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Quantum teleportation using active feed-forward between two Canary Islands
Xiao-song Ma, Thomas Herbst, Thomas Scheidl, Daqing Wang, Sebastian Kropatschek, William Naylor, Alexandra Mech, Bernhard Wittmann, Johannes Kofler, Elena Anisimova, Vadim Makarov, Thomas Jennewein, Rupert Ursin, Anton Zeilinger
TL;DR
Long-distance teleportation had generally lacked active real-time feed-forward, despite its importance for quantum information applications. This experiment used separate 143 km free-space quantum and classical links with specialized sources, detectors, and synchronization, achieving fidelities above the classical benchmark and confirming the teleportation process by tomography.
Problem
Long-distance quantum teleportation experiments generally lacked active real-time feed-forward of the Bell-state measurement result, an ingredient relevant to future quantum information applications.
Method
The experiment combined 143 km free-space quantum and classical links with Bell-state measurement, specialized photon sources, low-noise detectors, and entanglement-assisted clock synchronization.
Results
The average teleported-state fidelity was 0.863(38), and all observed fidelities significantly exceeded the classical average fidelity limit of 2/3.
Takeaways & Limitations
Active-feed-forward quantum teleportation was achieved over large free-space distances under real outdoor conditions.
Takeaways & Limitations
Severe weather and atmospheric turbulence imposed demanding requirements and caused link-attenuation fluctuations in the long-distance setup.
Abstract
from arXiv · showhide
Quantum teleportation [1] is a quintessential prerequisite of many quantum information processing protocols [2-4]. By using quantum teleportation, one can circumvent the no-cloning theorem [5] and faithfully transfer unknown quantum states to a party whose location is even unknown over arbitrary distances. Ever since the first experimental demonstrations of quantum teleportation of independent qubits [6] and of squeezed states [7], researchers have progressively extended the communication distance in teleportation, usually without active feed-forward of the classical Bell-state measurement result which is an essential ingredient in future applications such as communication between quantum computers. Here we report the first long-distance quantum teleportation experiment with active feed-forward in real time. The experiment employed two optical links, quantum and classical, over 143 km free space between the two Canary Islands of La Palma and Tenerife. To achieve this, the experiment had to employ novel techniques such as a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors, and entanglement-assisted clock synchronization. The average teleported state fidelity was well beyond the classical limit of 2/3. Furthermore, we confirmed the quality of the quantum teleportation procedure (without feed-forward) by complete quantum process tomography. Our experiment confirms the maturity and applicability of the involved technologies in real-world scenarios, and is a milestone towards future satellite-based quantum teleportation.
INTRODUCTION
The experiment addresses long-distance quantum teleportation with active classical feed-forward, using quantum and classical free-space links between La Palma and Tenerife. Alice’s Bell-state measurement determines Bob’s real-time correction needed to reproduce an unknown input polarization state.
- INTRODUCTION: Teleportation remained experimentally challenging because three-photon creation sharply lowers count rates, signal-to-noise ratio, and system stability.The protocol’s multiphoton requirements increase integration time and environmental demands compared with two-photon experiments.
- INTRODUCTION: 143 km free-space quantum and classical channels connect teleportation stations on La Palma and Tenerife.Alice and Charlie are located on La Palma, while Bob is on Tenerife.
- INTRODUCTION: Quantum teleportation transfers an unknown polarization state to Bob using shared entanglement and a classical Bell-state measurement result.Alice and Bob share photons 2 and 3, while Charlie supplies an unknown input state in photon 1.
- INTRODUCTION: Each Bell-state measurement outcome projects Bob’s photon onto the input state up to a corresponding unitary transformation.For |Ψ−⟩, Bob applies the identity; for |Ψ+⟩, he applies a π phase shift between horizontal and vertical polarization components.
EXPERIMENT
The experiment implements Bell-state analysis at Alice, transmits photon 3 over 143 km, and sends the measurement result to Bob through a separate classical link. Bob uses the received result to apply the required real-time polarization operation.
- EXPERIMENT: Alice identifies |Ψ−⟩ and |Ψ+⟩ using three-fold coincidence detection of photons 0, 1, and 2.The other two Bell states cannot be separately identified, giving the Bell-state measurement a 50% efficiency.
- EXPERIMENT: In the first stage, |Ψ−⟩ events required Bob to perform the identity operation while the received photon’s polarization was analyzed.These events test cases where the Bell-state result already leaves photon 3 in the input state.
