quantum metrology enhanced by continuous nondemolition measurement8m
In this work, we show that, by exploiting continuous quantum nondemolition measurement, it is possible to preserve quantum advantage in a frequency estimation (or magnetometry) measurement scheme even in the presence of independent dephasing noise, usually the most detrimental type of noise. We thus verify that such enhancement is preserved thanks to non-classical correlations, namely spin squeezing, which are dynamically generated by the measurement itself. Remarkably, our scheme does not require the preparation of any entangled, or non-classically correlated state of the probe: the probe is initialized in a classical coherent spin state and the resources required for the quantum enhancement are dynamically created during the conditional evolution. We moreover provide evidence that our results are robust and hold true in a wide range of noise intensities and even in the presence of inefficient measuring devices.
(University of Turku)
Continuous-time quantum walks in the presence of a quadratic perturbation7m
We address the properties of continuous-time quantum walks with Hamiltonians of the form $H= L + \lambda L^2$, being $L$ the Laplacian matrix of the underlying graph and being the perturbation $\lambda L^2$ motivated by its potential use to introduce next-nearest-neighbor hopping. We consider cycle, complete, and star graphs because paradigmatic models with low/high connectivity and/or symmetry. First, we investigate the dynamics of an initially localized walker. Then, we devote attention to estimating the perturbation parameter $\lambda$ using only a snapshot of the walker dynamics. Our analysis shows that a walker on a cycle graph is spreading ballistically independently of the perturbation, whereas on complete and star graphs one observes perturbation-dependent revivals and strong localization phenomena. Concerning the estimation of the perturbation, we determine the walker preparations and the simple graphs that maximize the Quantum Fisher Information. We also assess the performance of position measurement, which turns out to be optimal, or nearly optimal, in several situations of interest. Besides fundamental interest, our study may find applications in designing enhanced algorithms on graphs.
(Università di Modena e Reggio Emilia)
Super-resolution Optical Fluctuation Imaging—fundamental estimation theory perspective8m
We provide a rigorous quantitative analysis of super-resolution imaging techniques which exploit temporal fluctuations of luminosity of the sources in order to beat the Rayleigh limit. We define an operationally justified resolution gain figure of merit, that allows us to connect the estimation theory concepts with the ones typically used in the imaging community, and derive fundamental resolution bounds that scale at most as the fourth-root of the mean luminosity of the sources. We fine-tune and benchmark the performance of state-of-the-art methods, focusing on the cumulant based image processing techniques, taking into account the impact of limited photon number and sampling time.
(Faculty of Physics, University of Warsaw)
Quantum multiphase estimation in an integrated photonic circuit7m
Quantum Metrology is one of the most important quantum technologies where quantum resources are exploited to enhance the estimation of unknown parameters . In this context, since realistic scenarios generally involve more than one parameter, quantum multiparameter estimation is a central and very active research area. Nevertheless, in such relatively new field, several open questions are still present and experimental platforms able to perform multiparameter estimation protocols have to be developed. We realized a reconfigurable photonic integrated circuit, built through the femtosecond lase writing technique, able to perform simultaneous multiphase estimation with photonic quantum states. The circuit realizes a three arm interferometer and is highly tunable, so that the two independent phase shifts between the interfetometer's arms can be tuned. Firstly, we demonstrate quantum enhanced two-phases estimation by using two-photon probes . Then we provide a demonstration of a Bayesian adaptive protocol able to saturate, in the limited data regime, the sensitivity bound (Cramer-Rao bound) on the estimation of the two phases when single photon probes are employed .
 E. Polino, M. Valeri, N. Spagnolo, and F. Sciarrino, AVS Quantum Science 2, 024703 (2020).
 E. Polino, M. Riva, M. Valeri, R. Silvestri, G. Corrielli, A. Crespi, N. Spagnolo, R. Osellame, and F. Sciarrino, Optica 6, 288 (2019).
 M. Valeri, E. Polino, D. Poderini, I. Gianani, G. Corrielli, A. Crespi, R. Osellame, N. Spagnolo, and F. Sciarrino, arXiv preprint arXiv:2002.01232 (2020).
A machine learning approach to Bayesian parameter estimation7m
Bayesian estimation is a powerful theoretical paradigm for the operation of quantum sensors. However, the Bayesian method for statistical inference generally suffers from demanding calibration requirements, that have so far restricted its use to systems that can be explicitly modelled. In this theoretical study, we formulate parameter estimation as a classification task and use artificial neural networks to efficiently perform Bayesian estimation. We show that the network's posterior distribution is centred at the true (unknown) value of the parameter within an uncertainty given by the inverse Fisher information, representing the ultimate sensitivity limit for the given apparatus. When only a limited number of calibration measurements are available, our machine-learning based procedure outperforms naive calibration methods. Our machine-learning based procedure is model independent, and is thus well suited to a black box sensor where an explicit model is unavailable. Thus, our work paves the way for Bayesian quantum sensors that can take advantage of complex, potentially non-classical quantum states, which can significantly enhance the sensitivity of future devices.
(QSTAR, INO-CNR and LENS)
Bayesian multiphase estimation at the Heisenberg limit8m
Quantum (multi-)parameter estimation provides the central ingredient for many quantum technological tasks like, e.g., quantum computation or precision measurements. Previous work focussed mainly on single phase estimation at the fundamental limit, the Heisenberg limit, or on multiphase estimation at an optimal point. Here, we propose a quantum algorithm to measure d completely unknown phases and provide numerical evidence for Heisenberg limited precision of the algorithm. We show that the algorithm can outperform single phase estimation and discuss a possible quantum optical implementation.
