Can quantum teleportation run fast over city-scale fibre?
Researchers teleported single-photon quantum states across a campus fibre network several times per second with fidelity well above what any classical method could reach.
Source
Hertz-rate metropolitan quantum teleportation
Study at a glance
- Design
- Other — Three-station fibre teleportation experiment (Alice, Bob, Charlie) with a PPLN entangled-photon source, Bell-state measurement, active timing and polarisation feedback, and state tomography.
- N
- Physics experiment; results are photon coincidence counts, not a sample of units.
- Population
- Photonic time-bin qubits sent over deployed and spooled telecom fibre on a university campus
- Outcome
- Teleportation rate, teleportation fidelity (vs the classical limit), entanglement visibility and photon indistinguishability
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
Alice encoded time-bin qubits in weak laser pulses; Bob held one photon of an entangled pair made in a lithium-niobate waveguide and sent the other to Charlie, who performed a Bell-state measurement on Alice's photon and Bob's partner photon. Active feedback kept photon arrival times and polarisations matched despite fibre drifts. They checked the entanglement first, measured Hong-Ou-Mandel interference, then teleported several input states and reconstructed Bob's states with tomography, using the decoy-state method to estimate single-photon performance.
What they found
Teleportation ran at 7.1 ± 0.4 Hz over a total state-transfer distance of 64 km of fibre. The average fidelity measured by tomography was 86.4 ± 4.5%, and the decoy-state estimate for true single photons was at least 90.6 ± 2.6%, both above the classical limit of 2/3. Entanglement survived distribution, with two-photon interference visibilities of 94.3% and 93.5%.
The limits
What it doesn't show
Alice used attenuated laser pulses rather than genuine single photons, so the headline single-photon fidelity is an estimate from the decoy-state method, not a direct measurement. Much of the fibre was spooled in the lab rather than deployed in the field, and the Charlie-to-Bob distance was short, so this is not a demonstration over long free-running city links. Fidelity is limited by multiphoton events and remaining photon distinguishability, and the superposition states were more degraded than the pole states. A rate of a few hertz is still far below what practical networks need.
Key terms
- Quantum teleportation
- Transferring an unknown quantum state from one place to another using shared entanglement plus a measurement and a classical message; no matter is sent.
- Bell-state measurement
- A joint measurement that projects two photons onto one of the maximally entangled Bell states; its outcome tells the receiver how the state was transformed.
- Time-bin qubit
- A qubit encoded in whether a photon arrives in an early or late time slot, or a superposition of both.
- Fidelity
- How closely the received state matches the intended one (1 = perfect); classical strategies cannot exceed 2/3 on average for a qubit.
- Hong-Ou-Mandel interference
- A dip in coincidences when two indistinguishable photons meet at a beam splitter, used to test how identical they are.
- Decoy-state method
- Varying the mean photon number of laser pulses to bound how the system would perform with true single photons.
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Quiz yourself
What does Bob need besides his entangled photon to complete teleportation?
Common questions
Does teleportation send information faster than light?
No. Bob needs Charlie's classical measurement result, sent by an ordinary optical pulse, to know how his photon relates to Alice's state.
Why is 2/3 the key threshold?
Any classical measure-and-resend strategy can reach at most two-thirds average fidelity for a qubit, so beating it shows genuinely quantum transfer.
Why was active feedback needed?
Temperature and strain change fibre length and birefringence, making photons distinguishable in timing and polarisation, which would spoil the Bell-state measurement.
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