Controlled Rephasing of Single Collective Spin Excitations in a Cold Atomic Quantum Memory

Abstract

We demonstrate active control of inhomogeneous dephasing and rephasing for single collective atomic spin excitations (spin-waves) created by spontaneous Raman scattering in a quantum memory based on cold 87 Rb atoms. The control is provided by a reversible external magnetic field gradient inducing an inhomogeneous broadening of the atomic hyperfine levels. We demonstrate experimentally that active rephasing preserves the single photon nature of the retrieved photons. Finally, we show that the control of the inhomogeneous dephasing enables the creation of timeseparated spin-waves in a single ensemble, followed by a selective readout in time. This is an important step towards the implementation of a functional temporally multiplexed quantum repeater node. Atomic ensembles provide an efficient way of reaching the strong interaction between matter and light required for the implementation of quantum memories, without the need for high finesse cavities. In addition, they give the possibility to multiplex quantum information, which is desirable in several applications. In particular, in quantum repeater architectures, this alleviates the limitation due to the communication time between the nodes [28]. In atomic ensembles, quantum information is stored as collective atomic spin excitations, called spin-waves. Single spin-waves offer the important advantage that they can be efficiently transferred to single photons in a well defined spatio-temporal mode thanks to constructive interference between the involved atoms. In 2001, Duan, Cirac, Lukin and Zoller (DLCZ) proposed a protocol to implement a quantum repeater using the heralded creation of spin-waves in an atomic ensemble [5]. Several demonstrations of the building block of the protocol have been reported [12,16,29] including functional elementary segments of a quantum repeater [30,31]. Most of these demonstrations however, used only a single spin-wave per ensemble, although several ensembles have been already implemented in the same atom trap [32,33].A recent theoretical proposal has shown that the ability to precisely control the quantum state of single spinwaves would open new avenues for the realization of more efficient, temporally multiplexed quantum repeater architectures [34]. In particular, it has been proposed that the implementation of controlled dephasings and rephasings of single spin-waves would allow the creation of several time-separated single spin-waves in a single ensemble which could be selectively read out at different times. Rephasing protocols have also been proposed and implemented for optical collective atomic excitations, leading to the storage and retrieval of 64 weak optical modes in a rare-earth doped crystal [35], with pre-determined storage times. The capability to control the dephasing of spin-waves has also been used recently to implement a coherent optical pulse sequencer [36], efficient light storage for bright [37] and weak coherent pulses [17] using the gradient echo memory protocol [38], as well as storage of several temporal mo...

Generation of single photons with highly tunable wave shape from a cold atomic ensemble

“…For short read photon durations, we are still limited by noise due to higher-order components of the spin-wave, which can be addressed by reducing p w . In fact, the observed ≈0.4 is consistent with former measurements at similar values for p w and read pulse durations43. For longer read photon durations, we observe an increase of , which can be simply explained by a higher number of dark counts of the SPDs for longer read photon detection gates (see upper axis in Fig.…”

“…We verify numerically that in the absence of technical noise and considering infinite spin-wave coherence, for OD=50 an intrinsic retrieval efficiency of 80% can be achieved while maintaining control of the photon shape. The decrease of the efficiency at around 10 μs is due to dephasing of the spin-wave induced by atomic motion, spurious external magnetic field gradients43 and the finite read laser coherence time. In particular, our numerical simulations show clearly that in the absence of technical noise and in the limit of infinite spin coherence, the efficiency is kept constant (see orange diamonds and dashed line in Fig.…”

Wavevector multiplexed atomic quantum memory via spatially-resolved single-photon detection

“…By considering S photons in the given ROI, we calculate the conditional probability f ( κ ) of registering AS photon in the conjugate ROI in the AS arm, which gives us: where squaring is due to the two-dimensional character of the problem. Finally, to estimate the net coincidence probability, we additionally consider the total number of accidental coincidences, which is very well approximated by a product of probabilities in S and AS arms p S p AS 12 , 24 , 56 , 65 , 66 .…”

Nonclassical correlations between a C-band telecom photon and a stored spin-wave

“…For example if the atomic qubit was entangled with a converted photonic qubit (e.g. in polarization [37] or time-bin [38]) or in a two ensemble entanglement experiment [39] we can infer V max = (g (2) cw,r − 1)/(g (2) cw,r + 1). Here, it is assumed that the QFCD is phase-preserving [31,35].…”