A millisecond quantum memory for scalable quantum networks

Abstract

. Here we present an experimental investigation into extending the storage time of quantum memory for single excitations. We identify and isolate distinct mechanisms responsible for the decoherence of spin waves in atomic-ensemble-based quantum memories. By exploiting magnetic-field-insensitive statesso-called clock states-and generating a long-wavelength spin wave to suppress dephasing, we succeed in extending the storage time of the quantum memory to 1 ms. Our result represents an important advance towards long-distance quantum communication and should provide a realistic approach to large-scale quantum information processing.The quantum repeater with atomic ensembles and linear optics has attracted broad interest in recent years, as it holds promise to implement long-distance quantum communication and the distribution of entanglement over quantum networks. Following the protocol proposed in ref. 3 and the subsequent improved schemes 4-7 , significant experimental progress has been accomplished, including the coherent manipulation of the stored excitation in one 10,11 or two 14-16 atomic ensembles, the demonstration of memory-built-in quantum teleportation 17 and the realization of a building block of the quantum repeater 13,18 . In these experiments, the atomic ensembles serve as the storable and retrievable quantum memory for single excitations.Despite the advances achieved in manipulating atomic ensembles, long-distance quantum communication with atomic ensembles remains challenging owing to the short storage time of the quantum memory for single excitations. For example, for direct generation of entanglement between two memory qubits over a few hundred kilometres, we need a memory with a storage time of a few hundred microseconds. However, the longest storage time reported so far is of the order of only 10 µs (refs 10-13).It has long been believed that the short coherence time is mainly caused by the residual magnetic field 19,20 . Thereby, storing the collective state in the superposition of the first-order magnetic-field-insensitive states 21 , that is, the 'clock states', is suggested to inhibit this decoherence mechanism 19 . A numerical calculation shows that the expected lifetime is of the order of seconds in this case.Here we report on our investigation of prolonging the storage time of the quantum memory for single excitations. In the experiment, we find that using only the 'clock state' is not sufficient to obtain the expected long storage time. We further analyse, isolate and identify the distinct decoherence mechanisms, and thoroughly investigate the dephasing of the spin wave (SW) by varying its wavelength. We find that the dephasing of the SW is extremely sensitive to the angle between the write beam and detection mode, especially for small angles. On the basis of this finding, by exploiting the 'clock state' and increasing the wavelength of the SW to suppress the dephasing, we succeed in extending the storage time from 10 µs to 1 ms.The illustration of our experiment is depicted in Fig. 1a,b....

Quantum repeaters based on atomic ensembles and linear optics

“…In section II.A we have seen that the motion of the atoms is not a problem for the phases of the collective state, as long as a collinear configuration with k w = k s , and k r = k as is used. For all other configurations, the motion of the atoms will induce a dephasing that depends on the angle between k w and k s , as was nicely demonstrated experimentally in (Zhao et al, 2009a). The wavelength of the stored spin wave can be written as :…”

“…This preparation can be implemented with optical pumping techniques. A storage time of 1 ms has been recently demonstrated using this technique in a collinear configuration (Zhao et al, 2009a).…”

Quantum repeaters with imperfect memories: Cost and scalability

“…Memory decay is one of the most challenging problems in quantum repeater technology [17]. Its deteriorating effect, however, has not yet fully scrutinized.…”

Operating Spin Echo in the Quantum Regime for an Atomic-Ensemble Quantum Memory

“…In an atomic-ensemble quantum memory, inhomogeneous broadening due to ambient magnetic field, atomic random motion and interaction with host spins etc. severely limits the storage time [12,13]. One universal solution to overcome inhomogeneous broadening induced decoherence is to make use of the spin echo technique [14], where a series of π pulses are applied to reverse the phase evolution through population inversion.…”

“…As the spin wave is a collective exication over the whole ensemble, inhomogeneity of frequency or phase difference between the atomic levels of |g and |s for all atoms will distort the relative phase between each term in |Ψ gs , thus results in a distorted phase grating. Atomic motion is one dominant decoherence mechanism for atomic-ensemble quantum memories [12,24]. Within an atomic ensemble, the velocity v j of each atom varies from atom to atom, which will distort the original phase grating in |Ψ gs .…”