Improved Superconducting Qubit State Readout by Path Interference

High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting quantum circuit, the qubit state is usually determined by detecting the dispersive frequency shift of a microwave cavity from either transmission or reflection. We demonstrate the use of constructive interference between the transmitted and reflected signal to optimize the qubit state readout, with which we find a better resolved state discrimination and an improved qubit readout fidelity. As a simple and convenient approach, our scheme can be combined with other qubit readout methods based on the discrimination of cavity photon states to further improve the qubit state readout.

Frequency-tunable microwave quantum light source based on superconducting quantum circuits

A non-classical light source is essential for implementing a wide range of quantum information processing protocols,including quantum computing,networking,communication and metrology.In the microwave regime,propagating photonic qubits,which transfer quantum information between multiple superconducting quantum chips,serve as building blocks for large-scale quantum computers.In this context,spectral control of propagating single photons is crucial for interfacing different quantum nodes with varied frequencies and bandwidths.Here a deterministic microwave quantum light source was demonstrated based on superconducting quantum circuits that can generate propagating single photons,time-bin encoded photonic qubits and qudits.In particular,the frequency of the emitted photons can be tuned in situ as large as 200 MHz.Even though the internal quantum efficiency of the light source is sensitive to the working fre-quency,it is shown that the fidelity of the propagating photonic qubit can be well preserved with the time-bin encoding scheme.This work thus demonstrates a versatile approach to realizing a practical quantum light source for future distributed quantum computing.

Experimental repetitive quantum channel simulation

Universal control of quantum systems is a major goal to be achieved for quantum information processing,which demands thorough understanding of fundamental quantum mechanics and promises applications of quantum technologies. So far, most studies concentrate on ideally isolated quantum systems governed by unitary evolutions, while practical quantum systems are open and described by quantum channels due to their inevitable coupling to environment. Here, we experimentally simulate arbitrary quantum channels for an open quantum system, i.e. a single photonic qubit in a superconducting quantum circuit.The arbitrary channel simulation is achieved with minimum resource of only one ancilla qubit and measurement-based adaptive control. By repetitively implementing the quantum channel simulation,we realize an arbitrary Liouvillian for a continuous evolution of an open quantum system for the first time. Our experiment provides not only a testbed for understanding quantum noise and decoherence,but also a powerful tool for full control of practical open quantum systems.