Experimental demonstration of memory-enhanced quantum communication

Abstract

The ability to communicate quantum information over long distances is of central importance in quantum science and engineering1. Although some applications of quantum communication such as secure quantum key distribution2,3 are already being successfully deployed4,5,6,7, their range is currently limited by photon losses and cannot be extended using straightforward measure-and-repeat strategies without compromising unconditional security8. Alternatively, quantum repeaters9, which utilize intermediate quantum memory nodes and error correction techniques, can extend the range of quantum channels. However, their implementation remains an outstanding challenge10,11,12,13,14,15,16, requiring a combination of efficient and high-fidelity quantum memories, gate operations, and measurements. Here we use a single solid-state spin memory integrated in a nanophotonic diamond resonator17,18,19 to implement asynchronous photonic Bell-state measurements, which are a key component of quantum repeaters. In a proof-of-principle experiment, we demonstrate high-fidelity operation that effectively enables quantum communication at a rate that surpasses the ideal loss-equivalent direct-transmission method while operating at megahertz clock speeds. These results represent a crucial step towards practical quantum repeaters and large-scale quantum networks20,21.

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Fig. 1: Concept of memory-enhanced quantum communication.
Fig. 2: Realization of heralded spin–photon gate.
Fig. 3: Asynchronous BSMs using quantum memory.
Fig. 4: Performance of memory-assisted quantum communication.

Data availability

All data related to the current study are available from the corresponding author on reasonable request.

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Acknowledgements

We thank P. Stroganov, K. de Greve, J. Borregaard, E. Bersin, B. Dixon, R. Murphy and N. Sinclair for discussions and experimental help, V. Anant from PhotonSpot for providing SNSPDs, and J. MacArthur for assistance with electronics. This work was supported by the NSF, CUA, DoD/ARO DURIP, AFOSR MURI, ONR MURI, ARL, DOE and a Vannevar Bush Faculty Fellowship. Devices were fabricated at Harvard CNS, NSF award no. 1541959. M.K.B. and D.S.L. acknowledge support from an NDSEG Fellowship. R.R. acknowledges support from the Alexander von Humboldt Foundation. B.M. and E.N.K. acknowledge support from an NSF GRFP.

Author information

M.K.B., R.R., B.M., D.S.L., C.T.N., D.D.S. and M.D.L. planned the experiment, B.M. and E.N.K. fabricated the devices, M.K.B., R.R., B.M., D.S.L., C.T.N. and D.D.S. built the set-up, performed the experiment and analysed the data. All work was supervised by H.P., D.E., M.L. and M.D.L. All authors discussed the results and contributed to the manuscript. M.K.B., R.R., B.M., D.S.L. and C.T.N. contributed equally to this work.

Correspondence to M. D. Lukin.

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The authors declare no competing interests.

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Peer review information Nature thanks Josh Nunn and Wolfgang Tittel for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Experimental schematic.

a, Control flow of experiment. HSDIO (National Instruments) is a digital signal generator that synchronizes the experiment. Opt (MW) AWG is a Tektronix AWG7122B 5 GS/s (Tektronix AWG70001a 50 GS/s) arbitrary waveform generator used to generate photonic qubits (microwave control signals). All signals are recorded on a time-tagger (TT, PicoQuant HydraHarp 400). b, Fibre network used to deliver photons to and collect photons from the memory device, including elements for polarization control and diagnostic measurements of coupling efficiencies using photodiodes M1, M2 and MC. c, Preparation of optical fields. The desired phase relation between lock and qubit paths is ensured by modulating AOMs using phase-locked RF sources with a precise 1.8 MHz frequency shift between them. The AM (amplitude modulator) and ΦM (phase modulator) are used to define the photonic qubits.

Extended Data Fig. 2 Characterization of device cooperativity.

a, Cavity reflection spectrum far-detuned (blue) and on resonance (red) with SiV centre. Blue solid line is a fit to a Lorentzian, enabling extraction of linewidth κ = 21.8 GHz. Red solid line is a fit to a model used to determine the single-photon Rabi frequency g = 8.38 ± 0.05 GHz and shows the onset of a normal mode splitting. b, Measurement of SiV linewidth far detuned (Δc = 248 GHz) from cavity resonance. Red solid line is a fit to a Lorentzian, enabling extraction of natural linewidth γ = 0.123 GHz.

Extended Data Fig. 3 Microwave characterization of spin-coherence properties.

a, Optically detected magnetic resonance spectrum of the qubit transition at ~12 GHz split by coupling to a nearby 13C. b, Rabi oscillations, read out via the population in the |↑〉 state (p) showing π time of τR = 30 ns. A π time of 32 ns is used for experiments reported in the main text. c, XY8-1 dynamical decoupling signal (unnormalized) as a function of total time T, showing coherence lasting on the timescale of several hundred microseconds. d, XY8-8 dynamical decoupling signal (normalized) revealing a region of high fidelity at the relevant value of 2τ = 142 ns. e, Fidelity of spin state after a dynamical decoupling sequence with varying numbers of π pulses (Nπ; blue points). Red point (diamond) is under illumination with 〈nm = 0.02.

Extended Data Fig. 4 Measurements on a single time-bin qubit in Z and X bases.

a, Example of optical pulses sent in the experiment described in Fig. 2d. b, Time trace of detected photons on the + detector (see Fig. 2a) when the pulses shown in a are sent directly into the TDI. The first and last peaks correspond to late and early photons taking the long and short paths of the TDI, which enable measurements in the Z basis, {|e〉,|l〉}. The central bin corresponds to the late and early components overlapping and interfering constructively to come out of the + port, equivalent to a measurement of the time-bin qubit in the |+x〉 state. A detection event in this same timing window on the other detector (not shown) would constitute a |−x〉 measurement. In this measurement, the TDI was left unlocked, so we observe no interference in the central window.

Extended Data Fig. 5 Performance of memory device versus channel loss.

a, Enhancement of memory-based approach compared to direct-transmission approach, keeping N = 124 fixed and varying 〈nm in order to vary the effective channel transmission probability, pA→B. At high pA→B (larger 〈nm), rs approaches 0 owing to increased QBER arising from undetected scattering of a third photon. b, Left, plot of QBER for same sweep of 〈nm shown in a. Right, plot of QBER while sweeping N in order to vary loss. These points correspond to the same data shown in Fig. 4. At lower pA→B (larger N), microwave-induced heating-related dephasing leads to increased QBER. Vertical error bars, 68% confidence interval; horizontal error bars, s.d. of the systematic power fluctuations.

Extended Data Table 1 High-level experimental sequence
Extended Data Table 2 Main experimental sequence
Extended Data Table 3 Truth table of asynchronous BSM protocol
Extended Data Table 4 Quantum-memory-based advantage

Supplementary information

Supplementary Information

Details about the theoretical description of Bell-state measurement and analysis of the quantum communication experiment.

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Bhaskar, M.K., Riedinger, R., Machielse, B. et al. Experimental demonstration of memory-enhanced quantum communication. Nature 580, 60–64 (2020). https://doi.org/10.1038/s41586-020-2103-5

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