Quantum Memory Protocol Research Articles

Nuclear quantum memory for hard x-ray photon wave packets.

Optical quantum memories are key elements in modern quantum technologies to reliably store and retrieve quantum information. At present, they are conceptually limited to the optical wavelength regime. Recent advancements in x-ray quantum optics render an extension of optical quantum memory protocols to ultrashort wavelengths possible, thereby establishing quantum photonics at x-ray energies. Here, we introduce an x-ray quantum memory protocol that utilizes mechanically driven nuclear resonant 57Fe absorbers to form a comb structure in the nuclear absorption spectrum by using the Doppler effect. This room-temperature nuclear frequency comb enables us to control the waveform of x-ray photon wave packets to a high level of accuracy and fidelity using solely mechanical motions. This tunable, robust, and highly flexible system offers a versatile platform for a compact solid-state quantum memory at room temperature for hard x-rays.

Quantum optical memory for entanglement distribution

Optical photons are powerful carriers of quantum information, which can be delivered in free space by satellites or in fibers on the ground over long distances. Entanglement of quantum states over long distances can empower quantum computing, quantum communications, and quantum sensing. Quantum optical memories are devices designed to store quantum information in the form of stationary excitations, such as atomic coherence, and are capable of coherently mapping these excitations to flying qubits. Quantum memories can effectively store and manipulate quantum states, making them indispensable elements in future long-distance quantum networks. Over the past two decades, quantum optical memories with high fidelities, high efficiencies, long storage times, and promising multiplexing capabilities have been developed, especially at the single-photon level. In this review, we introduce the working principles of commonly used quantum memory protocols and summarize the recent advances in quantum memory demonstrations. We also offer a vision for future quantum optical memory devices that may enable entanglement distribution over long distances.

Implementation of a Quantum Memory Protocol Based on the Revival of Silenced Echo in Orthogonal Geometry at the Telecommunication Wavelength

An optical quantum memory protocol has been implemented on the basis of the revival of silenced echo at the telecommunication wavelength for signal light fields with a small number of photons. To this end, a long-lived (>1 s) absorption line has been initialized and the orthogonal geometry of the propagation of the signal and rephasing fields has been chosen. An efficiency of revival of (17 ± 1)% has been reached for the orthogonal polarization components of a signal pulse at a storage time of 60 μs. The input pulse contains ~38 photons on average, the revived echo signal includes ~6 photons, and the signal-to-noise ratio is 1.3.

Memory and Transduction Prospects for Silicon T Center Devices

The T center, a silicon-native spin-photon interface with telecommunication-band optical transitions and long-lived microwave qubits, offers an appealing new platform for both optical quantum memory and microwave-to-optical telecommunication-band transduction. A wide range of quantum memory and transduction schemes could be implemented withT center ensembles with sufficient optical depth, with advantages and disadvantages that depend sensitively on the ensemble properties. In this work we characterize T center spin ensembles to inform device design. We perform the first T ensemble optical depth measurement and calculate the improvement in center density or resonant optical enhancement required for efficient optical quantum memory. We further demonstrate a coherent microwave interface by coherent population trapping and Autler-Townes splitting. We then determine the most promising microwave and optical quantum memory protocol for such ensembles. By estimating the memory efficiency both in free space and in the presence of a cavity, we show that efficient optical memory is possible with reasonable optical density forecasts. Finally, we formulate a transduction proposal and discuss the achievable efficiency and fidelity.Received 13 September 2022Accepted 13 March 2023DOI:https://doi.org/10.1103/PRXQuantum.4.020308Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCoherent population trappingElectromagnetically induced transparencyQuantum information with hybrid systemsQuantum information with solid state qubitsQuantum interconnectsQuantum memoriesQuantum networksQuantum repeatersQuantum InformationAtomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Variance-based sensitivity analysis of Λ -type quantum memory

The storage and retrieval of photonic quantum states, quantum memory, is a key resource for a wide range of quantum applications. Here we investigate the sensitivity of $\mathrm{\ensuremath{\Lambda}}$-type quantum memory to experimental fluctuations and drift. We use a variance-based approach, focusing on the effects of fluctuations and drift on memory efficiency. We consider shot-to-shot fluctuations of the memory parameters, and separately we consider longer timescale drift of the control field parameters. We find the parameters that a quantum memory is most sensitive to depend on the quantum memory protocol being employed, where the observed sensitivity agrees with physical interpretation of the protocols. We also present a general framework that is applicable to other figures of merit beyond memory efficiency. These results have practical ramifications for quantum memory experiments.