(QSTAR, CNR-INO and University of Napoli)
Critical Quantum Metrology with a Finite-Component Quantum Phase Transition7m
Physical systems close to a quantum phase transition exhibit a divergent susceptibility, suggesting that an arbitrarily high precision may be achieved by exploiting quantum critical systems as probes to estimate a physical parameter. However, such an improvement in sensitivity is counterbalanced by the closing of the energy gap, which implies a critical slowing down and an inevitable growth of the protocol duration. In this contribution, we present different metrological protocols that exploit the superradiant phase transition of the quantum Rabi model, a finite-component system composed of a single two-level atom interacting with a single bosonic mode. We show that, in spite of the critical slowing down, critical quantum optical probes can achieve a quantum-enhanced time scaling of the sensitivity in frequency-estimation protocols.
(Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche IFN-CNR)
Discrimination of Ohmic thermal baths by quantum dephasing probes8m
The discrimination of structured baths at different temperatures by dephasing quantum probes is studied. The exact reduced dynamic is derived, and the minimum error probability is evaluated by three different kinds of quantum probes, namely a qubit, a qutrit, and a quantum register made of two qubits. The results indicate that dephasing quantum probes are useful in discriminating low values of temperature and that lower probabilities of error are achieved for intermediate values of the interaction time. A qutrit probe outperforms a qubit one in the discrimination task, whereas a register made of two qubits does not offer any advantage compared to two single qubits used sequentially.
Invited: Atoms and Molecules
Integrated Organic Molecules for Quantum Technologies15m
The realization of a robust and scalable nanophotonic platform which efficiently integrates quantum emitters as on-demand sources of non-classical light is crucial to the successful development of photonic quantum technologies. However, conventional strategies to on-chip integration, based on lithographic processes in semiconductors, typically introduce dephasing effects which broaden the transition linewidth of the emitter and are detrimental for its coherence properties. Moreover, such fabrication techniques are intrinsically limited to planar geometries and in this sense are difficult to scale up to a big number of integrated emitters. In the present contribution we demonstrate an alternative platform based on molecules that preserve near-Fourier-limited fluorescence even when embedded in polymeric photonic structures . Deterministic integration is achieved in three-dimensions via direct laser writing (DLW) around selected molecular emitters, with a fast, inexpensive and scalable fabrication process. In particular, organic molecules of dibenzoterrylene (DBT) are embedded in anthracene (Ac) nanocrystals (NCs), which have shown photostable single-photon emission, near to lifetime-limited linewidths at 3K , and are especially suitable for the integration in polymeric devices. We integrate DBT:Ac NCs via DLW on different substrates and at variable heights in different polymeric designs. Enhanced light extraction is achieved in a micro-dome solid-immersion-lens, reporting unprecedented detected count rates for a single cold molecule and efficient coupling to a single-mode fiber. The proposed technology may represent an important step in the integration of single emitters into robust quantum protocols based on molecules, including arrays of indistinguishable single-photon sources. In this latter merit, we will also discuss the possibility of using an all-optical approach to independently shift the transition frequency of individual emitters and bring them into resonance, with spacial resolution at the micron level and maintaining the coherent spectral properties .
 M. Colautti et al., Adv. Quantum Technol., (2020) doi:10.1002/qute.202000004
 S. Pazzagli et al., ACSNano 12, 4295–4303 (2018).
 M. Colautti et al., Under Review
(CNR-Ino - European Laboratory for Non-Linear Spectroscopy (LENS))
Adiabatic Quantum Operations with UltraStrongly Coupled Artificial Atoms15m
Ultrastrong coupling (USC) between light and matter has been recently achieved in architectures of solid state artificial atoms coupled to cavities. Such architectures may provide new building blocks for quantum state processing, where ultrafast operations could be performed. However faster dynamics has a cost. Indeed USC breaks the symmetry associated with the conservation of the number of excitations, leading to a series of new physical effects of great fundamental interest but detrimental for quantum state processing. In particular the highly entangled nature of the eigenstates, dressed by a potentially very large number of virtual photons, leads to leakage of excitation via the dynamical Casimir effect (DCE) and via decay. In this work we analyze quantum operations between two artificial atoms ultrastrongly coupled to a cavity, operating as a virtual bus. We show that an adiabatic protocol similar to STIRAP may overcome the problem of leakage. Ideally the cavity is never populated and thus it is expected to greatly reduce the impact of DCE and decay. We show that high fidelity operations can be performed for moderate couplings in the USC regime and fidelities higher than the ones obtained in the strong coupling regime (SC, where the rotating wave approximation holds) can be obtained. Moreover, optimal control theory allows for properly crafted controls that extend the high fidelity region to even larger couplings. The protocol is extremely robust agaist DCE, in the absence of decoherence yields almost 100% fidelity for remote state transfer and multiqubit entanglement generation. It is also resilient to decay due to leakage from the cavity, which is the main decoherence mechanism for present USC architectures. In this more realistic scenario it is seen that for larger coupling (entering the deep strong coupling regime) the fidelity decreases due to the interplay between decoherence and DCE. Our results suggest that adiabatic manipulations, may be a promising tool for quantum state processing in the USC regime.
