Abstract The ability to efficiently realize storage and readout of optical squeezed states plays a key roll in continuous-variables quantum information processing. Here we study the quantum memory for squeezed state of propagating light in atoms based on the hybrid photon echo re-phasing. The optical quantum state is recorded in two sublevels of the ground state of an atomic ensemble to realize long-lived quantum memory. Taking into account the noise effect due to atomic decay, our estimation indicates that high fidelities larger than the classical fidelity threshold 81.5% are obtainable even with currently available techniques. Moreover, our result shows that the decay rate of atoms restricts the maximal fidelity. Our work provides some practical guidance for the realization of efficient and faithful photon-echo-based memory for squeezed light.
Abstract The ability to efficiently realize storage and readout of optical squeezed states plays a key roll in continuous-variables quantum information processing. Here we study the quantum memory for squeezed state of propagating light in atoms based on the hybrid photon echo re-phasing. The optical quantum state is recorded in two sublevels of the ground state of an atomic ensemble to realize long-lived quantum memory. Taking into account the noise effect due to atomic decay, our estimation indicates that high fidelities larger than the classical fidelity threshold 81.5% are obtainable even with currently available techniques. Moreover, our result shows that the decay rate of atoms restricts the maximal fidelity. Our work provides some practical guidance for the realization of efficient and faithful photon-echo-based memory for squeezed light.
Quantum memory of light is not only the building block of constructing large-scale quantum computer, but also the kernel component of quantum repeater for quantum networks, which makes long distance quantum communication come true. Due to the inevitable optical losses, squeezed vacuum generated from optical parametric amplifier becomes squeezed thermal state of light, which is no longer the minimum uncertainty state. Therefore quantum memory of squeezed thermal state of optical field is the key step towards the implementation of quantum internet. Atomic ensemble is one of ideal quantum memory media, as a result of high optical depth and good atomic coherence. Electromagnetically induced transparency (EIT) is one of mature approaches to quantum state mapping between non-classical optical fields and atomic spin waves. In atomic ensembles, the EIT can on-demand map quantum state between quadratures of light and spin waves of atomic ensemble, i.e., controlled quantum memory. Here the condition of quantum memory for squeezed thermal state of light is investigated according to the fidelity benchmark of quantum memory. The fidelity benchmark of quantum memory is the maximum fidelity which can be reached by classical methods, and it is quantum memory if the memory fidelity is higher than the fidelity benchmark of quantum memory. By numerically calculating the fidelity benchmark of quantum memory for different kinds of squeezed thermal states of light and the dependence of memory fidelity on the memory efficiency, we obtain the minimum memory efficiency which can realize quantum memory for squeezed thermal state of light. The quantum memory can be easily obtained by increasing squeezing parameter r. The thermal state fluctuation is sensitive to the realization of quantum memory. The required minimum memory efficiency is lower, when smaller thermal state fluctuation is employed in experiment by reducing the optical losses in optical parametric amplifier. On the other hand, quantum memory fidelity benchmark is high for small squeezing parameter and large optical depth, which requires high memory efficiency. And atomic memory efficiency can be increased by utilizing optical cavity to enhance the interaction between light and atom or atomic ensemble with high optical depth. For example, the fidelity benchmark is 0.80, when squeezing parameter r is 0.35 and thermal state fluctuation is 2.38 dB. Thus quantum memory can be realized if the memory efficiency is larger than 4.34%. Our work can provide the direct reference for experimental design of continuous variable quantum memory, quantum repeater, and quantum computer based on atomic ensembles.
The three-pulse photon echo is a well-known technique to store intense light pulses in an inhomogeneously broadened atomic ensemble. This protocol is attractive because it is relatively simple and it is well suited for the storage of multiple temporal modes. Furthermore, it offers very long storage times, greater than the phase relaxation time. Here, we consider the three-pulse photon echo in both two- and three-level systems as a potential technique for the storage of light at the single-photon level. By explicit calculations, we show that the ratio between the echo signal corresponding to a single-photon input and the noise is smaller than one. This severely limits the achievable fidelity of the quantum state storage, making the three-pulse photon echo unsuitable for single-photon quantum memory.
