. 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 memories are regarded as one of the fundamental building blocks of linear-optical quantum computation [1] and long-distance quantum communication [2]. A long standing goal to realize scalable quantum information processing is to build a long-lived and efficient quantum memory. There have been significant efforts distributed towards this goal. However, either efficient but short-lived [3,4] or long-lived but inefficient quantum memories [5][6][7] have been demonstrated so far. Here we report a high-performance quantum memory in which long lifetime and high retrieval efficiency meet for the first time. By placing a ring cavity around an atomic ensemble, employing a pair of clock states, creating a longwavelength spin wave, and arranging the setup in the gravitational direction, we realize a quantum memory with an intrinsic spin wave to photon conversion efficiency of 73(2)% together with a storage lifetime of 3.2(1) ms. This realization provides an essential tool towards scalable linearoptical quantum information processing.A high-performance quantum memory is of crucial importance for large-scale linear-optical quantum computation[1], distributed quantum computing, and long-distance quantum communication [2]. The lifetime and the retrieval efficiency of a quantum memory are two important quantities that determine the scalability of realistic quantum information protocols. For a certain quantum information task, e.g. creating a large-scale cluster state [8] or distributing entanglement through the quantum repeater protocol [9-12], the time overhead T r is inversely proportional to a power law of the retrieval efficiency R, T r ∝ R −n , where n is determined by the scale of the quantum computation or the communication distance. In order to implement one of those tasks, the lifetime of the quantum memory must be larger than this time overhead. To satisfy this condition, one has to improve the lifetime of the quantum memory and reduce the time overhead by improving the retrieval efficiency. Besides, different protocols also set thresholds on the retrieval efficiency and lifetime. For example, in loss-tolerant linear-optical quantum computation the minimum retrieval efficiency required is 50% [13] and in long-distance quantum communication distributing en-tanglement over 1000 km requires a communication time of at least 3.3 ms.Quantum memories for light have been demonstrated with atomic ensembles [14][15][16], solid state systems [17,18], and single atoms [19]. With these quantum memories, the principle of some quantum information protocols have been demonstrated, e.g., functional quantum repeater nodes were realized with atomic ensembles [20,21]. However, due to the low retrieval efficiency and short lifetime, the implementation of further steps is extremely difficult. Therefore, in recent years, many efforts have been devoted towards improving the retrieval efficiency and the lifetime of the quantum memories and significant progress has been achieved. However, an efficient and long-lived quantum memory remai...
A single photon source is realized with a cold atomic ensemble ( 87 Rb atoms). In the experiment, single photons, which is initially stored in an atomic quantum memory generated by Raman scattering of a laser pulse, can be emitted deterministically at a time-delay in control. It is shown that production rate of single photons can be enhanced by a feedback circuit considerably while the single-photon quality is conserved. Thus our present single-photon source is well suitable for future large-scale realization of quantum communication and linear optical quantum computation.PACS numbers: 03.67. Hk, 32.80.Pj, 42.50.Dv Although weak coherent beams can be used as a pseudo single-photon source, the advent of quantum information processing (QIP) has placed stringent requirements on single photons either on demand or heralded [1]. In particular, secure quantum cryptography [2] and linear optical quantum computing [3] depend on the availability of such single-photon sources. Different approaches have been attempted in the last decade to develop the on-demand single-photon source, such as the implementations with quantum dots [4,5], single atoms and ions [6,7], and color centers [8]. However, all of them are confronted with different challenges. For example, the single-atom implementation provides spectrally narrow single photons with a well defined spatial mode, but the main challenge is the manipulation of single atoms, which requires sophisticated and expensive setups [6]. Although quantum dots present many advantages as potential source of single photons, e.g. high single-photon rate, the requirement of spectral filtering entails inevitable losses. Additionally, it is a major problem for preparing truly identical sources due to inhomogeneities in both the environment of the emitters and the emitters itself [9]. The stability of color centers is excellent, even at room temperature. However, the high peak intensities of a pulsed excitation can lead to complex and uncontrollable dark states [1]. So it has been taken as a formidable task to develop a promising single-photon source.Moreover, an important challenge in distributed QIP is the controllable transfer of quantum state between flying qubit and macroscopic matter. Remarkably, as shown in a recent proposal for long-distance quantum communication with atomic ensembles [10], it is possible to implement both a single-photon source on demand and the controllable transfer of quantum state between photonic qubit and macroscopic matter, provided that proper feedback is applied to achieve the classical feed-forward ability. Such feed-forward ability is a crucial requirement in linear optics QIP [3,10]. In other words, it must be, in principle, possible to detect when an operation has succeeded by performing some appropriate measurement on ancilla photons. This information can then be feed-forwarded for conditional future operations on the photonic qubits to achieve efficient QIP.Recently, significant experimental progresses have been achieved in demonstration of quantum ...
We create independent, synchronized single-photon sources with built-in quantum memory based on two remote cold atomic ensembles. The synchronized single photons are used to demonstrate efficient generation of entanglement. The resulting entangled photon pairs violate a Bell's inequality by 5 standard deviations. Our synchronized single photons with their long coherence time of 25 ns and the efficient creation of entanglement serve as an ideal building block for scalable linear optical quantum information processing.
We report the experimental demonstration of a quantum memory for collective atomic states in a far-detuned optical dipole trap. Generation of the collective atomic state is heralded by the detection of a Raman scattered photon and accompanied by storage in the ensemble of atoms. The optical dipole trap provides confinement for the atoms during the quantum storage while retaining the atomic coherence. We probe the quantum storage by cross-correlation of the photon pair arising from the Raman scattering and the retrieval of the atomic state stored in the memory. Non-classical correlations are observed for storage times up to 60 µs.PACS numbers: 03.67. Hk, 37.10.Gh, 42.50.Dv A quantum memory, a storage device for quantum states, is requisite to a scalable quantum repeater [1] for the realization of long-distance quantum communication [2]. In the quantum repeater protocol, the tranmission channel is divided into several segments with lengths comparable to the channel attenuation length. Entanglement is then generated and purified [3] for short distances before being extended to a longer distance by entanglement swapping [4]. The entanglement creation, purification, and connection are probabilistic, thereby requiring the successfully entangled segment state to be stored in a quantum memory while waiting for the others to generate. Once the entanglement is distributed over the transmission channel, it can be used for quantum teleportation [5] or cryptography [6]. A quantum memory with long storage time is therefore crucial to achieve scalable quantum communication networks with a manageable time overhead.Various schemes were proposed for implementing quantum repeaters [7,8,9] in which the scalability stems from the entanglement between a sent photon and the quantum state stored in the quantum memory. The quantum state is stored in a collective state of an atomic ensemble where a superposition between two ground states is shared among all the atoms. The key issue to a quantum memory is that the stored state, which could be later read out by converting into another photon, keeps its quantum correlation with the sent photon. This correlation also allows an arbitrary state to be written into the quantum memory by quantum teleportation.Significant progress has been made toward realization of quantum repeaters in recent years. Non-classical correlation has been observed between Raman scattered photons and the consequent collective excitations in an atomic ensemble [10,11,12,13,14]. Number-state entanglement has also been generated between two ensembles of atoms [15,16]. Most recently, quantum teleportation with a built-in quantum memory [17] and entanglement swapping [18] have been demonstrated. Despite these advances, the storage times of the quantum memories reported to date are primarily limited by inhomogeneous broadening of the ground state due to magnetic fields [19]. As a result, the quadrupole magnetic field of the magneto-optical trap (MOT) used to confine the atoms is switched off during the storage. Neverthele...
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