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Experimental quantum repeater without quantum memory
Zheng-Da Li, Rui Zhang, Xu-Fei Yin, Li-Zheng Liu, Yi Hu, Yu-Qiang Fang, Yue-Yang Fei, Xiao Jiang, Jun Zhang, Li Li, Nai-Le Liu, Feihu Xu, Yu-Ao Chen, Jian-Wei Pan
TL;DR
Conventional repeaters use intermediate quantum memories to retain entanglement until link outcomes are known, while all-photonic repeaters seek to replace those memories with graph-state measurements. This experiment uses GHZ states and passive-choice measurement in a 12-photon interferometer; it reports a 2 × 2 parallel all-photonic repeater demonstrated at down-conversion probability 0.0483 ± 0.0001.
Problem
Conventional repeaters use intermediate quantum memories to retain entanglement until neighboring-link outcomes are heralded and to enable selective Bell measurements.
Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6Method
The experiment replaces intermediate repeater graph states with GHZ states and uses passive-choice measurement to switch between entangling and disentangling operations.
Source: PASSIVE IMPLEMENTATION OF THE… · p. 6Results
The 2 × 2 parallel structured all-photonic quantum repeater is reported as fully demonstrated at down-conversion probability 0.0483 ± 0.0001.
Source: DETAIL RESULTS WITH DOWN-CONVERSION… · p. 11Takeaways & Limitations
Repeater graph states can replace intermediate quantum memories for selective Bell measurement, while strict shared quantum output states still require end-node memories.
Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6Strict shared quantum output states still require memories at Alice and Bob to determine which photons are entangled.
Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6
Abstract
from arXiv · showhide
Quantum repeaters -- important components of a scalable quantum internet -- enable the entanglement to be distributed over long distances. The standard paradigm for a quantum repeater relies on a necessary demanding requirement of quantum memory. Despite significant progress, the limited performance of quantum memory makes practical quantum repeaters still a great challenge. Remarkably, a proposed all-photonic quantum repeater avoids the need for quantum memory by harnessing the graph states in the repeater nodes. Here we perform an experimental demonstration of an all-photonic quantum repeater using linear optics. By manipulating a 12-photon interferometer, we implement a 2-by-2 parallel all-photonic quantum repeater, and observe an 89% enhancement of entanglement-generation rate over the standard parallel entanglement swapping. These results open a new way towards designing repeaters with efficient single-photon sources and photonic graph states, and suggest that the all-photonic scheme represents an alternative path -- parallel to that of matter-memory-based schemes -- towards realizing practical quantum repeaters.
MEMORY-BASED AND ALL-PHOTONIC QUANTUM REPEATERS
Standard memory-based repeaters use intermediate quantum memories to retain entanglement until neighboring links are heralded, enabling selective Bell measurements. All-photonic repeaters use repeater graph states and local measurements for this switching, though strict shared quantum outputs still require end-node memories.
MEMORY-BASED AND ALL-PHOTONIC QUANTUM REPEATERS
- Intermediate quantum memories retain entanglement until neighboring-link success or failure is heralded, then enable Bell measurements only on successfully entangled pairs.This selective measurement is the role the all-photonic scheme aims to replace at intermediate nodes.Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6
- Repeater graph states use local feed-forward measurements to connect distant photons after a successful BSM or break off a photon after failure.The heralding signals are local to each node, so their waiting time is zero in principle.Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6
- For a strictly shared quantum output state, both schemes still require memories at Alice and Bob to determine which photons are entangled.In this experiment, post-selection determines the final entanglement instead.Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6
- For protocols needing only shared correlated information, delay-choice entanglement swapping can remove end-node memories in the all-photonic scheme.Alice and Bob can measure their local qubits before receiving classical signals from intermediate nodes.Source: MEMORY-BASED AND ALL-PHOTONIC QUANTUM… · p. 6
PASSIVE IMPLEMENTATION OF THE SELECTIVE BSM
The implementation substitutes GHZ states and passive-choice measurement for repeater graph states and active feed-forward. The PCM switches between Bell measurement and X-basis projection according to its detection pattern, while imperfect efficiency contributes estimated errors.
