Two-photon interference of weak coherent laser pulses recalled from separate solid-state quantum memories

TL;DR

This paper demonstrates that solid-state quantum memories can faithfully store and recall weak laser pulses without degrading their wavefunction, enabling high-visibility two-photon interference crucial for quantum information processing.

Contribution

It shows for the first time that quantum memories can preserve the entire photonic wavefunction, allowing two-photon interference after storage and retrieval.

Findings

Near-theoretical maximum interference visibility achieved

Quantum memories preserve the entire photonic wavefunction

Suitable for advanced quantum information applications

Abstract

Quantum memories for light, which allow the reversible transfer of quantum states between light and matter, are central to the development of quantum repeaters, quantum networks, and linear optics quantum computing. Significant progress has been reported in recent years, including the faithful transfer of quantum information from photons in pure and entangled qubit states. However, none of these demonstrations confirm that photons stored in and recalled from quantum memories remain suitable for two-photon interference measurements, such as C-NOT gates and Bell-state measurements, which constitute another key ingredient for all aforementioned applications of quantum information processing. Using pairs of weak laser pulses, each containing less than one photon on average, we demonstrate two-photon interference as well as a Bell-state measurement after either none, one, or both pulses have been reversibly mapped to separate thulium-doped titanium-indiffused lithium niobate (Ti:Tm:LiNbO3) waveguides. As the interference is always near the theoretical maximum, we conclude that our solid-state quantum memories, in addition to faithfully mapping quantum information, also preserves the entire photonic wavefunction. Hence, we demonstrate that our memories are generally suitable for use in advanced applications of quantum information processing that require two-photon interference.