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Abstract: The current generation of quantum machines can perform universal quantum logic operations, but the number of successive operations is limited by noise, imperfections, and decoherence. My work focuses on improving the quality of quantum measurement in superconducting circuits, which is induces the largest errors at the moment (on order 1%). In the first part of the seminar, I will discuss new tomographic methods to experimentally reconstruct the complete quantum process behind a quantum non-demolition (QND) measurement [1,2]. This provides valuable information to identify measurement errors, and we show how to use this to improve the calibration of dispersive superconducting qubit, quantify more precisely the QNDness and back-action of the measurement, as well as to characterize multi-qubit parity measurements needed in quantum error correcting codes. In the second part of the seminar, I will propose a new method to improve the performance of directional quantum limited amplification for superconducting qubit readout and detection of broadband microwave signals. This is based on a new proposal for doing a topological Josephson travelling wave parametric amplifier (topological JTWPA) by using a global pump on Josephson junction array [3,4]. Tuning the spatial dependence of pump’s phase, we can break time-reversal symmetry, allowing the device to enter a topological amplifying steady-state phase]. In this regime, microwave signals are unidirectionally and exponentially amplified along the JJ array with all back-reflections exponentially suppressed [5]. With state-of-art technology the device can be fully integrated on-chip and reach near quantum limited amplification with large gain (>20 dB), large reverse isolation (<-20 dB), large bandwidth (~1 GHz), and large robustness to disorder.
References: [1] L. Pereira, J.J. García-Ripoll, TR, “Complete physical characterization of QND measurements via tomography”, Phys. Rev. Lett. 129, 010402 (2022). [2] L. Pereira, J.J. García-Ripoll, TR, “Parallel QND measurement tomography of multi-qubit quantum devices”, npj Quantum Information 9, 22 (2023).
[3] TR, A. Gómez-León, J.J. García-Ripoll, A. González-Tudela, D. Porras, “Directional Josephson traveling-wave parametric amplifier via non-Hermitian topology”, arXiv:2207.13728.
[4] A. Gómez-León, TR, A. González-Tudela, D. Porras, “Non-Hermitian topological phases in traveling-wave parametric amplifiers”, Quantum 7, 1016 (2023).
[5] TR, J.J. García-Ripoll, D. Porras, “Topological input-output theory for directional amplification”, Phys. Rev. A 103, 033513 (2021).
