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Based on new obtained analytical results, the main properties of photon echo quantum memory protocols are analysed and discussed together with recently achieved experimental results. The main attention is paid to studying the influence of spectral dispersion and nonlinear interaction of light pulses with resonant atoms. The distinctive features of the effect of spectral dispersion on the quantum storage of broadband signal pulses in the studied echo protocols are identified and discussed. Using photon echo area theorem, closed analytical solutions for echo protocols of quantum memory are obtained, describing the storage of weak and intense signal pulses, allowing us to find the conditions for the implementation of high efficiency in the echo protocols under strong nonlinear interaction of signal and control pulses with atoms. The key existing practical problems and the ways to solve them in realistic experimental conditions are outlined. We also briefly discuss the potential of using the considered photon echo quantum memory protocols in a quantum repeater.

We propose a quantum memory based on the pre-created long-lived macroscopic quantum coherence. It is shown that the proposed approach provides new physical properties and methods for retrieval of the signal light fields and improvement of the basic parameters of quantum memory. We demonstrate how the pre-created coherence can enable quantum storage with low quantum noise, programmable and on demand retrieval of signal light fields in atomic ensembles with natural inhomogeneous broadening. The feasibility of implementing this proposal in various crystals doped with rare earth ions, as well as in atomic gases with a Raman transition indicates a new way for the development of optical quantum memory.

We develop the integrated efficient multiresonator quantum memory scheme based on a system of three interacting resonators coupled through a common resonator to an external waveguide via switchable coupler. It is shown that high-precision parameter matching based on the step-by-step optimization makes it possible to efficiently store the signal field and on demand retrieval the signal at specified time moments. Possible experimental implementations and practical applications of the proposed quantum memory scheme are discussed., Comment: 7 pages, 4 figures

We derive the area theorem for light pulses interacting with inhomogeneously broadened ensemble of two-level atoms in a single-mode optical waveguide and present its analytical solution for Gaussian-type modes, which demonstrates the significant difference from the formation of $2\pi$ pulses by plane waves. We generalize this theorem to the description of photon echo and apply it to the two-pulse (primary) echo and the revival of silenced echo (ROSE) protocol of photon echo quantum memory. For the first time, we implemented ROSE protocol in a single-mode laser-written waveguide made of an optically thin crystal $Tm^{3+}:Y_3Al_5O_{12}$. The experimental data obtained are satisfactorily explained by the developed theory. Finally, we discuss the obtained experimental results and possible applications of the derived pulse area approach.

Microwave quantum memory promises advanced capabilities for noisy intermediate-scale superconducting quantum computers. Existing approaches to microwave quantum memory lack complete combination of high efficiency, long storage time, noiselessness and multi-qubit capacity. Here we report an efficient microwave broadband multimode quantum memory. The memory stores two spectral modes of single photon level microwave radiation in on-chip system of eight coplanar superconducting resonators. Single mode storage shows a power efficiency of up to $60\pm 3\%$ at single photon energy and more than $73\pm 3\%$ at higher intensity. The demonstrated efficiency is an order of magnitude larger than the previously reported multimode microwave quantum memory. The noiseless character of the storage is confirmed by coherent state quantum process tomography. The demonstrated results pave the way to further increase in efficiency and hence building a practical multimode microwave memory for superconducting quantum circuits., Comment: 14 pages, 8 figures

We seek to design experimentally feasible broadband, temporally multiplexed optical quantum memory with near-term applications to telecom bands. Specifically, we devise dispersion compensation for an impedance-matched narrow-band quantum memory by exploiting Raman processes over two three-level atomic subensembles, one for memory and the other for dispersion compensation. Dispersion compensation provides impedance matching over more than a full cavity linewidth. Combined with one second spin-coherence lifetime the memory could be capable of power efficiency exceeding 90% leading to 106 modes for temporal multiplexing. Our design could lead to significant multiplexing enhancement for quantum repeaters to be used for telecom quantum networks.

The single photon detectors currently used in quantum communication schemes impose considerable restrictions on signal registration and dark count rates, require cooling to low temperatures, and are relatively expensive. Alternative approaches have recently been proposed that are based on the homogeneous and heterogeneous detection of quantum signals and can be used with conventional photodetectors. This work studies the possibility of obtaining a heterogeneous detection scheme in a subcarrier wave (also known as homodyne, self-homodyne or self-heterodyne detection methods for side band frequencies) quantum communication system that could be used to create quantum networks.

We propose a cascade scheme of a superefficient broadband quantum memory consisting of four high-Q ring resonators forming a controllable frequency comb and interacting with long-lived spin systems and with a common waveguide. Using the transfer function giving extended matching conditions, the optimization of all spectroscopic parameters of the system for quantum memory in the resonator is carried out. It was shown that our quantum memory scheme does not impose large restrictions on the parameters of losses in resonators and allows to achieve super high efficiency 99.99 % in a wide frequency range., Comment: arXiv admin note: substantial text overlap with arXiv:1711.07014

We propose a scheme of a universal block of broadband quantum memory consisting of three ring microresonators forming a controllable frequency comb and interacting with each other and with a common waveguide. We find the optimal parameters of the microresonators showing the possibility of highly efficient storage of light fields on this memory block and we demonstrate the procedure for gluing several memory blocks for increasing spectral range of the composite quantum memory while maintaining high efficiency.

