Loss-tolerant concatenated Bell-state measurement with encoded coherent-state qubits for long-range quantum communication

TL;DR

This paper introduces a concatenated Bell-state measurement scheme using encoded coherent-state qubits that significantly improves success probability and robustness against photon loss, enabling efficient long-range quantum communication.

Contribution

It proposes a hardware-efficient CBSM scheme with modified parity encoding for coherent states, enhancing failure suppression and dephasing resistance in quantum communication.

Findings

Success probability approaches unity at low photon loss rates.

Enables quantum communication over distances exceeding 1000 km.

Performance compares favorably with existing methods.

Abstract

The coherent-state qubit is a promising candidate for optical quantum information processing due to its nearly-deterministic nature of the Bell-state measurement (BSM). However, its non-orthogonality incurs difficulties such as failure of the BSM. One may use a large amplitude ($\alpha$) for the coherent state to minimize the failure probability, but the qubit then becomes more vulnerable to dephasing by photon loss. We propose a hardware-efficient concatenated BSM (CBSM) scheme with modified parity encoding using coherent states with reasonably small amplitudes ($|\alpha| \lessapprox 2$), which simultaneously suppresses both failures and dephasing in the BSM procedure. We numerically show that the CBSM scheme achieves a success probability arbitrarily close to unity for appropriate values of $\alpha$ and sufficiently low photon loss rates (e.g., $\lessapprox 5\%$). Furthermore, we verify that the quantum repeater scheme exploiting the CBSM scheme for quantum error correction enables one to carry out efficient long-range quantum communication over 1000 km. We show that the performance is comparable to those of other up-to-date methods or even outperforms them for some cases. Finally, we present methods to prepare logical qubits under modified parity encoding and implement elementary logical operations, which consist of several physical-level ingredients such as generation of Schr\"odinger's cat state and elementary gates under coherent-state basis. Our work demonstrates that the encoded coherent-state qubits in free-propagating fields provide an alternative route to fault-tolerant information processing, especially long-range quantum communication.