Intra-atomic frequency-comb-based photonic quantum memory using single-atom-cavity setup

On-demand and efficient storage of photons is an essential element in quantum information processing and long-distance quantum communication. Most of the quantum memory protocols require bulk systems in order to store photons. However, with the advent of integrated photonic chip platforms for quantum information processing, on-chip quantum memories are highly sought after. In this paper, we propose a protocol for multimode photonic quantum memory using only a single-atom-cavity setup. We show that a single atom containing a frequency comb coupled to an optical cavity can store photons efficiently. Further, this scheme can also be used to store polarization states of light. As examples, we show that rubidium and cesium atoms coupled to nanophotonic waveguide cavities can serve as promising candidates to realize our scheme. This provides the possibility of a robust and efficient on-chip quantum memory to be used in integrated photonic chips.

Протокол оптической памяти ROSE в волноводе кристаллa Tm : YAG

In this work, optical spectroscopy of thulium ions in a single-mode optical waveguide fabricated in a Tm3+:Y3Al5O12 crystal using the femtosecond laser printing method was carried out and an optical quantum memory protocol was demonstrated in a revival of silenced echo scheme. An analysis of the experimental data indicates the presence of instantaneous spectral diffusion at a thulium ion concentration of less than 0.01%, a weak effect of imperfections in the formed waveguide on the lifetime of the optical memory, and indicates the possibility of achieving a high efficiency of input signal recovery in the implemented waveguide scheme of the protocol.

Spin-Echo Silencing Using a Current-Biased Frequency-Tunable Resonator

The ability to control microwave emission from a spin ensemble is a requirement of several quantum memory protocols. Here, we demonstrate such ability by using a resonator whose frequency can be rapidly tuned with a bias current. We store excitations in an ensemble of rare-earth ions and suppress on demand the echo emission ("echo silencing") by two methods: (1)detuning the resonator during the spin rephasing, and (2)subjecting spins to magnetic field gradients generated by the bias current itself. We also show that spin coherence is preserved during silencing.

Toward a Quantum Memory in a Fiber Cavity Controlled by Intracavity Frequency Translation.

We propose a quantum memory protocol based on trapping photons in a fiber-integrated cavity, comprised of a birefringent fiber with dichroic reflective end facets. Photons are switched into resonance with the fiber cavity by intracavity Bragg-scattering frequency translation, driven by ancillary control pulses. After the storage delay, photons are switched out of resonance with the cavity, again by intracavity frequency translation. We demonstrate storage of quantum-level THz-bandwidth coherent states for a lifetime up to 16 cavity round trips, or 200ns, and a maximum overall efficiency of 73%.

Optimization of broadband Λ -type quantum memory using Gaussian pulses

Optical quantum memory, the ability to store photonic quantum states and retrieve them on demand, is an essential resource for emerging quantum technologies and photonic quantum information protocols. Simultaneously achieving high efficiency and high-speed, broadband operation is an important task necessary for enabling these applications. We investigate the optimization of a large class of optical quantum memories based on resonant and near-resonant interaction with ensembles of $\mathrm{\ensuremath{\Lambda}}$-type level systems with the restriction that the temporal envelope of all optical fields must be Gaussian, which reduces experimental complexity. Through this optimization we demonstrate an experimentally simple path to saturation of the protocol-independent storage efficiency bound that is valid for a wide range of memory bandwidths, including those that are broadband and high speed. Examining the resulting optimal Gaussian control field parameters, we find a continuous transformation between three physically distinct resonant quantum memory protocols. We compare this Gaussian optimization scheme with standard shape-based optimization.

Coherent Control in the Ground and Optically Excited States of an Ensemble of Erbium Dopants

Ensembles of erbium dopants can realize quantum memories and frequency converters that operate in the minimal-loss wavelength band of fiber optical communication. Their operation requires the initialization, coherent control, and readout of the electronic spin state. In this work, we use a split-ring microwave resonator to demonstrate such control in both the ground and optically excited state. The presented techniques can also be applied to other combinations of dopant and host and may facilitate the further development of quantum memory protocols and sensing schemes.

