Non-local Physics with Superconducting Circuits

Abstract

Entanglement and non-locality constitute two of the defining features of quantum physics. The non-local nature of certain entangled systems can be demonstrated in a Bell test, which experimentally rules out local descriptions of the effect. Moreover, non-local entanglement, proven via Bell tests, serves as a novel resource in quantum information processing, enabling tasks impossible with only classical means. Specifically, such applications include testing the correct working of quantum devices with minimal assumptions about them, distributing a provably private key between parties in a quantum network, or generating certified random bits. Bell test experiments considered fully robust for testing fundamental physics and for the aforementioned applications must be free of loopholes, i.e. additional assumptions on the experimental implementation. These experiments are technically hard to achieve, as they impose strict quality requirements and tight technical constraints on the experimental setup. Such loophole-free Bell tests have so far been demonstrated with polarization-encoded photons, NV-centers in diamond, and neutral atoms. In this thesis, we realize a loophole-free Bell test with superconducting circuits, a prime contender for building large-scale quantum computing systems. We place a specific focus on closing the locality loophole, which is the necessity to prevent classical communication between the two parties involved in each repetition of a Bell test. This requires to entangle two superconducting circuits at a space-like separated distance of tens of meters given the Bell test protocol time of about 100 ns, a particularly challenging feat given the confines of dilution refrigerators in experiments involving microwave-frequency based superconducting circuits. Addressing this challenge, we present a unique 30-meter-long cryogenic microwave quantum link system, and we discuss its operation in detail. We further summarize the other critical building blocks of the experiment, such as fast, high-fidelity readout, random measurement basis choice, and precise synchronization and time control of two remote electronic setups. Combining these tools in a single experimental system, we present the first loophole-free Bell test with superconducting quantum bits, a landmark experiment in the field of circuit quantum electrodynamics. Furthermore, we demonstrate the use of a loophole-free Bell inequality violation for a task in quantum information processing with superconducting circuits. Specifically, we implement device-independent self-testing of a superconducting circuit system, showcasing the first successful use of the resource of non-local entanglement on a platform at the forefront of quantum computing. The results presented in this thesis pave the way for using non-local quantum effects with superconducting circuits to power further, novel quantum communication and distributed quantum computing algorithms, and for fundamental physics research.