On-chip storage of broadband photonic qubits in a cavity-protected rare-earth ensemble

TL;DR

This paper demonstrates a cavity-protected quantum memory using a dense ensemble of neodymium ions coupled to a nanophotonic resonator, enabling near-perfect decoherence suppression and high-fidelity broadband photonic qubit storage.

Contribution

It introduces a novel cavity protection technique that suppresses decoherence in solid-state ensembles, enabling ultra-fast, high-fidelity broadband quantum storage and transfer of photonic qubits.

Findings

Near-complete suppression of decoherence via cavity protection.

High-fidelity (98.7%) retrieval of broadband photonic qubits.

Demonstration of ultra-fast transfer (~50 GHz bandwidth) of photonic frequency qubits.

Abstract

Ensembles of solid-state optical emitters enable broadband quantum storage and transduction of photonic qubits, with applications in high-rate optical quantum networks for secure communications, global time-keeping, and interconnecting future quantum computers. To realize coherent quantum information transfer using ensembles, spin rephasing techniques are currently used to mitigate fast decoherence resulting from inhomogeneous broadening. Here we use a dense ensemble of neodymium rare-earth ions strongly coupled to a nanophotonic resonator to demonstrate that decoherence of a single photon excitation is near-completely suppressed via cavity protection- a new technique for accessing the decoherence-free subspace of collective coupling. The protected Rabi oscillations between the cavity field and the atomic superradiant state thereby enable ultra-fast transfer of photonic frequency qubits (~50 GHz bandwidth) into the ions, followed by retrieval with 98.7% fidelity. By coupling the superradiant excitation to other long-lived rare-earth spin states, this technology will enable broadband, always-ready quantum memories and fast optical-to-microwave transducers.