Optically addressable nuclear spins in a solid with a six-hour coherence time

Abstract

Space-like separation of entangled quantum states is a central concept in fundamental investigations of quantum mechanics and in quantum communication applications. Optical approaches are ubiquitous in the distribution of entanglement because entangled photons are easy to generate and transmit. However, extending this direct distribution beyond a range of a few hundred kilometres to a worldwide network is prohibited by losses associated with scattering, diffraction and absorption during transmission. A proposal to overcome this range limitation is the quantum repeater protocol, which involves the distribution of entangled pairs of optical modes among many quantum memories stationed along the transmission channel. To be effective, the memories must store the quantum information encoded on the optical modes for times that are long compared to the direct optical transmission time of the channel. Here we measure a decoherence rate of 8 × 10(-5) per second over 100 milliseconds, which is the time required for light transmission on a global scale. The measurements were performed on a ground-state hyperfine transition of europium ion dopants in yttrium orthosilicate ((151)Eu(3+):Y2SiO5) using optically detected nuclear magnetic resonance techniques. The observed decoherence rate is at least an order of magnitude lower than that of any other system suitable for an optical quantum memory. Furthermore, by employing dynamic decoupling, a coherence time of 370 ± 60 minutes was achieved at 2 kelvin. It has been almost universally assumed that light is the best long-distance carrier for quantum information. However, the coherence time observed here is long enough that nuclear spins travelling at 9 kilometres per hour in a crystal would have a lower decoherence with distance than light in an optical fibre. This enables some very early approaches to entanglement distribution to be revisited, in particular those in which the spins are transported rather than the light.

Towards highly multimode optical quantum memory for quantum repeaters

“…We use the yellow 7 F 0 → 5 D 0 transition at 580.04 nm, which has an extremely narrow optical homogeneous broadening [40] at cryogenic temperatures. It also presents long spin coherence times [41][42][43]. The isotopically enriched 151 Eu 3+ doping results in a larger optical depth as compared to a natural abundance of Eu 3+ isotopes [44].…”

Ground-to-satellite quantum teleportation

“…Further, teleportation of quantum logic gates, a key element in distributed quantum computing schemes, is also possible assisted by shared multiparticle entangled states 5,6 . Given the rapid progress on long-live quantum memories 26 and efficient light-matter interface 27 , more sophisticated space-scale teleportation can be realized and is expected to play a key role in the future distributed quantum internet. The highest loss is ~52 dB at a distance of 1400 km, when the satellite is at 14.5°a ngle.…”

Memory-assisted quantum key distribution resilient against multiple-excitation effects

“…The photon is emitted due to a quantum interference effect between each absorption line of the comb. Ensembles of rare-earth-ions are particularly suited for atomic frequency comb quantum memories due to the long coherence times of both the optical (100s of microseconds [62,63]) and spin (up to milliseconds [62,63] or even seconds [64]) transitions in conjunction with level structures that allow for efficient atomic frequency combs over ∼MHz bandwidths [11,62,63].…”

“…Taking into consideration the technical challenges of obtaining both high efficiency and low noise in a rare-earth-ion-doped crystal-based atomic frequency comb system, in section 3 we explore the possibility of using several (spectral) modes to overcome the no-memory bound. Note that coherence times of 6 hours [64] and one minute [67] have been measured using magnetically-insensitive ground-level transitions of 151 Eu:Y 2 SiO 5 and Pr:Y 2 SiO 5 , respectively. However, it has yet to be shown that these coherence times can be combined with the possibility of efficient and broadband storage, hence these transitions may not be suitable for MA-MDI-QKD.…”