Linear Stark effect in Y3Al5O12:Tm3+ crystal and its application in the addressable quantum memory protocol

We report an observation of the linear Stark effect in a ${\mathrm{Tm}}^{3+}:{\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ crystal with the distribution of the Stark coefficient over the ion ensemble. We associate this effect with local lattice distortions near the positions of ${\mathrm{Tm}}^{3+}$ ions. Using this effect, the addressable storage of a series of weak light pulses in a cavity-assisted scheme of the revival of silenced echo quantum memory protocol is implemented. In this memory scheme, we also demonstrate storage of a light pulse on the few photon level. The application of an optical resonator makes it possible to increase the memory efficiency in this crystal and to reduce the minimal number of photons in the input signal pulse to 5.6 for the signal-to-noise ratio of 1 in the retrieved echo pulse. The results are in good agreement with the theoretical analysis. The possible ways of the further improvement of the implemented memory scheme are also discussed.

Storing short single-photon-level optical pulses in Bose–Einstein condensates for high-performance quantum memory

Large-scale quantum networks require quantum memories featuring long-lived storage of non-classical light together with efficient, high-speed and reliable operation. The concurrent realization of these features is challenging due to inherent limitations of matter platforms and light–matter interaction protocols. Here, we propose an approach to overcome this obstacle, based on the implementation of the Autler–Townes-splitting (ATS) quantum-memory protocol on Bose–Einstein condensate (BEC) platform. We demonstrate a proof-of-principle of this approach by storing short pulses of single-photon-level light as a collective spin-excitation in a rubidium-BEC. For 20 ns long-pulses, we achieve an ultra-low-noise memory with an efficiency of 30% and lifetime of 15 μs. The non-adiabatic character of the ATS protocol (leading to high-speed and low-noise operation) in combination with the intrinsically large atomic densities and ultra-low temperatures of the BEC platform (offering highly efficient and long-lived storage) opens up a new avenue toward high-performance quantum memories.

Optical and spin manipulation of non-Kramers rare-earth ions in a weak magnetic field for quantum memory applications

Rare-earth ion doped crystals have proven to be solid platforms for implementing quantum memories. Their potential use for integrated photonics with large multiplexing capability and unprecedented coherence times is at the core of their attractiveness. The best performances of these ions are, however, usually obtained when subjected to a DC magnetic field, but consequences of such fields on the quantum memory protocols have only received little attention. In this paper, we focus on the effect of a DC bias magnetic field on the population manipulation of non-Kramers ions with nuclear quadrupole states, both in the spin and optical domains, by developing a simple theoretical model. We apply this model to explain experimental observations in a $^{151}\mathrm{Eu}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal, and highlight specific consequences on the atomic frequency comb spin-wave protocol. The developed analysis should allows predicting optimal magnetic field configurations for various protocols.

The Effect of Diffraction on a Pulse of Squeezed Light in the Protocol of a Multimode Resonant Quantum Memory Based on a Thermal Atomic Ensemble

The effect of diffraction on the conservation of the quantum state of a quadrature-squeezed light pulse in an ensemble of thermal atoms for a collinear configuration of light fields has been considered. The motion of atoms leads to the fact that when both the signal and control fields propagate in the same direction as when writing, there is an uncompensated phase incursion even in the case of forward reading. This phase incursion leads to the fact that the squeezed quadrature of the light pulse is mixed with a stretched one. As a result, the pulse squeezing can decrease significantly. The analyses of the effect of diffraction for different configurations of the multimode resonant quantum memory protocol has been performed and the ways to reduce this effect to a possible minimum have been found.

Understanding of Collective Atom Phase Control in Modified Photon Echoes for a Near-Perfect Storage Time-Extended Quantum Memory.

A near-perfect storage time-extended photon echo-based quantum memory protocol has been analyzed by solving the Maxwell–Bloch equations for a backward scheme in a three-level system. The backward photon echo scheme is combined with a controlled coherence conversion process via controlled Rabi flopping to a third state, where the control Rabi flopping collectively shifts the phase of the ensemble coherence. The propagation direction of photon echoes is coherently determined by the phase-matching condition between the data (quantum) and the control (classical) pulses. Herein, we discuss the classical controllability of a quantum state for both phase and propagation direction by manipulating the control pulses in both single and double rephasing photon echo schemes of a three-level system. Compared with the well-understood uses of two-level photon echoes, the Maxwell–Bloch equations for a three-level system have a critical limitation regarding the phase change when interacting with an arbitrary control pulse area.

