Loss-tolerant parity measurement for distant quantum bits

TL;DR

This paper introduces a loss-tolerant parity measurement scheme for distant qubits that maintains coherence and enables high-fidelity entanglement despite transmission losses, using a superposition of coherent states as a probe.

Contribution

The authors develop a novel parity measurement protocol that is resilient to channel losses, allowing deterministic entanglement generation between distant qubits.

Findings

High-fidelity entanglement achieved despite significant transmission loss

Transmission loss mainly reduces measurement strength, not coherence

Repeated weak measurements enable robust entanglement

Abstract

We propose a scheme to measure the parity of two distant qubits, while ensuring that losses on the quantum channel between them does not destroy coherences within the parity subspaces. This capability enables deterministic preparation of highly entangled qubit states whose fidelity is not limited by the transmission loss. The key observation is that for a probe electromagnetic field in a particular quantum state, namely a superposition of two coherent states of opposite phases, the transmission loss stochastically applies a near-unitary back-action on the probe state. This leads to a parity measurement protocol where the main effect of the transmission losses is a decrease in the measurement strength. By repeating the non-destructive (weak) parity measurement, one achieves a high-fidelity entanglement in spite of a significant transmission loss.