We show that, under certain circumstances, an optical field in a two-mode squeezed vacuum (TMSV) state can propagate through a lossy atomic medium without degradation or evolution. Moreover, the losses give rise to that state when a different state is initially injected into the medium. Such a situation emerges in a {\Lambda}-type atomic system, in which both optical transitions are driven by strong laser fields that are two-photon resonant with the respective signal modes. Then the interactions of the two signal modes with the ground-state atomic coherence interfere destructively, thereby ensuring the preservation of the TMSV with a particular squeezing parameter. This mechanism permits unified interpretation of recent experimental results and predicts new phenomena.

We produced optical waveguides in the 167Er3+ :7LiYF4 crystal with diameters ranging from 30 to 100 mkm by using the depressed-cladding approach with femtosecond laser. These waveguides were studied (both inside and outside) by stationary and coherent spectroscopy on the 809 nm optical transitions between the hyperfine sublevels of 4I15/2 and 4I9/2 multiplets of 167Er3+ ions. It was found that the spectra of 167Er3+ were slightly broadened and shifted inside the waveguides compared to the bulk crystal spectra. We managed to observe a two-pulse photon echo on this transition and determined phase relaxation times for each of waveguides. The experimental results show that the created crystal waveguides doped by rare-earth ions can be used in optical quantum memory and integrated quantum schemes.

The optical scheme is proposed for realization of controlled phase quantum gate based on the off-resonant interaction of photon with three-level atom in single mode QED-cavity. Possible physical implementation of this scheme is discussed.

We investigated the optimal spectral design of periodic structure of narrow lines (AFC-structure) within the inhomogeneously broadened atomic transition which are created for the implementation of broadband AFC photon echo quantum memory.The influence of the spectral design on the suppression of negative dispersion effects in the AFC-echo retrieval was studied for different spectroscopic parameters of atomic media. The maps of the assigned spectral quantum efficiency have been constructed for the created AFC-structures characterized by different spectral design, finesse and optical depth. Based on the performed analysis, we discuss the possible ways for experimental implementation of highly efficient broadband AFC-protocol.

Single-photon detectors based on avalanche photodiodes (SPAD) are key elements of many modern highly sensitive optical systems. One of the bottlenecks of such detectors is a afterpulsing effect which limits a detection rate and requires an optimal hold-off time. In this letter we propose a novel approach for statistical analysis of SPAD dark counts and we demonstrate its usefulness for the search of the experimental condition where the afterpulsing effect can be strongly eliminated. This approach exploits a sequence of ranked time intervals between the dark counts and does not contain a complex mathematical analysis of the experimental data. We show that the approach can be efficiently applied for a small number of the dark counts and it seems to be very beneficial for practical characterization of SPAD devices.

In this paper we experimentally demonstrated a broadband microwave scheme suitable for the multiresonator quan- tum memory-interface. The microwave scheme consists of the system of composed mini-resonators strongly inter- acting with a common broadband resonator coupled with the external microwave waveguide. We have implemented the controllable tuning of the mini-resonator frequencies and coupling of the common resonator with the external waveguide for the implementation of the impedance matched quantum storage. The storage of microwave pulses with an efficiency of 16.3% has been shown experimentally at room temperature. The possible properties of the proposed scheme for mini-resonators with high-Q at low temperatures are discussed. The obtained results pave the way for the implementation of superefficient broadband microwave quantum memory-interface., Comment: 5 pages, 2 figures, Third International Conference on Quantum Technologies (ICQT-2017), LPHYS'17 Workshop

In this work we report about the fundamental correspondence between the high efficiency of the broadband quantum memory on multifrequency absorbers in the single-mode cavity and the topology of the observed frequency lines. We have found the spectral-topological matching condition by using the multiparametric optimization of the absorber characteristics which makes it possible to improve considerably the spectral efficiency of quantum memory under conditions of a real experiment. The elaborated approach opens the possibility for the creation of the broadband quantum interface consisting of a small number of the absorbers and characterized by the super-high quantum efficiency of more than 99.9%., Comment: 4 pages, 2 figures

The dynamics of the interaction between microcavities connected to a common waveguide in a multiresonator quantum memory circuit is investigated. Optimum conditions are identified for the use of quantum memory and a dynamic picture of the exchange of energy between different microcavities is obtained.

In this paper we present universal broadband multi-resonator quantum memory (MR-QM) based on the spatial-frequency combs of the microresonators coupled with a common waveguide. Here we found new Bragg-type impedance matching condition for the coupling of the microresonators with a waveguide field which provides an efficient broadband quantum storage. The obtained analytical solution for the microresonator fields enables sustainable parametric control of all the memory characteristics.

We demonstrate that a photon echo can be implemented by all-optical means using an array of on-chip high-finesse ring cavities whose parameters are chirped in such a way as to support equidistant spectra of cavity modes. When launched into such a system, a classical or quantum optical signal -- even a single-photon field -- becomes distributed between individual cavities, giving rise to prominent coherence echo revivals at well-defined delay times, controlled by the chirp of cavity parameters. This effect enables long storage times for high-throughput broadband optical delay and quantum memory., Comment: 7 pages, 5 figures