Antiferromagnetic cavity optomagnonics

Currently, there is a growing interest in studying the coherent interaction between magnetic systems and electromagnetic radiation in a cavity, prompted partly by possible applications in hybrid quantum systems. We propose a multimode cavity optomagnonic system based on antiferromagnetic insulators, where optical photons couple coherently to the two homogeneous magnon modes of the antiferromagnet. These have frequencies typically in the THz range, a regime so far mostly unexplored in the realm of coherent interactions, and which makes antiferromagnets attractive for quantum transduction from THz to optical frequencies. We derive the theoretical model for the coupled system, and show that it presents unique characteristics. In particular, if the antiferromagnet presents hard-axis magnetic anisotropy, the optomagnonic coupling can be tuned by a magnetic field applied along the easy axis. This allows to bring a selected magnon mode into and out of a dark mode, providing an alternative for a quantum memory protocol. The dynamical features of the driven system present unusual behavior due to optically induced magnon-magnon interactions, including regions of magnon heating for a red detuned driving laser. The multimode character of the system is evident in a substructure of the optomagnonically induced transparency window.

Improved light-matter interaction for storage of quantum states of light in a thulium-doped crystal cavity

We design and implement an atomic frequency comb quantum memory for 793 nm wavelength photons using a monolithic cavity based on a thulium-doped Y$_3$Al$_5$O$_{12}$ (Tm:YAG) crystal. Approximate impedance matching results in the absorption of approximately $90\%$ of input photons and a memory efficiency of (27.5$\pm$ 2.7)% over a 500 MHz bandwidth. The cavity enhancement leads to a significant improvement over the previous efficiency in Tm-doped crystals using a quantum memory protocol. In turn, this allows us for the first time to store and recall quantum states of light in such a memory. Our results demonstrate progress toward efficient and faithful storage of single photon qubits with large time-bandwidth product and multi-mode capacity for quantum networking.

Conversion and storage of modes with orbital angular momentum in a quantum memory scheme

The paper studies the Raman quantum memory protocol as applied to quantum light with orbital angular momentum. The memory protocol is implemented on an ensemble of three-level cold atoms with the $\Lambda$- configuration of energy levels. The possibility of storing quantum statistics of light with an orbital momentum is analysed in the case when the driving field could be treated as a plane wave. The efficiency analysis shows that examined storage/retrieval processes do not cause the efficiency decreasing compared with the spatial multimode memory protocol considered in [Golubeva et al. 2012]]. We also present an effective transformation of the orbital angular momentum of a quantum field on a memory cell using the driving field with orbital angular momentum.

Research progress of quantum memory

Quantum technologies, for example, quantum communication and quantum computation, promise spectacular quantum enhanced advantages beyond what can be done classically. However, quantum states, as the element of quantum technologies, are very fragile and easily get lost to the environment, and meanwhile, their generation and quantum operations are mostly probabilistic. These problems make it exponentially hard to build long-distance quantum channels for quantum communication and large quantum systems for quantum computing. Quantum memory allows quantum states to be stored and retrieved in a programmable fashion, therefore providing an elegant solution to the probabilistic nature and associated limitation by coordinating asynchronous events. In the past decades, enormous advances in quantum memory have been made by developing various storage protocols and their physical implementations, and the quantum memory has gradually evolved from the initial conceptual demonstration to a nearly practical one. Aiming at being practicable for efficient synchronisation and physical scalability, an ideal quantum memory should meet several key features known as high efficiency, low noise level, large time bandwidth product (lifetime divided by pulse duration) and operating at room temperature. Here, we present the research status and development trends of this field by introducing some typical storage protocols. Among these protocols, a room-temperature broadband quantum memory is the most attractive due to its simplicity and practicability. However, at room temperature, noise becomes dominant and is a bottleneck problem that has impeded the realization of a real room-temperature broadband quantum memory in the last decades. Recently, the noise problem has been solved in two memory protocols, i.e. FORD (far off-resonance Duan-Lukin-Cirac-Zoller) protocol and ORCA (off-resonant cascaded absorption) protocol. In this paper, the working principles, the merits and demerits of various quantum memory protocols are illustrated. Furthermore, the approaches to eliminating noise and the applications of quantum memory are summarized.