(Dipartimento di Fisica e Astronomia “Ettore Majorana”, Universitá di Catania)
Achieving Heisenberg scaling with maximally entangled states: an analytic upper bound for the attainable root mean square error15m
In this talk we explore the possibility of performing Heisenberg limited quantum metrology of a phase, without any prior, by employing only maximally entangled states. Starting from the estimator introduced by Higgins et al. in New J. Phys. 11, 073023 (2009), the main result discussed in the talk is an analytical upper bound on the associated Mean Squared Error which is monotonically decreasing as a function of the square of the number of quantum probes used in the process. The analyzed protocol is non-adaptive and requires in principle (for distinguishable probes) only separable measurements. From the practical point of view, at difference with the previous works, where it was required the states sizes to grow as the powers of two, we are often able to extract Heisenberg Scaling from an arbitrary sequence of entangled states sizes, possibly realizable in a laboratory. We also explore how the strategy changes in presence of probe loss or fluctuations of the phase.
Bound states in the continuum in arrays of quantum emitters15m
Quantum emitters coupled to EM fields in waveguides provide a controllable and experimentally feasible testbed to observe interesting physical phenomena; in particular, the emergence of bound states in the continuum opens new possibilities to generate states with specific entanglement properties. We characterize analytically the bound states for any number of emitters, showing that the finite spacing between the emitters and the structure of the field dispersion relation become relevant and yield nonperturbative effects.
(University of Bari & INFN)
Quantum computing model of an artificial neuron with continuously valued input data15m
Artificial neural networks have been proposed as potential algorithms that could benefit from being implemented and run on quantum computers. In particular, they hold promise to greatly enhance Artificial Intelligence tasks, such as image elaboration or pattern recognition. The elementary building block of a neural network is an artificial neuron, i.e. a computational unit performing simple mathematical operations on a set of data in the form of an input vector. Starting from the design of a previously introduced quantum artificial neuron , which fully exploits the use of superposition states to encode binary valued input data, during the talk it will be shown how the implementation of the quantum neuron can be further generalized to accept continuous- instead of discrete-valued input vectors, without increasing the number of qubits . This further step is crucial to allow for a direct application of gradient descent based learning procedures, which would not be compatible with binary-valued data encoding.
 Tacchino, F., Macchiavello, C., Gerace, D. et al. (2019). An artificial neuron implemented on an actual quantum processor. npj Quantum Inf 5, 26. https://doi.org/10.1038/s41534-019-0140-4
 Mangini, S., Tacchino, F., Gerace, D., Macchiavello, C. and Bajoni, D. (2020). Quantum computing model of an artificial neuron with continuously valued input data. Machine Learning: Science and Technology. https://doi.org/10.1088/2632-2153/abaf98
(University of Pavia)
Fighting qubit loss in topological QEC codes: theory and experiment15m
The loss of qubits poses one of the fundamental obstacles towards large-scale and fault-tolerant quantum information processors. In this work, we design and characterize a complete toolbox for a full cycle of qubit loss detection and correction on a minimal instance of a topological surface code. This includes a quantum non-demolition measurement of a qubit loss event that conditionally triggers a restoration procedure, mapping the logical qubit onto a new encoding on the remaining qubits. The demonstrated methods, implemented here in a trapped-ion quantum processor, are applicable to other quantum computing architectures and codes, including leading 2D and 3D topological quantum error correcting codes. These tools complement previously demonstrated techniques to correct computational errors, and in combination constitute essential building blocks for complete and scalable quantum error correction.
The Frustration of being Odd7m
A central tenant in the classification of phases is that boundary conditions cannot affect the bulk properties of a system. In our works, we show striking, yet puzzling, evidence of a clear violation of this assumption. We study some exactly solvable spin chains, mappable to free fermions, in a ring geometry with an odd number of sites. In such a setting, even at finite sizes, we are able to calculate directly the spontaneous magnetizations that are traditionally used as order parameters to characterize the system's phases. We find that boundary conditions can destroy local order, change it, and even induce a new quantum phase transition.
Main references: https://iopscience.iop.org/article/10.1088/1367-2630/aba064 https://arxiv.org/abs/2002.07197
Bulk detection of time-dependent topological transitions in quenched chiral models8m
The topology of one-dimensional chiral systems is captured by the winding number of the Hamiltonian eigenstates. We proved that this invariant can be read-out by measuring the Mean Chiral Displacement of a single-particle wavefunction that is connected to a fully localized one via a unitary and translation-invariant map. Remarkably, this implies that the Mean Chiral Displacement can detect the winding number even when the underlying Hamiltonian is quenched between different topological phases. We confirm experimentally these results in a photonic quantum walk, realized in the transverse-momentum space of structured light.
Francesco Di Colandrea
(Università degli Studi di Napoli Federico II )
Witnesses of coherence and dimension from multiphoton indistinguishability tests7m
The quantum interference  is a useful tool for the characterization of the single photon sources, for quantum computing and for quantum communication. In particular, the indistinguishability and the superposition are the key elements of the quantum interference and for this reason, it is worth to develop better methods for their identification and quantification. We present an indistinguishability test for a multiphoton state based on two-photon Hong-Ou-Mandel tests. Our approach  consists of an interferometer that allows measure simultaneously the three photon overlaps on a four photon state produced by a spontaneous parametric down conversion(SPDC) source. As shown in Ref. , we quantify the indistinguishability from the obtained value measured overlaps. Starting from these measurements we infer precise bounds for the unmeasured overlaps. For this purpose, we assume two different models for the multiphoton state: generally mixed four-photon state , and the tensor product of pure single-photon state . Each model provides different inequalities for the unmeasured overlaps. Furthermore, changing the number and the arrangement of HOM tests performed between pair of photon, i.e having access to different pairwise overlaps, other information can be retrieved on the system. From the same inequalities, we also can formulate a coherence witness and dimension witness based on this overlaps estimation, as shown in Ref. . This basis-independent coherence witness attests that it is not possible to diagonalize the state on a given reference basis. We experimentally test this new coherence witness. In order to have a three photon state in input and measure the three pairwise possible overlaps, we rearrange our interferometer. To obtain the coherence witness violation, we tune the different input state by photon polarization. We also experimentally measure the Hilbert space dimension witness which attests if the space dimension of each photon state is up to two. For this purpose, we use the photon delay times as degree of freedom. In this way, we have three qudit states that violate the dimension witness. Our results confirm the validity of these novel coherence and dimension witnesses and they provide a complete characterization of the single photon sources. Moreover, the identification of an undesired degree of freedom makes the dimension witness very useful for quantum cryptography.
