Observation of eight-photon entanglement

Journal name:
Nature Photonics
Year published:
Published online


The creation of increasingly large multipartite entangled states is not only a fundamental scientific endeavour in itself1, 2, 3, but is also the enabling technology for quantum information4, 5. Tremendous experimental effort has been devoted to generating multiparticle entanglement with a growing number of qubits6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16. So far, up to six spatially separated single photons10, 11, 12, 13, 14 have been entangled based on parametric downconversion17. Multiple degrees of freedom of a single photon have been exploited to generate forms of hyper-entangled states18, 19. Here, using new ultra-bright sources of entangled photon pairs20, an eight-photon interferometer and post-selection detection, we demonstrate for the first time the creation of an eight-photon Schrödinger cat state1 with genuine multipartite entanglement. The ability to control eight individual photons represents a step towards optical quantum computation, and will enable new experiments on, for example, quantum simulation21, 22, topological error correction23 and testing entanglement dynamics under decoherence24.

At a glance


  1. Experimental scheme for generating eight-photon Schrodinger cat states.
    Figure 1: Experimental scheme for generating eight-photon Schrödinger cat states.

    a, Left: an initial photon pair is generated by a non-collinear type-II PDC17 and passes through a pair of birefringent compensators (not shown) consisting of a 1 mm BBO crystal and a HWP. After one photon's polarization is rotated by 90° using the HWP, the two photons are superposed on a PBS. Right: the principle of an interferometric Bell-state synthesizer20. Here, the photon leaves the BBO crystal with ‘e’ and ‘o’ polarizations with different spectral widths, which are separated by the PBS and detected by different detectors. This effectively disentangles the timing information from the polarization information of the photon pair, generating high-fidelity entangled photons. b, Left: an interferometer combining four incoming e-polarized photons (each from an entangled pair) with three PBSs. With coincidence detection, the four pairs of entangled photons are transformed into the eight-photon Schrödinger cat state ( equation (1)). Right: graph state representation of the process of engineering the four photon pairs into the eight-photon cat state. The graph state can be thought of as being constructed by first preparing the qubits at each vertex in the state |+right fence = (|Hright fence + |Vright fence)/√2 and then applying controlled phase gates between pairs of neighbouring qubits.

  2. Experimental set-up.
    Figure 2: Experimental set-up.

    Ultraviolet laser pulses with a central wavelength of 390 nm, pulse duration of 120 fs and repetition rate of 76 MHz successively pass through four BBO crystals to produce four PDC photon pairs, which are further combined on four interferometric Bell-state synthesizers, each consisting of a HWP and a PBS. The distance between the first and the last BBO crystal is  ~ 1.3 m. The photons in mode 1 and 4 are then combined on PBS1, photons 5 and 8 on PBS2, and finally photons 4′ and 8′ on PBS3. Technically, much effort is made to ensure good spatial and temporal overlap in the seven interferences involved in this set-up and to keep them stable in a temperature-stabilized laboratory. We use high-precision PBSs with extinction ratios of >1,000:1 and a beam deviation of <3 arcseconds. Different lens settings are used for beam profile matching when overlapping on the PBSs. The photons are detected by 16 single-photon detectors (quantum efficiency, >60%), and a complete set of 256 eightfold coincidence events are simultaneously registered by a homemade FPGA-based coincidence unit.

  3. Experimental results for the eight-photon Schrodinger cat state.
    Figure 3: Experimental results for the eight-photon Schrödinger cat state.

    a, Coincidence counts measured in the |Hright fence/|Vright fence basis accumulated for 40 h. b, Expectation values of M⊗8k, each derived from a complete set of 256 sixfold coincidence events in the basis of |Hright fence ± eikπ/8 |Vright fence. The setting of M0 is measured in 25 h, and the remaining seven settings are measured in 15 h. Error bars indicate one standard deviation deduced from propagated Poissonian counting statistics of the raw detection events.


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  1. Shanghai Branch, National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Shanghai, 201315, China

    • Xing-Can Yao,
    • Tian-Xiong Wang,
    • Ping Xu,
    • He Lu,
    • Ge-Sheng Pan,
    • Xiao-Hui Bao,
    • Cheng-Zhi Peng,
    • Chao-Yang Lu,
    • Yu-Ao Chen &
    • Jian-Wei Pan


X.-C.Y., X.-H.B., Y.-A.C. and J.-W.P. conceived and designed the research. X.-C.Y., T.-X.W., P.X., H.L., G.-S.P. and C.-Z.P. carried out the experiment. X.-H.B. programmed the FPGA logic. C.-Y.L. contributed theoretical analysis tools. X.-C.Y. and Y.-A.C. analysed the data. X.-C.Y., C.-Y.L., Y.-A.C. and J.-W.P. wrote the manuscript. C.-Y.L., Y.-A.C. and J.-W.P. supervised the project.

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