In continuous-variable quantum information processing, it is crucial to develop high-efficiency and broadband quantum memory of squeezed light, which enables the storage of full-bandwidth information. Here, we present a quantum memory of squeezed light with up to 24 MHz bandwidth, which is at least 12 times that of previous narrowband resonant memory systems, via a far-off resonant Raman process. We achieve output squeezing of as high as 1.0 dB with fidelity above 92% and a memory efficiency of 80%, corresponding to an end-to-end efficiency of 64.2%, when input squeezing is 1.6 dB. The lowest excess noise of 0.025 shot-noise-unit in the memory system is estimated by the noisy channel model which is benefited from optimizing quantum memory performance with a backward retrieval strategy. Our results represent a breakthrough in high-performance memory for squeezed states within tens of MHz-level bandwidth, which has potential applications in high-speed quantum information processing.
Long-lived quantum memories are essential components of a long-standing goal of remote distribution of entanglement in quantum networks. These can be realized by storing the quantum states of light as single-spin excitations in atomic ensembles. However, spin states are often subjected to different dephasing processes that limit the storage time, which in principle could be overcome using spin-echo techniques. Theoretical studies suggest this to be challenging due to unavoidable spontaneous emission noise in ensemble-based quantum memories. Here, we demonstrate spin-echo manipulation of a mean spin excitation of 1 in a large solid-state ensemble, generated through storage of a weak optical pulse. After a storage time of about 1 ms we optically read-out the spin excitation with a high signal-to-noise ratio. Our results pave the way for long-duration optical quantum storage using spin-echo techniques for any ensemble-based memory.
The non-interacting and high-speed nature of light makes it an ideal carrier of information that is essential for transmission of quantum information. Indeed, many proposals and demonstrations of quantum cryptography rely on the use of fibre-optic networks. Construction of a memory that can store light and preserve its quantum properties will be useful in a range of quantum information systems such as secure quantum communication and quantum computation. This is why a quantum memory for light is a remarkable objective. The key to quantum memory is to store the probability amplitude of the possible outcomes of measurement but without measurement. An important criterion for a quantum memory is that the efficiency of the recall must exceed 50%. This is the crucial no-cloning limit for security of information, since it guarantees that nobody can access the information by copying it. This benchmark is important because any kind of deterministic amplification of quantum information is fundamentally impossible. On-demand retrieval of information and ability to controllably manipulate the quantum information are also important for quantum applications. When light is absorbed by atoms, it is actually possible to reverse the absorption process. In our memory system: light is absorbed by an ensemble of atoms and, using careful conditioning and control, we can cause the stored light to be regenerated and released at a later time. This is done by applying a gradient of magnetic field along the atomic ensemble that is the basis for our optical memory. To recall the light we flip the sign of the gradient field. This kind of reversible absorption is called photon echo, hence the name of our scheme: The Gradient Echo Memory (GEM). This simple protocol is used in our experiment and can be applied to a range of different atomic systems. We have extended the GEM protocol and experimentally implemented a memory using three-level atoms. We used an off-the-shelf Rb vapour cell operating above room temperature as the memory medium. In this realisation, we broke the efficiency record with 87% recall of the input light pulse. Moreover, through complete state tomography of coherent states we have demonstrated the ability of our memory to noiselessly store quantum states of light. We have also demonstrated that the memory can store a string of pulses and then recall the pulses ondemand in arbitrary order allowing re-sequencing of the stored information. Furthermore, we have shown that pulses could be time-compressed, time-stretched or split into multiple smaller pulses and selectively recalled in several pieces. This technique enables the construction of an optical random-access memory for quantum information. Moreover, the scheme to manipulate the spectral properties of optical data, stored inside the memory, has been introduced. We have also investigated the possibility of obtaining large nonlinear phase shifts between single photons inside the memory. Such strong interactions can be used for the implementation of universal quantum gates.
We have performed a theoretical analysis of the photon-echo-based quantum memory realized recently in A. L. Alexander et al., Phys. Rev. Lett. 96, 043602 (2006) in the medium with linear correlation between atomic spatial coordinates and frequency detunings of the inhomogeneously broadened transition. A more general vision on the physical picture of temporal and spatial light-atoms dynamics has been obtained in the analytical solutions, taking into account the atomic phase relaxation, inhomogeneous broadening, and arbitrary parameters of the probe light pulses. We have evaluated preferable experimental conditions for the storage of incoming data light field modes where quantum memory efficiency and fidelity have been found. Physics of high efficiency in the forward geometry of the quantum memory and advantages for the realization of more universal long-lived storage on this basis are clarified.