 Flamini, F., Spagnolo, N., & Sciarrino, F. “Photonic quantum information processing: a review.” Reports on Progress in Physics, 82(1), 016001 (2018).
 Giordani, T., Brod, D. J., Esposito, C., Viggianiello, N., Romano, M., Flamini, F., ... & Sciarrino, F. (2020). Experimental quantification of four-photon indistinguishability. New Journal of Physics.
 D. J. Brod, et al., “Witnessing genuine multiphoton indistinguishability,” Phys. Rev. Lett. 122, 063602 (2019).
 D. J. Brod and E. F. Galvão, “Quantum and classical bounds for unknown two-state overlaps”
Vacancy-like dressed states in topological waveguide QED8m
We identify a class of dressed atom-photon states forming at the same energy of the atom at any coupling strength. As a hallmark, their photonic component is an eigenstate of the bare photonic bath with a vacancy in place of the atom. The picture allows to formalize and re-interpret all quantum optics phenomena where atoms behave as perfect mirrors, connecting in particular dressed bound states (BS) in the continuum (or BIC) with geometrically-confined photonic modes in a waveguide. Most notably, when applied to photonic lattices, the framework allows to formulate for the first time a general criterion to predict atom-photon dressed BS in lattices with topological properties by putting them in one-to-one correspondence with photonic bound modes whose occurrence is ruled by the known Atland-Zirnbauer classification. The criterion is applied to predict new classes of dressed BS in the photonic Creutz-ladder and Haldane models. In the latter case, states with non-zero local photon flux occur where an atom is dressed by a photon orbiting around it, a phenomenon so far unexplored in quantum optics.
(University of Palermo)
The complete spectrum of spin-1/2 XXZ chain at root of unity7m
We have studied the complete spectrum of spin-1/2 XXZ chain at root of unity, i.e. a paradigmatic model of quantum integrability. Making use of transfer matrix fusion relation, we constructed a family of 2-parameter transfer matrices, which help us obtain all the eigenstates in terms of Bethe roots. This elucidates a long-standing problem dated from the debate between McCoy and Baxter. We also exemplify the applications in the thermodynamic limit, which explains the rôle of quasi-local Z charge in the presence of quantum quenches and generalised hydrodynamics.
(Institute for Theoretical Physics, University of Amsterdam)
D-Wave as a generator of structural models in materials science8m
A tool capable to efficiently generate realistic structural models of disordered systems has been a goal of material science for many years. We show the feasibility of quantum annealing in exploring the energy landscape of materials that deviate from the ideal crystalline phase, specifically vacancy defects in graphene and disordered silicon. By mapping the competing interactions onto quadratic unconstrained binary optimization problems (QUBO), our approach guarantees access to all the arrangements of the multiple defects on the graphene sheet respecting the relative formation energies. In the case of silicon a large portion of the structural models with an increasing disorder is encoded in the low energy spectrum of the QUBO formulation and detected in the annealing process. Our approach reproduces known results and provides a stepping-stone towards applications of quantum annealing to more complex problems of physical-chemical interest.
(Central Michigan University)
Modeling order-disorder phase transitions with a quantum annealer7m
Quantum annealers have grown in complexity to the point that devices with few thousand qubits are approaching capacities to tackle material science problems. Starting from a representation of crystal structures in terms of networks, we develop models of order-disorder phase transitions for two prototypical classes of materials (entropy stabilized alloys and perovskites) that are directly implementable on the D-Wave devices. Cost functions are built to encode the ordered phase, while disordered phases appear as excited states in the spectrum of the classical Ising Hamiltonian, which accounts for the competing interactions in the material. Taking advantage of the statistical nature of the quantum annealing, we explore the energy landscape and generate all the structural models for each step of the order-disorder phenomenon. Besides providing a correct description in terms of critical temperatures, our model allows us to access a wide range of structural models, overcoming some limitations of classical methods.
(University of Southern California)
Mitigating quantum errors with Mitiq8m
This talk introduces an open-source package for error-mitigation in quantum computation using zero-noise extrapolation. Error-mitigation techniques improve computational performance (with respect to noise) using minimal overhead in quantum resources by relying on a mixture of quantum sampling and classical post-processing techniques. Our error-mitigation package interfaces with multiple quantum computing software stacks, and we demonstrate improved performance on a variety of benchmarks performed on IBM and Rigetti quantum processors. We describe the library using code snippets to demonstrate usage and discuss features, support, and contribution guidelines.We also report on how cloud-based interactive workshops have helped develop the library with feedback from the research community.
