Nondestructive detection of photonic qubits
Abstract One of the biggest challenges in experimental quantum information is to sustain the fragile superposition state of a qubit 1 . Long lifetimes can be achieved for material qubit carriers as memories 2 , at least in principle, but not for propagating photons that are rapidly lost by absorption, diffraction or scattering 3 . The loss problem can be mitigated with a nondestructive photonic qubit detector that heralds the photon without destroying the encoded qubit. Such a detector is envisioned to facilitate protocols in which distributed tasks depend on the successful dissemination of photonic qubits 4,5 , improve loss-sensitive qubit measurements 6,7 and enable certain quantum key distribution attacks 8 . Here we demonstrate such a detector based on a single atom in two crossed fibre-based optical resonators, one for qubit-insensitive atom–photon coupling and the other for atomic-state detection 9 . We achieve a nondestructive detection efficiency upon qubit survival of 79 ± 3 per cent and a photon survival probability of 31 ± 1 per cent, and we preserve the qubit information with a fidelity of 96.2 ± 0.3 per cent. To illustrate the potential of our detector, we show that it can, with the current parameters, improve the rate and fidelity of long-distance entanglement and quantum state distribution compared to previous methods, provide resource optimization via qubit amplification and enable detection-loophole-free Bell tests.