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The development of a single-photon counter based on superconducting qubits holds great promise for detecting weak and elusive signals, such as axions and high-frequency gravitational waves (HFGWs). Integrating such a detector into the INFN QUAX experiment could significantly enhance its sensitivity in the search for axions.
As part of the INFN Qub-It project, this work focuses on designing an itinerant single-photon counter using two or more superconducting qubits dispersively coupled to a common storage cavity. By leveraging the quantum non-demolition (QND) technique, this approach aims to enhance sensitivity while reducing dark count rates. The design is informed by state-of-the-art two-dimensional (2D) and three-dimensional (3D) schemes.
In the initial phase, superconducting transmon qubits —designed to resolve photon numbers inside a 3D cavity—were fabricated (TII, CNR-IFN) and fully characterized through experimental measurements in a 10mK cryogenic environment at the INFN National Laboratory of Frascati. To refine the design, simulations of a 3D device consisting of a transmon qubit inside a 3D cavity were conducted using Ansys HFSS and Python-based energy participation ratio (PyEPR) analysis. Several key system parameters were calculated, and experimental tests on the fabricated superconducting qubits validated the simulations, confirming the high sensitivity and stability required for photon counting within the target frequency range.
Building on this foundation, we extended our design to a two-qubit system within the same cavity to exploit “quantum coincidence,” reducing the false positive rate. First, we derived the equations governing the quantum state dynamics and analyzed the system’s time evolution using QuTiP, a quantum toolbox in Python. This provided insights into optimizing control parameters to achieve near-unity detection efficiency for the final state |11>, corresponding to simultaneous photon detection by both qubits. Subsequently, Ansys HFSS and PyEPR analysis were used to simulate the two-qubit system, focusing on qubit-cavity interactions, individual qubit properties, and couplings with dedicated readout cavities.
This work presents the experimental development, the design parameters and expected performance of the device, along with its anticipated impact on enhancing sensitivity in axion detection experiments.