Invited: Condensed Matter
Semiconductor qubits based on hole spins in CMOS devices and edge-states in Hall interferometers15m
Among the possible frameworks to encode the quantum bit, semiconductor-based implementations possibly present the highest potential in terms of scalability and compatibility with current nanoelectronics industry. In this talk, I will outline two different platforms for the realization of the qubit in semiconductor devices, and present the numerical approach we adopted for their characterization in full-scale simulations. The first approach, pursued within the recently started IQubits EU project , focuses on the use and engineering of fabricated CMOS devices to implement hole/electron spin qubits. The experimental characterization of a 22-nm FDSOI MOSFETs  has proved the formation of a double hole/electron quantum dot in the Si/SiGe channel, which potentially enables the monolithic integration of the control and readout circuitry on the same die . The hole states are controlled by the top and back gates, while the spin is manipulated by electric-dipole spin resonance. Within a multiscale approach, we compute the single-hole states by diagonalizing the k·p Luttinger-Kohn Hamiltonian, starting from a realistic confining potential, simulated by means of the “Ginestra” software . Prospects for scalability are included, starting from the simulation of double quantum dot systems in Si/SiGe MOSFETS, with a particular emphasis on the effects of Coulomb interactions and correlations. The second approach exploits topologically-protected edge states in the Integer Quantum Hall regime for a flying implementation of the quantum bit, with a coherence length up to 10 micrometers . The qubit propagates at the edges of a confined 2DEG, while the inter-channel scattering rotates their state. Hall interferometers implement this manipulation protocol to realize single and two-qubit operation on the fly. We present our proposal for a Hall conditional phase shifter and show its feasibility for phase rotation up to π . The exact two-electron wavefunction is evolved in the full-scale 2D potential of the device, where single-charges are injected as Gaussian wavepackets of edge states. Our numerical approach involves HPC techniques to include exactly the interplay between Coulomb repulsion and the device geometry, whose tuning is crucial for logic operations.
 S. Bonen et al., IEEE Electron Device Letters, 40 127-130 (2019).
 M. J. Gong et al., 2019 IEEE Radio Frequency Integrated Circuits Symposium (RFIC), Boston, MA, USA, pp. 111-114 (2019).
 P. Roulleau et al., Phys. Rev. Lett. 100, 126802 (2008); E Bocquillon et al., Science 339 6123 (2013).
 L. Bellentani, G. Forghieri, P. Bordone and A. Bertoni, Phys. Rev. B 102, 035417 (2020).
Vibrational modulation of electronic transitions in Copper Germanate: a theoretical model15m
The dynamics of complex materials can be conveniently investigated through pump-probe techniques. Here we present a simple quantum theoretical model that is used to interpret the results of a recently performed pump-probe experiment on Copper Germanate . In this context, in order to study the electron-phonon coupling in the material, lattice vibrations are excited by an infrared pump pulse and d-d electronic transitions are probed with a visible pulse.
 A. Marciniak, et al. arXiv:2003.13447
(University of Nottingham)
Full control of nonlinear processes in Josephson parametric amplifiers for readout of superconducting qubits15m
In the last decade, Superconducting Quantum Circuits (SQCs) based on Josepshon Junctions (JJs) showed that a working quantum processor can be successfully built and operated to perform Quantum Information Processing (QIP) on a system made of many superconducting qubits. The key ingredient to reach such an achievement are the endless possibilities allowed by SQCs in order to efficiently control and readout superconducting qubits, and the research on these interface devices is at the edge of technological advances in QIP. Efficient readout of superconducting qubits requires coherent amplification of single microwave photon signals while preserving a high Signal to Noise Ratio (SNR) in order to reach high single-shot readout fidelity. This task can be achieved by driving parametric processes in superconducting nonlinear oscillators, allowing coherent energy transfer between a strong pump and a single microwave photon signal that carries the result of a Quantum Non-Demolition measurement on a qubit. Thanks to these techniques, it is possible to build Quantum Limited Amplifiers (QLA) that can generate well detectable signals with SNR being degraded only by the least amount imposed by quantum mechanical principles. A QLA can be built around Josephson microwave circuits that synthesize nonlinear Hamiltonians with the required characteristics, and established devices are nowadays used in every superconducting quantum computing experiment. However, the trigonometric nature of the Josephson nonlinearities makes their independent control a challenging task. We show how is possible to improve the synthesis capabilities of Josephson Hamiltonians with a “Gradiometric SNAIL Parametric Amplifier” (G-SPA), a novel Josephson parametric amplifier that allows complete tuning of its nonlinearities via in-situ magnetic fluxes. Our approach expands the tunability range of the parametric processes, allowing independent choice of their participation in the treatment of single photon signals and opening to many new applications for these devices.
