Entangling Different-Color Photons via Time-Resolved Measurement and Active Feed Forward

Abstract

Entangling independent photons is not only of fundamental interest but also of crucial importance for quantum information science. Two-photon interference is a major method to entangle independent identical photons. If two photons are color-different, perfect two-photon coalescence cannot happen anymore, which makes the entangling of color-different photons difficult to realize. In this letter by exploring and developing time-resolved measurement and active feed-forward, we have entangled two independent photons of different colors for the first time. We find that entanglement with a varying form can be identified for different two-photon temporal modes through time-resolved measurement. By using active feed-forward we are able to convert the varying entanglement into uniform. Adopting these measures, we have successfully entangled two photons with a frequency separation of 16 times larger than their linewidths. In addition to its fundamental interest, our work also provides an approach to solve the frequency mismatch problem for future quantum networks. Entangling independent photons through two-photon interference [1,2] is ubiquitous in photonic quantum information experiments [3][4][5]. When two identical photons are superimposed on a beam-splitter, the probabilities of both photons are transmitted or both are reflected interfere with each other and result in two-photon coalescence. Such a two-photon interference effect has been first observed by Hong, Ou and Mandel [6]. From a more fundamental point of view, this interference is due to the bosonic nature of photons [7]. Two identical photons have a symmetric wave function, thus their spacial wave function has to be symmetric, which leads to photon coalescence after passing through a beam-splitter. Therefore, only an anti-symmetric two-photon state will lead to a coincidence between different output ports of a beamsplitter, which constitutes the physical basis of measuring Bell states and entangling independent photons. What if the input photons are color-different, can we still make Bell-state measurement and entangle independent photons as usual, for instance in the degrees of polarization, time-bin and momentum?Entangling color-different photons also has strong practical applications. In quantum networking [8], photons from separate quantum systems are often different in color due to various reasons. For instance, in the condensed matter systems such as quantum dots, nitrogen vacancy (NV) centers, photons from two separate emitters are usually different in frequency due to their different local environments [9,10]. For all quantum systems, when they are moving with a high-speed (e.g. in a satellite or an airplane), the Doppler effect will give rise to significant frequency shifts for the emitting photons. Besides, the strong interest in hybrid quantum networking by combining the advantages of each physical system also necessitates the entangling operation between color-different photons. Preliminary studies have been carried out on the quantum beat of two ...

Entanglement of distinguishable quantum memories

“…In order to set the phase of the entangled memory state when t r is sufficiently small, either ∆t max must also be small or ∆t used to feedforward a phase adjustment to the memory state [10]. By restricting ∆t max 2π/∆ω, the maximum possible entangled memory state phase evolution is ∆ω∆t max .…”

“…This requirement of photon indistinguishability is a major impediment to the construction of heterogeneous quantum networks and also hinders the entanglement of similar memories whose emission frequencies differ due to variations in local environment or fabrication. Recently, photons with frequencies that differ by many linewidths have been entangled using time-resolved detection and active feedforward [10]. Here, we extend this technique to entangle distinguishable remote quantum memories by interference of distinguishable photons.…”

“…This decoherence can be reduced by postselecting entanglement events in which ∆t 2π/∆ω at the cost of entanglement rate [15]. Alternatively, if ∆ω is known, ∆t can be fed forward to subsequent operations to maintain high-fidelity, constantphase memory entanglement without sacrificing rate [10].In this Rapid Communication, we entangle two remote quantum memories that emit distinguishable photons by utilizing time-resolved detection of photonic polarization qubits. We reveal the time-dependent nature of the resulting entangled memory state and observe that temporally filtering detection events results in maximized state fidelity.…”

“…This decoherence can be reduced by postselecting entanglement events in which ∆t 2π/∆ω at the cost of entanglement rate [15]. Alternatively, if ∆ω is known, ∆t can be fed forward to subsequent operations to maintain high-fidelity, constantphase memory entanglement without sacrificing rate [10].…”

Quantum Erasure Cryptography

“…One notable exception is Zhao et al recently employing a frequency eraser to entangle, for the first time, different-color photons [6].…”