Detecting an Itinerant Optical Photon Twice without Destroying It.

Nondestructive quantum measurements are central for quantum physics applications ranging from quantum sensing to quantum computing and quantum communication. Employing the toolbox of cavity quantum electrodynamics, we here concatenate two identical nondestructive photon detectors to repeatedly detect and track a single photon propagating through a 60 m long optical fiber. By demonstrating that the combined signal-to-noise ratio of the two detectors surpasses each single one by about 2 orders of magnitude, we experimentally verify a key practical benefit of cascaded nondemolition detectors compared to conventional absorbing devices.

Quantum Teleportation between Remote Qubit Memories with Only a Single Photon as a Resource.

Quantum teleportation enables the deterministic exchange of qubits via lossy channels. While it is commonly believed that unconditional teleportation requires a preshared entangled qubit pair, here we demonstrate a protocol that is in principle unconditional and requires only a single photon as an ex-ante prepared resource. The photon successively interacts, first, with the receiver and then with the sender qubit memory. Its detection, followed by classical communication, heralds a successful teleportation. We teleport six mutually unbiased qubit states with average fidelity F[over ¯]=(88.3±1.3)% at a rate of 6 Hz over 60 m.

Nondestructive detection of photonic qubits.

One of the biggest challenges in experimental quantum information is to sustain the fragile superposition state of a qubit1. Long lifetimes can be achieved for material qubit carriers as memories2, at least in principle, but not for propagating photons that are rapidly lost by absorption, diffraction or scattering3. 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 qubits4,5, improve loss-sensitive qubit measurements6,7 and enable certain quantum key distribution attacks8. 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 detection9. 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.

A quantum-logic gate between distant quantum-network modules.

The big challenge in quantum computing is to realize scalable multi-qubit systems with cross-talk-free addressability and efficient coupling of arbitrarily selected qubits. Quantum networks promise a solution by integrating smaller qubit modules to a larger computing cluster. Such a distributed architecture, however, requires the capability to execute quantum-logic gates between distant qubits. Here we experimentally realize such a gate over a distance of 60 meters. We employ an ancillary photon that we successively reflect from two remote qubit modules, followed by a heralding photon detection, which triggers a final qubit rotation. We use the gate for remote entanglement creation of all four Bell states. Our nonlocal quantum-logic gate could be extended both to multiple qubits and many modules for a tailor-made multi-qubit computing register.

Therapy of the neonatal abstinence syndrome with tincture of opium or morphine drops.

BACKGROUND: Treating opioid-addicted women with methadone in pregnancy increased the number of newborns suffering from neonatal abstinence syndrome (NAS). High-pitch crying, insomnia, tremor, myoclonic jerks, vomiting, diarrhoea and poor weight gain were reported symptoms, which were evaluated using the Finnegan (F)-score. Earlier phenobarbital or paregoric had been used to suppress symptoms. We surveyed the administration of pure mu-agonist morphine (MO) in comparison to the alcoholic opioid mixture in tincture of opium (TO). Thirty-three newborns were included in the survey, after informed consent by their parents. RESULTS: NAS started 3-5 days after delivery and lasted for 27 or 30 days (mean) in the TO and MO groups, respectively. In either of the tested parameters, we found no significant differences between the two groups (2P < 0.05). The maximum F-score was similar in both groups, but the dose to suppress NAS was higher in the MO group (0.6-0.5 mg/day; total dose 61.6-42.7 mg of morphine). The duration of the therapy was longer in the MO than in the TO group (37.5-32.4 days). On the other hand the weight gain was better in the MO group than in the TO group (25-19 g/day), but was reduced in both groups compared with healthy newborns. CONCLUSIONS: Morphine is suitable to treat NAS in a similar manner as tincture of opium, but avoids unwanted effects of the alcoholic extracts with various alkaloids in the tincture of opium and allows better weight gain of the newborns.