(University of Naples Federico II)
Career Panel: Publishing
Career panel: Publishing1h
Katiuscia Cassemiro (Physical Review X Quantum - American Physical Society)
Jean-Sébastien Caux (University of Amsterdam and SciPost)
P32 - Luca Fasolo (Politecnico di Torino and Istituto Nazionale di Ricerca Metrologica)
A quantum model for rf-SQUIDs based metamaterials enabling 3WM and 4WM Travelling Wave Parametric Amplification
P33 - Wandearley De Silva Dias (Universidade Federal de Alagoas)
Aperiodic space-inhomogeneous quantum walks: Localization properties, energy spectra, and enhancement of
P34 - Loris Maria Cangemi (University of Naples Federico II)
Violation of TUR in a periodically driven quantum work-to-work converter
P35 - Anderson Buarque (Universidade Federal de Alagoas)
Self-trapped quantum walks
P36 - Gustavo Martin Bosyk (Instituto de Física La Plata, CONICET-UNLP & Università degli Studi di Cagliari)
Generalized coherent vector: definition and applications
P37 - Fadwa Benabdallah (LPHE-Modeling and Simulation, Faculty of Sciences, Mohammed V University in Rabat)
Quantum discord based on linear entropy and thermal entanglement of qutrit-qubit spin chain under influence of
the external magnetic field
P38 - Revanth Badveli (Birla Institute of Technology and Science, Pilani)
Compressed Sensing Quantum State Tomography: An Alternate Approach
P39 - Utkarsh Azad (International Institute of Information Technology, Hyderabad)
Quantum Neural Networks - Towards an era of Quantum-Assisted Machine Learning
P40 - Giorgio Zarantonello (Leibniz Universität Hannover)
Towards fault-tolerant quantum computation based on near-field microwaves with trapped ions
P41 - Giacomo Guarnieri (Trinity College Dublin)
Quantum fluctuations hinder finite-time information erasure near the Landauer limit
P42 - Jan Tuziemski (Stockholm University).
Relative decoherence in quantum reference frames
P43 - Gabriel Matos (University of Leeds)
Quantifying the efficiency of state preparation via quantum variational eigensolvers
P44 - Kartikeya Rambhatla (Shiv Nadar University)
Adaptive phase estimation through a genetic algorithm
P45 - Manik Banik (Indian Institute of Science Education and Research, Thiruvananthapuram, India)
Quantum Advantage in Shared Randomness Processing
P46 - Marcos Basso (Federal University of Santa Maria)
An uncertainty view on complementarity and a complementarity view on uncertainty
P47 - Colin Benjamin (National Institute of Science education and research)
Testing quantum speedups in exciton transport through a photosynthetic complex using quantum stochastic walks
Anyonic molecules in atomic fractional quantum Hall liquids: a quantitative probe of fractional charge and anyonic statistics15m
We study the quantum dynamics of massive impurities embedded in a strongly interacting two-dimensional atomic gas driven into the fractional quantum Hall (FQH) regime under the effect of a synthetic magnetic field. For suitable values of the atom-impurity interaction strength, each impurity can capture one or more quasi-hole excitations of the FQH liquid, forming a bound molecular state with novel physical properties. An effective Hamiltonian for such molecules is derived within the Born-Oppenheimer approximation, which provides renormalized values for the effective mass, charge and statistics of such anyonic molecules by combining the finite mass of the impurity and the fractional charge and anyonic statistics of the quasi-holes. The anyonic statistics is shown to provide a long-range Aharonov-Bohm-like interaction between molecules. The resulting relative phase of the direct and exchange scattering channels can be thus extracted from the angular position of the interference fringes in the scattering cross section of a pair of colliding molecules. Different configurations providing direct and quantitative insight on the fractional charge and the anyonic statistics of quasi-hole excitations in FQH liquids are highlighted for both cold atoms and photonic systems.
Alberto Muñoz de las Heras
(University of Trento)
Entanglement of formation of mixed many-body quantum states using tensor networks15m
In this talk, we will present an algorithm to compute the entanglement of formation for mixed many-body quantum states by using tensor networks. Indeed, we will introduce a new tensor network ansatz --- the Tree Tensor Operator --- which leads to a very convenient description of density matrices. Our results will focus on thermal states of the quantum Ising chain in transverse field, for which we could consider up to 22 spins.
(Università di Padova)
There is only one time15m
We draw a picture of physical systems that allows us to recognize what is this thing called "time" by requiring consistency not only with our notion of time but also with the way time enters the fundamental laws of Physics, independently of one using a classical or a quantum description. Elements of the picture are two non-interacting and yet entangled quantum systems, one of which acting as a clock, and the other one doomed to evolve. The setting is based on the so called "Page and Wootters (PaW) mechanism", and updates, with tools from Lie-Group and large-N quantum approaches. The overall scheme is quantum, but the theoretical framework allows us to take the classical limit, either of the clock only, or of the clock and the evolving system altogether; we thus derive the Schrödinger equation in the first case, and the Hamilton equations of motion in the second one. Suggestions about possible links with general relativity and gravity are also put forward.
(Università degli studi di Firenze)
Nonclassical steering and the Gaussian steering triangoloids15m
We fully characterize the mechanism by which nonclassicality according to the Glauber P-function can be conditionally generated on one mode of a two-mode Gaussian quantum state by generic Gaussian measurements on the other mode. For two-mode squeezed thermal states (TMSTs), we visualize the whole set of conditional states constructing Gaussian steering triangoloids and we show that nonclassicality can be induced in this way if and only if the initial state is EPR-steerable. In the more general case, we recognize two types of quantum correlations: weak and strong nonclassical steering, the former being independent of entanglement, and the latter implying EPR steerability. We show that EPR-steering and weak/strong nonclassical steering merge precisely for TMSTs, and we discuss applications of this result to one-sided device-independent quantum key distribution and noisy propagation of twin-beam states.
(Università degli Studi di Milano)
Witnesses of non-classicality beyond quantum theory15m
I discuss general argument to show that if a physical system can mediate locally the generation of entanglement between two quantum systems, then it itself must be non-classical. Remarkably, the argument does not assume any classical or quantum formalism to describe the mediating physical system: the result follows from general information-theoretic principles, drawn from the recently proposed constructor theory of information. This argument provides the indispensable theoretical basis for recently proposed tests of non-classicality in gravity, based on witnessing gravitationally-induced entanglement in quantum probes.