- EXPERIMENT: For |Ψ+⟩ events, Alice sends the result over a classical free-space channel and Bob applies a π phase shift between |H⟩ and |V⟩.The result is encoded in 1064 nm laser pulses, converted to a TTL trigger, and used to drive an electro-optical modulator.
- EXPERIMENT: Separate time-tagging units record the Bell-state outcomes and relevant detection events at Alice’s and Bob’s stations.The timing records support identification of corresponding events across the two locations.
SIGNIFICANT EXPERIMENTAL CHALLENGES IN REAL LIFE
The 143 km outdoor experiment faced severe signal-to-noise and environmental challenges. The researchers combined improved photon sources, low-noise detectors, clock synchronization, tracking, and tomography to make the experiment feasible.
- SIGNIFICANT EXPERIMENTAL CHALLENGES IN REAL LIFE: Extremely low signal-to-noise ratios in real outdoor conditions required a combination of cutting-edge techniques.The authors describe this combination as necessary to make the experiment possible.
- SIGNIFICANT EXPERIMENTAL CHALLENGES IN REAL LIFE: A frequency-uncorrelated entangled-photon source increased the signal while ultra-low-noise detectors and entanglement-assisted synchronization reduced noise.The source maintained entanglement quality without narrow-band filters, while the detector and clock techniques improved detection and timing conditions.
- SIGNIFICANT EXPERIMENTAL CHALLENGES IN REAL LIFE: Atmospheric turbulence and harsh weather caused link-attenuation fluctuations and delayed the experimental realization by nearly one year.Reported conditions included rapid temperature changes, sand storms, rain, fog, strong wind, and snow.
- SIGNIFICANT EXPERIMENTAL CHALLENGES IN REAL LIFE: State tomography collected 605 four-fold coincidence counts over about 6.5 hours for four teleported states.The reconstructed density matrices were compared with ideal states, with uncertainties estimated using a Poissonian-error Monte Carlo routine.
- SIGNIFICANT EXPERIMENTAL CHALLENGES IN REAL LIFE: A bidirectional closed-loop tracking system maintained the quantum and classical free-space links under turbulent atmospheric conditions.Beacon lasers provided pointing references for both transmitter and receiver telescopes.
EXPERIMENTAL RESULTS
The experiment demonstrated quantum teleportation over a 143 km free-space channel, including real-time feed-forward, with state fidelities above the classical limit and process tomography confirming the teleportation operation.
- Teleportation without feed-forward: The four input states |H⟩, |V⟩, |P⟩, and |L⟩ were used for state tomography without feed-forward.The reconstructed density matrices characterized the teleported states.
- Teleportation without feed-forward: 0.863(38) average state fidelity exceeded the classical average fidelity limit of 2/3 despite 28.1–39.0 dB link attenuation.The four measured fidelities were 0.890(42), 0.865(46), 0.845(27), and 0.852(37).
- Process tomography: 0.710(42) process fidelity was 5 standard deviations above the classical process-fidelity limit of 0.5.Quantum process tomography found the identity component χ00 to be dominant.
- Real-time feed-forward: Real-time feed-forward applied the required π phase shift after Bob received Alice’s |Ψ+⟩12 Bell-state measurement result.The correction rotated the polarization appropriately for the tested states.
- Real-time feed-forward: 0.760(50) and 0.800(37) were the fidelities for feed-forward teleportation of |P⟩ and |R⟩, respectively, both above the classical fidelity bound.The experiment used the |P⟩ and |R⟩ states to test the phase-shift correction required after the |Ψ+⟩12 Bell-state outcome.
- Overall result: The results show that active feed-forward quantum teleportation can operate over 143 km of free space under real outdoor conditions.The authors identify this as important for future quantum-information applications.
CONCLUDING COMMENTS
Real-time feed-forward enabled unambiguous quantum teleportation across 143 km between La Palma and Tenerife. The demonstrated performance supports both ground-based and satellite-based free-space teleportation and future space-based quantum networks.
- 143 km: Real-time feed-forward enabled unambiguous quantum teleportation from La Palma to Tenerife over a free-space channel.
- Ground-based and satellite-based free-space quantum teleportation were shown to be feasible.
- The setup achieved coincidence production rates and fidelities capable of coping with optical-link attenuation expected in ground-to-LEO-satellite transmission.
- The experiment represents a crucial step toward space-based quantum networks requiring space-to-ground quantum communication.
- The implemented technologies are expected to provide key building blocks for future long-distance ground-communication experiments.