A simple digital phase-shifter for multiple-quantum NMR

Jonathan A. Jones

Ioannis Krikidis

In this letter, we study the analogue pre/post-coding vector design for a point-to-point multiple-input multiple-output (MIMO) system with 1-bit phase shifters. Specifically, we focus on the signal-to-noise ratio (SNR) maximization problem which corresponds to a combinatorial NP-hard due to the binary phase resolution. Two classical computation heuristics are proposed i.e., i) an 1-bit real-valued approximation of the optimal digital designs, and ii) an alternating optimization where a Rayleigh quotient problem is solved at each iteration. An iterative quantum annealing (QA)-based heuristic is also investigated, which outperforms classical counterparts and achieves near-optimal performance while ensuring polynomial time complexity. Experimental results in a real-world D-WAVE QA device validate the efficiency of the proposed QA approach.

Junqin Cao, Zhixin Chen, Yaxin Wang +5 more

With the development of research on integrated photonic quantum information processing, the integration level of the integrated quantum photonic circuits has been increasing continuously, which makes the calibration of the phase shifters on the chip increasingly difficult. For the calibration of multiple cascaded phase shifters that is not easy to be decoupled, the resources consumed by conventional brute force methods increase exponentially with the number of phase shifters, making it impossible to calibrate a relatively large number of cascaded phase shifters. In this work, we experimentally validate an efficient method for calibrating cascaded phase shifters that achieves an exponential increase in calibration efficiency compared to the conventional method, thus solving the calibration problem for multiple cascaded phase shifters. Specifically, we experimentally calibrate an integrated quantum photonic circuit with nine cascaded phase shifters and achieve a high-precision calibration with an average fidelity of 99.26%.

Jonathan A. Jones

Xin Tao, Shijie Wei, Julen Simon Pedernales +2 more

Scott Potter McHaffie

This thesis investigates whether a programmable photonic integrated circuit can remain useful even after some of its thermo-optic phase shifters have become permanently disabled. The approach is based on a digital twin: a differentiable model of the chip is trained from experimental current-output data and then used to solve an inverse-programming problem to recreate desired optical intensity transformations. The method is demonstrated on an 8 × 8 photonic circuit for both classical and quantum tasks. The trained twin accurately modeled the experimental setup, achieving a test fidelity greater than 0.99 for all disabled-phase-shifter configurations. Using the twin as a differentiable forward model to solve for currents, a mean fidelity of 0.9914 ± 0.0001 was measured over 100 target intensity transformations of the form T=|U|², where U is a Haar-random unitary. When any of five individual phase shifters were disabled this mean fidelity remained above 0.9739 ± 0.0002, and under cumulative disabled phase shifters the fidelity decreased to 0.8100 ± 0.0006 at 28 disabled phase shifters. A postselected dual-rail CNOT gate was also implemented and the highest postselected fidelity of 0.8730 ± 0.0097 was obtained for 20 disabled phase shifters, indicating that damage could act as a regularizer for two-photon interference.