PASSIVE IMPLEMENTATION OF THE SELECTIVE BSM
- GHZ states replace repeater graph states, with Bell measurement establishing entanglement and X-basis measurement disentangling an unwanted qubit.The GHZ state is local-unitary equivalent to a repeater graph state.Source: PASSIVE IMPLEMENTATION OF THE… · p. 6
- The PCM performs a BSM when coincident detections occur at both CPBS outputs and an X-basis projection when detection occurs at only one output.This passive switching avoids an active feed-forward gate to decide which photon is connected or disconnected.Source: PASSIVE IMPLEMENTATION OF THE… · p. 6
- The PCM is built from a circular PBS, two normal PBSs, and four single-photon detectors.A circular PBS is illustrated in Fig. 6.Source: PASSIVE IMPLEMENTATION OF THE… · p. 6, Figure 6 · p. 8
- At 38% average system efficiency, estimated error rates are 1.04% and 1.45% for down-conversion probabilities 0.0344 and 0.0483, respectively.The authors state these errors can be ignored in the experiment.Source: PASSIVE IMPLEMENTATION OF THE… · p. 7
THE CORRESPONDING FINAL STATES WITH DIFFERENT RESULTS OF FOUR PCMS
Table I associates different PCM outcomes with the corresponding final states.
THE CORRESPONDING FINAL STATES WITH DIFFERENT RESULTS OF FOUR PCMS
- Table I maps different PCM results to their respective final states.The table caption gives the mapping's scope but does not state the individual outcomes.Source: Table I · p. 8
THE SIGNAL-TO-NOISE RATIO OF 12 PHOTONS
The experiment uses a twelve-photon interferometer and measures its Z-basis photon distribution to assess manipulation of twelve photons. Under 12-fold coincidence, only the all-horizontal and all-vertical components are observed, with a 1420:1 signal-to-noise ratio.
THE SIGNAL-TO-NOISE RATIO OF 12 PHOTONS
- The setup yields a 12-photon coincidence rate of 1.2 h−1 with a 700 mW pump and 30 nm-FWHM bandpass filters.The filters are placed before the single-photon detectors to remove small sidebands.Source: THE SIGNAL-TO-NOISE RATIO OF 12 PHOTONS · p. 9
- 1420:1 is the signal-to-noise ratio for the desired components relative to non-desired components in the measured Z^⊗12 distribution.Only the |H^⊗12〉 and |V^⊗12〉 components are observed under 12-fold coincidence.Source: THE SIGNAL-TO-NOISE RATIO OF 12 PHOTONS · p. 10, Figure 7 · p. 10
- The twelve-photon interferometer is characterized by measuring its photon distribution in the Z^⊗12 basis.The measured distribution is shown in Fig. 7.Source: THE SIGNAL-TO-NOISE RATIO OF 12 PHOTONS · p. 10
DETAIL RESULTS WITH DOWN-CONVERSION PROBABILITY p = 0.0483
At p = 0.0483 ± 0.0001, the reported final-pair fidelity accompanies the demonstration of a 2 × 2 parallel all-photonic repeater. Additional characterization at p = 0.049 reports four-photon GHZ and final-pair measurements.
DETAIL RESULTS WITH DOWN-CONVERSION PROBABILITY p = 0.0483
- At p = 0.049, four-photon GHZ-state tomography yielded fidelity F = 0.877, indicating genuine entanglement.The reconstructed density matrix is shown in Fig. 8 for p = 0.0483 ± 0.0001.Source: DETAIL RESULTS WITH DOWN-CONVERSION… · p. 10, Figure 8 · p. 11
- The entanglement-generation-rate ratio between the all-photonic repeater and conventional scheme is influenced by down-conversion probability p; an additional data point was acquired at p = 0.049.The data point is shown in Fig. 4a in the main text.Source: DETAIL RESULTS WITH DOWN-CONVERSION… · p. 10
- At p = 0.049, final-pair fidelities were 0.542 ± 0.012, 0.537 ± 0.010, 0.549 ± 0.012, and 0.554 ± 0.010 for pairs 1&11, 4&11, 1&10, and 4&10, respectively.The measured fractions were taken in XX, YY, and ZZ bases; the experiment registered 22 h eight-fold coincidences in each basis.Source: DETAIL RESULTS WITH DOWN-CONVERSION… · p. 10, Figure 9 · p. 11
- 0.546 ± 0.006 is the calculated final entangled-pair fidelity for the 2 × 2 parallel structured all-photonic repeater at p = 0.0483 ± 0.0001.The authors describe this as a full demonstration of the repeater.Source: DETAIL RESULTS WITH DOWN-CONVERSION… · p. 11