Speaker
Description
The quantum repeater (QR) [1] is a fundamental building block for the realization of large, long-distance quantum networks. By dividing a transmission link into segments of entangled quantum memories and cells generating asynchronously entangled photons [2], it is possible to overcome the exponential loss of direct transmission.
We report on the implementation of a quantum repeater cell with free-space-coupled photons from two $^{40}$Ca$^+$ ions in the same Paul trap acting as memories. Ion-photon entanglement is generated asynchronously by controlled emission of single photons from the individually addressed ions into separate single-mode fibers. Photon-photon entanglement with 77.8(6)% average fidelity is then generated by applying a Mølmer–Sørensen gate and state projection of the ions.
The advantage of this protocol is highlighted by a 100-fold improvement of the photon pair probability compared to a synchronously operated QR cell, resulting in a single-attempt probability of $9.76(2)\cdot10^{-5}$ and a photon-pair detection rate of $11.34(2)$s$^{-1}$ [3].
A QR segment connects individual QR cells to form a quantum repeater link. We demonstrate the implementation of such a segment in the same setup with two $^{40}$Ca$^+$ quantum memories that are entangled by entanglement swapping of free-space-coupled single photons after generating ion-photon entanglement [4].
Entanglement of the two memories is verified by a parity measurement using two $\pi/2$ rotation pulses on the ions after photonic coincidence detection. To demonstrate the possible use for heterogeneous systems in which the second memory emits at a different wavelength, and to reduce attenuation for long-distance communication, the photons are converted to the telecom C band using polarization-preserving quantum frequency conversion [5] before their detection.
The parity oscillation shows a peak-to-peak value of 1.36(15), with a value larger than one being sufficient to prove entanglement [6]. This corresponds to a fidelity of better than 68(8)%. We produce entangled memories at a rate of 4.7 per day, which will be enhanced in the future by the use of an optical resonator.
[1] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998)
[2] P. van Loock et al., Adv. Quantum Technol., 3: 1900141 (2020)
[3] M. Bergerhoff et al., Phys. Rev. A 110, 032603 (2024)
[4] P. Baumgart, et. al. Optica Quantum 2.0 Conference and Exhibition, paper QTh4A.3 (2025)
[5] E. Arenskötter et al., npj Quantum Inf 9, 34 (2023).
[6] L. Slodicka et al., Phys. Rev. Lett. 110, 083603 (2013)
| Academic level | PhD student |
|---|