Nondestructive detection of photonic qubits

Qubits encoded in single photons are very useful to distribute quantum information over remote locations, but at the same time are also very fragile objects. The loss of photonic qubits (through absorption, diffraction or scattering) is actually the main limitation in the maximum reachable quantum communication distance. In this context, the nondestructive detection of photonic qubits is a great scientific challenge that can help tracking the qubit transmission and mitigate the loss problem. Such a detector is envisioned to improve loss-sensitive qubit measurements [1], facilitate protocols in which distributed tasks depend on the successful dissemination of photonic qubits [2], and also enable certain quantum key distribution attacks [3]. We recently implemented such a detector [4] with a single atom coupled to two crossed fiber-based optical resonators, one for qubit-insensitive atom–photon coupling and the other for atomic-state detection. We achieve a nondestructive detection efficiency of 79(3) % conditioned on the survival of the photonic qubit, a photon survival probability of 31(1) %, and we preserve the qubit information with a fidelity of 96.2(0.3) %. To illustrate the potential of our detector we show that it can provide an advantage for long-distance entanglement and quantum-state distribution, resource optimization via qubit amplification, and detection-loophole-free Bell tests.



[1] N. Gisin et al., Phys. Rev. Lett. 105, 070501 (2010)

[2] K. Boone et al., Phys. Rev. A 91, 052325 (2015)

[3] G. Brassard et al., Phys. Rev. Lett. 85, 1330–1333 (2000)

[4] D. Niemietz et al., Nature 591, 570–574 (2021)