Violating Bell's inequality in gate-defined quantum dots

TL;DR

This paper demonstrates violation of Bell's inequality in gate-defined quantum dots by achieving high-fidelity entangled states through advanced calibration, even at elevated temperatures and with long entanglement lifetimes.

Contribution

The study introduces heralded initialization and gate set tomography to significantly improve entanglement fidelity in quantum dots, enabling Bell inequality violation.

Findings

Bell state fidelity of 97.17% without readout correction

Bell signal of 2.731 close to the maximum of 2.828

Entanglement persists at 1.1 K and 100 μs lifetime

Abstract

Superior computational power promised by quantum computers utilises the fundamental quantum mechanical principle of entanglement. However, achieving entanglement and verifying that the generated state does not follow the principle of local causality has proven difficult for spin qubits in gate-defined quantum dots, as it requires simultaneously high concurrence values and readout fidelities to break the classical bound imposed by Bell's inequality. Here we employ heralded initialization and calibration via gate set tomography (GST), to reduce all relevant errors and push the fidelities of the full 2-qubit gate set above 99 %, including state preparation and measurement (SPAM). We demonstrate a 97.17 % Bell state fidelity without correcting for readout errors and violate Bell's inequality with a Bell signal of S = 2.731 close to the theoretical maximum of $2\sqrt{2}$. Our measurements exceed the classical limit even at elevated temperatures of 1.1 K or entanglement lifetimes of 100 $\mu s$.