(University of Oxford)
Contributed: Quantum Information
Characterising quantum correlations of fixed dimension7m
We give a converging semidefinite programming hierarchy of outer approximations for the set of quantum correlations of fixed dimension. Starting from the Navascués-Pironio-Acín (NPA) hierarchy for general quantum correlations, we identify additional semidefinite constraints for any fixed dimension, leading to analytical bounds on the convergence speed of the resulting hierarchy. Additionally, we provide an algorithm, built upon our hierarchy, able to compute additive approximations on the value of two-player free games with an assisting quantum system of fixed dimension, and a given number of questions |Q| and answers |A|. The computational time of our algorithm scales polynomially in |Q| and quasi-polynomially in |A|, thereby improving on previously known approximation algorithms for which worst-case run-time guarantees were at best exponential in |Q||A|. To derive our analytical bounds on the convergence of the hierarchy, we make a connection to the quantum separability problem and employ, as our main technical tool, an improved multipartite quantum de Finetti theorem with linear constraints.
Robust entanglement preparation against noise by controlling spatial indistinguishability8m
Initialization of composite quantum systems into highly entangled states is usually a must to enable their use for quantum technologies. However, unavoidable noise in the preparation stage makes the system state mixed, hindering this goal. Here, we address this problem in the context of identical particle systems within the operational framework of spatially localized operations and classical communication (sLOCC). We define the entanglement of formation for an arbitrary state of two identical qubits. We then introduce an entropic measure of spatial indistinguishability as an information resource. Thanks to these tools we find that spatial indistinguishability, even partial, can be a property shielding non-local entanglement from preparation noise, independently of the exact shape of spatial wave functions. These results prove quantum indistinguishability is an inherent control for noise-free entanglement generation.
(University of Palermo)
Entanglement of identical particles7m
Entanglement is a well defined and useful notion for distinguishable particles. It provides a framework of locality and can be used as a resource in quantum information and communication protocols. However, for identical particles, no universal accepted definition exists. The symmetrization principle makes identical particle states look entangled when written in first quantization notation. In particular, the state of two hydrogen atoms, one on the moon and one on the earth, which have never met each other cannot be written as a tensor product. However, because of the symmetrization, one also has to restrict the allowed operators on the Hilbert space. This means that one cannot easily infer nontrivial correlations in the systems by comparing the states of identical particles with their distinguishable counterparts. In my seminar, I want to address the question of entanglement for identical particles, with the particular example of the aforementioned hydrogen atoms. We will use recently developed guiding principles to motivate a useful notion of entanglement and show how to apply it on this given problem.
Variational hybrid quantum-classical optimization represents one of the most promising avenue to show the advantage of nowadays noisy intermediate-scale quantum computers in solving hard problems, such as finding the minimum-energy state of a Hamiltonian or solving some machine-learning tasks. In these devices noise is unavoidable and impossible to error-correct, yet its role in the optimization process is not much understood, especially from the theoretical viewpoint. Here we consider a minimization problem with respect to a variational state, iteratively obtained via a parametric quantum circuit, taking into account both the role of noise and the stochastic nature of quantum measurement outcomes. We show that the accuracy of the result obtained for a fixed number of iterations is bounded by a quantity related to the Quantum Fisher Information of the variational state. Using this bound, we study the convergence property of the quantum approximate optimization algorithm under realistic noise models, showing the robustness of the algorithm against different noise strengths.
Manipulation and reconstruction of high-dimensional states7m
The capability to control and manipulate high dimensional quantum states has become relevant in several fields ranging from the probing of fundamentals of quantum mechanics to the development of safer encryption algorithms. Various engineering techniques of high dimensional quantum states have been proposed, but they strongly depend on the experimental platform and do not provide a general scheme. Here, we experimentally demonstrate an engineering protocol based on the Quantum Walk (QW) dynamic encoding the walker state in the orbital angular momentum (OAM) degree of freedom and the coin state in the spin angular momentum (SAM). The QW dynamic allows the implementation of a platform-independent scheme to engineer qudit states encoded in the walker system. Each step of the 5-steps QW is composed of a set of wave-plates that manipulate the coin state and a peculiar device, the q-plate, that can conditionally change the OAM according to the polarization. Consequently, the walker dynamics are controlled by a suitable choice of step-dependent coin operators. Moreover, decode the information stored in the OAM states is challenging experimentally and theoretically. Indeed, the platforms proposed envisage additional instruments, such as interferometry and spatial filtering, that introduce damaging noise and loss. In this regard, we characterized structured beams where the helicoidal wavefront is coupled with a not uniform distribution of the polarization on the transverse plane (Vector Vortex Beam), by using both supervised and unsupervised machine learning techniques. In particular, we obtained optimal results characterizing 15 experimental classes using both Convolutional Neural Network or Support Vector Machine supported by Principal Component Analysis (PCA). The regression task is addressed too, leveraging PCA to reconstruct a specific class of Vector Vortex Beams.
(Università di Roma "Sapienza")
The quantum Wasserstein distance of order 18m
We propose a generalization of the Wasserstein distance of order 1 to the quantum states of n qudits. Our proposal recovers the classical Wasserstein distance for quantum states diagonal in the canonical basis, hence the distance between vectors of the canonical basis coincides with the Hamming distance. Our distance is invariant with respect to permutations of the qudits and unitary operations acting on one qudit and is additive with respect to the tensor product. Our main result is a continuity bound for the von Neumann entropy with respect to our distance, which significantly strengthens the best continuity bound with respect to the trace distance. We also propose a generalization to quantum observables of the Lipschitz constant for functions, which allows us to compute our distance with a semidefinite program. We prove a quantum version of Marton's transportation inequality and a quantum Gaussian concentration inequality for the spectrum of quantum Lipschitz observables. Moreover, we explore the contraction coefficient with respect to our distance of the n-th tensor power of a one-qudit quantum channel and of shallow quantum circuits. Our distance can have a large impact in quantum information, quantum computation and quantum machine learning, and we discuss several possible applications.
