Deterministic entanglement of superconducting qubits by parity measurement and feedback

TL;DR

This paper demonstrates a high-fidelity, deterministic entanglement generation between superconducting qubits using a parity measurement and feedback control, advancing quantum error correction capabilities.

Contribution

It presents the first realization of a parity meter that creates entanglement for both measurement outcomes and implements feedback to deterministically generate entanglement.

Findings

Achieved 77% concurrence in entanglement via parity measurement.

Transformed probabilistic entanglement into deterministic with 66% fidelity.

Enabled key components for active quantum error correction.

Abstract

The stochastic evolution of quantum systems during measurement is arguably the most enigmatic feature of quantum mechanics. Measuring a quantum system typically steers it towards a classical state, destroying any initial quantum superposition and any entanglement with other quantum systems. Remarkably, the measurement of a shared property between non-interacting quantum systems can generate entanglement starting from an uncorrelated state. Of special interest in quantum computing is the parity measurement, which projects a register of quantum bits (qubits) to a state with an even or odd total number of excitations. Crucially, a parity meter must discern the two parities with high fidelity while preserving coherence between same-parity states. Despite numerous proposals for atomic, semiconducting, and superconducting qubits, realizing a parity meter creating entanglement for both even and odd measurement results has remained an outstanding challenge. We realize a time-resolved, continuous parity measurement of two superconducting qubits using the cavity in a 3D circuit quantum electrodynamics (cQED) architecture and phase-sensitive parametric amplification. Using postselection, we produce entanglement by parity measurement reaching 77% concurrence. Incorporating the parity meter in a feedback-control loop, we transform the entanglement generation from probabilistic to fully deterministic, achieving 66% fidelity to a target Bell state on demand. These realizations of a parity meter and a feedback-enabled deterministic measurement protocol provide key ingredients for active quantum error correction in the solid state.