Meter-scale Microwave Quantum Networks for Superconducting Circuits

Abstract

A Bell test is a hardware agnostic experimental procedure which can reject classes of physical theories and be used to certify randomness or secure communications with untrusted devices. Violating a Bell inequality without any of the major loopholes - detection, freedom-of-choice, and locality - is an outstanding challenge which has been achieved only in a handful of experiments, using either NV centers, trapped ions, or optical photons. However, Bell tests with superconducting circuits, a top-contending quantum computing platform, have so far ignored the locality loophole due to the difficulty to entangle physically remote systems without an optical photon interface. In this thesis, I report the work we have done at the Quantum Device Lab towards realizing a loophole-free Bell test using superconducting qubits entangled using microwave photons. To minimize the distance required between the two Bell parties to close the locality loophole, we have developed a new readout parameter optimization procedure enabling the discrimination of a transmon qubit state with high fidelity in a record speed. We have also designed and realized a modular cryogenic link technology to connect superconducting circuits housed in separate dilution refrigerators and separated by a distance of up to 30 m with a milli-Kelvin temperature, lossless waveguide, which acts as a microwave quantum bus between the remote quantum systems. Using a microwave-activated sideband transition, we demonstrate the possibility to transfer qubit excitation via the successive emission and absorption of a single microwave photon with time-reversal-symmetric envelope, propagating within the cryogenic link. We also demonstrate a simple method to unconditionally reset the transmon qubits in record time and fidelity using this very sideband emission. Using the photon transfer scheme, we transfer qubit states, generate entangled states, and violate Bell's inequality with predetermined measurement settings, using two transmon qubits separated by up to 30 m. In the same experimental setup, we demonstrate random measurement-basis choice in less than 30 ns, and high-fidelity qubit readout in 50 ns. Therefore, this setup should be able to realize a signifiant loophole-free Bell test, with repetition rates exceeding 10 kHz. This would put yet-unachieved device-independent tasks within reach and would demonstrate the potential to use microwave photons for realizing local area quantum networks.