Quantum teleportation across a metropolitan fibre network

Journal name:
Nature Photonics
Volume:
10,
Pages:
676–680
Year published:
DOI:
doi:10.1038/nphoton.2016.180
Received
Accepted
Published online

If a photon interacts with a member of an entangled photon pair via a Bell-state measurement (BSM), its state is teleported over principally arbitrary distances onto the pair's second member1. Since 1997, this puzzling prediction of quantum mechanics has been demonstrated many times2. However, with two exceptions3, 4, only the photon that received the teleported state, if any, travelled far, while the photons partaking in the BSM were always measured close to where they were created. Here, using the Calgary fibre network, we report quantum teleportation from a telecom photon at 1,532 nm wavelength, interacting with another telecom photon after both have travelled several kilometres and over a combined beeline distance of 8.2 km, onto a photon at 795 nm wavelength. This improves the distance over which teleportation takes place to 6.2 km. Our demonstration establishes an important requirement for quantum repeater-based communications5 and constitutes a milestone towards a global quantum internet6.

At a glance

Figures

  1. Aerial view of Calgary.
    Figure 1: Aerial view of Calgary.

    Alice ‘A’ is located in Manchester, Bob ‘B’ at the University of Calgary and Charlie ‘C’ in a building next to City Hall in Calgary downtown. The teleportation distance—in our case the distance between Charlie and Bob—is 6.2 km. All fibres belong to the Calgary telecommunication network but, during the experiment, they only carry signals created by Alice, Bob or Charlie and were otherwise ‘dark’. Imagery ©2016 Google. Map data ©2016 Google.

  2. Schematics of the experimental set-up.
    Figure 2: Schematics of the experimental set-up.

    a, Alice's set-up. An intensity modulator (IM) tailors 20-ps-long pulses of light at an 80 MHz rate out of 10-ns-long, phase-randomized laser pulses at 1,532 nm wavelength. Subsequently, a widely unbalanced fibre interferometer with Faraday mirrors (FMs), active phase control (see Methods) and path-length difference equivalent to 1.4 ns travel time difference creates pulses in two temporal modes or bins. Following their spectral narrowing by means of a 6-GHz-wide fibre Bragg grating (FBG) and attenuation to the single-photon level, the time-bin qubits are sent to Charlie via a deployed fibre—referred to as a quantum channel (QC)—featuring 6 dB loss. b, Bob's set-up. Laser pulses at 1,047 nm wavelength and 6 ps duration from a mode-locked laser are frequency-doubled (SHG) in a periodically poled lithium-niobate (PPLN) crystal and passed through an actively phase-controlled Mach–Zehnder interferometer (MZI) that introduces the same 1.4 ns delay as between Alice's time-bin qubits. Spontaneous parametric downconversion (SPDC) in another PPLN crystal and pump rejection using an interference filter (not shown) results in the creation of time-bin entangled photon pairs28 at 795 and 1,532 nm wavelength with mean probability μSPDC up to 0.06. The 795 nm and 1,532 nm (telecommunication-wavelength) photons are separated using a dichroic mirror (DM) and subsequently filtered to 6 GHz by a Fabry–Perot (FP) cavity and an FBG, respectively. The telecom photons are sent through deployed fibre featuring 5.7 dB loss to Charlie, and the state of the 795 nm wavelength photons is analysed using another interferometer (again introducing a phase-controlled travel-time difference of 1.4 ns) and two single-photon detectors based on silicon avalanche photodiodes (Si-APD) with 65% detection efficiency. c, Charlie's set-up. A beamsplitter (BS) and two WSi superconducting nanowire single-photon detectors29 (SNSPD), cooled to 750 mK in a closed-cycle cryostat and with 70% system detection efficiency, allow the projection of biphoton states (one from Alice and one from Bob) onto the |ψ〉 Bell state. To ensure indistinguishability of the two photons at the BSM, we actively stabilize the photon arrival times and polarization, the latter involving a polarization tracker and polarizing beamsplitters (PBSs), as explained in the Methods. Various synchronization tasks are performed through deployed fibres, referred to as classical channels (CC) and aided by dense-wavelength division multiplexers (DWDM), photodiodes (PDs), arbitrary waveform generators (AWGs) and field-programmable gate arrays (FPGAs) (for details see Methods).

  3. Indistinguishability of photons at Charlie.
    Figure 3: Indistinguishability of photons at Charlie.

    a, Fluctuations of the count rate of a single SNSPD at the output of Charlie's BS with and without polarization feedback. b, Orange filled circles: the change in the generation time of Alice's qubits that is applied to ensure they arrive at Charlie's BSM at the same time as Bob. Green open squares: coincidence counts per 10 s with timing feedback engaged, showing locking to the minimum of the HOM dip (see Methods and Supplementary Section 3 for details). Inset: rate of coincidences between counts from SNSPDs as a function of arrival time difference, displaying a HOM dip30 when photon-arrival times at the BS are equal. The dashed red line shows the coincidence rate in the case of completely distinguishable photons. All error bars (one standard deviation) are calculated assuming Poissonian detection statistics.

  4. Density matrices of four states after teleportation.
    Figure 4: Density matrices of four states after teleportation.

    Shown are the real and imaginary parts of the reconstructed density matrices for four different input states created at Alice. The mean photon number per qubit is μA = 0.014 and the mean photon pair number is μSPDC = 0.045. The state labels denote the states expected after teleportation.

  5. Individual and average fidelities of four teleported states with expected (ideal) states, measured using quantum state tomography (QST) and the decoy-state method (DSM).
    Figure 5: Individual and average fidelities of four teleported states with expected (ideal) states, measured using quantum state tomography (QST) and the decoy-state method (DSM).

    For the DSM we set μSPDC = 0.06. Error bars (one standard deviation) are calculated assuming Poissonian detection statistics and using Monte Carlo simulation. Count rates for both methods are provided in Supplementary Section 7. The larger degradation of |+〉 and |+i〉 states is due to the limited quality of the BSM (Supplementary Section 5) and imperfect interferometers. Neither cause an effect for |e〉 and |l〉 states.

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Author information

  1. These authors contributed equally to this work

    • Raju Valivarthi,
    • Marcel.li Grimau Puigibert,
    • Qiang Zhou &
    • Gabriel H. Aguilar

Affiliations

  1. Institute for Quantum Science and Technology, and Department of Physics & Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

    • Raju Valivarthi,
    • Marcel.li Grimau Puigibert,
    • Qiang Zhou,
    • Gabriel H. Aguilar,
    • Daniel Oblak &
    • Wolfgang Tittel
  2. National Institute of Standards and Technology, Boulder, Colorado 80305, USA

    • Varun B. Verma &
    • Sae Woo Nam
  3. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA

    • Francesco Marsili &
    • Matthew D. Shaw

Contributions

The SNSPDs were fabricated and tested by V.B.V., F.M., M.D.S. and S.W.N. The experiment was conceived and guided by W.T. The set-up was developed, measurements were performed, and the data were analysed by R.V., M.G.P., Q.Z., G.H.A. and D.O. The manuscript was written by W.T., R.V., M.G.P., Q.Z., G.H.A. and D.O.

Competing financial interests

The authors declare no competing financial interests.

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