Generic aspects of the resource theory of coherence7m
The class of incoherent operations induces a pre-order on the set of quantum pure states. We study the maximal success probability of incoherent conversion between pairs of n-dimensional random pure states chosen independently, and find an explicit formula for its large-n asymptotic distribution. Our analysis shows that the statistics of the maximal conversion probability can be determined by the behaviour of the extreme values.
Contextuality-by-default for behaviours in compatibility scenarios8m
The compatibility-hypergraph approach to contextuality (CA) and the contextuality-by-default approach (CbD) are usually seen as products of entirely different views on how physical measurements and measurement contexts should be understood: the latter is based on the idea that a physical measurement has to be seen as a collection of random variables, one for each context containing that measurement, while the imposition of the non-disturbance condition as a physical requirement in the former precludes such interpretation of measurements. The aim of our work is to show that the main idea behind CbD is already implicit in CA and to introduce in the latter important ideas which arise from the former. We introduce in CA the non-degeneracy condition, which is the analogous of consistent connectedness, and prove that this condition is, in general, weaker than non-disturbance. The set of non-degenerate behaviours defines a polytope, therefore one can characterize non-degeneracy using linear inequalities. We introduce the idea of extended contextuality for behaviours and prove that a behaviour is non-contextual in the standard sense iff it is non-degenerate and non-contextual in the extended sense. Finally, we use extended scenarios and behaviours to shed new light on our results.
Mechanism of decoherene-free coupling between giant atoms15m
Giant atoms are a new paradigm of quantum optics going beyond the usual local coupling. Building on this, a new type of decoherence-free (DF) many-body Hamiltonians was shown in a broadband waveguide. Here, these are incorporated in a general framework (not relying on master equations) and contrasted to dispersive DF Hamiltonians with normal atoms: the two schemes are shown to correspond to qualitatively different ways to match the same general condition for suppressing decoherence. Next, we map the giant atoms dynamics into a cascaded collision model (CM), providing an intuitive interpretation of the connection between non-trivial DF Hamiltonians and coupling points topology. The braided configuration is shown to implement a scheme where a shuttling system subject to periodic phase kicks mediates a DF coupling between the atoms. From the viewpoint of CMs theory, this shows a collision model where ancillas effectively implement a dissipationless, maximally-entangling two-qubit gate on the system.
(Università degli Studi di Palermo, Dipartimento di Fisica e Chimica - Emilio Segrè)
Decoherence-Free Rotational Degrees of Freedom for Quantum Applications15m
Quantum metrology and sensing represent promising near term applications of quantum technologies as they require the control of a few or even single quantum systems. However, a central challenge common to all quantum sensor designs is the necessity to achieve both robustness to environmental noise and, at the same time, high sensitivity to a signal of interest. This task becomes even more challenging when considering massive particles whose translational degrees of freedom are highly susceptible to first order (gradient) fluctuations of external fields, thus inhibiting the generation of long-lived macroscopic quantum superpositions. In this talk, we address this challenge by designing the shape of rigid bodies such that their rotational degrees of freedom can be made robust against decoherence from distant sources, while at the same time allowing for interaction with signals from nearby sources. To this end we introduce a systematic method, based on the mathematical theory of spherical t-designs, to construct rigid bodies whose rotational states are degenerate up to a desired order of the multipole expansion of their energy in a perturbing potential. This allows for the generation of long-lived macroscopic quantum superpositions of rotational degrees of freedom and the robust generation of entanglement between two or more such solids with applications in robust quantum sensing and precision metrology as well as quantum registers.
Quantum Zeno and anti-Zeno effect in non-Markovian decay process of single-photon polarization states15m
The quantum Zeno effect is a feature of quantum-mechanical systems allowing a system's time evolution to be freezed, or at least slowed down, by measuring the system frequently enough [1-5]. On the contrary, it is also possible to exploit frequent measurements to accelerate the system's evolution, obtaining the quantum anti-Zeno effect. In my presentation, I will describe an experiment investigating quantum Zeno and anti-Zeno effects in the non-Markovian decay process of single-photon polarization states. In our implementation, we simulate a noisy quantum channel exploiting a set of half wave-plates introducing correlated random phase shifts between the vertical and horizontal polarization components. Each phase shift represents a stochastic process defined by a random variable, sampled each time by a specific probability distribution depending on the previous phase shifts (non-Markovian behavior). This stochastic polarization dephasing leads to a decay of the probability to find the system in its initial state. To induce the Zeno effect, we perform repeated measurements by inserting a polarizer between subsequent wave-plates. By controlling the interplay between the application of a sequence of repeated measurements and the probability distribution characterizing the noise of the channel, it is possible to induce on the quantum state both Zeno or anti-Zeno effect. This experiment represents a proof of principle of a technique allowing to control the dynamics of a quantum system in any realistic physical scenario affected by time-correlated noise. In real scenarios, the randomness on the phase can be due to imperfections of the measurement apparatus or to the interaction with an external reservoir, usually entailing non-Markovianity  in the observed quantum system.
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(INRiM, Università di Torino)