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arXiv:2610.24098 · IEEE International Conference on Quantum Computing and Engineering (QCE)

Performance of a Hot Rb Vapour-Based Portable Quantum Memory

Kenneth Gregory, Konrad Socha, Khaled Mnaymneh, Connor Kupchak

quant-phSubmitted Oct 16, 2026• Announced Oct 20, 2026• 1 citation
arXivHTMLPDF

Abstract

Quantum repeaters will be paramount to the success of the future quantum internet. Presently however, the losses incurred in fibre communications are unsustainable when fragile quantum states are transmitted over long distances. The most effective repeater nodes will require incorporation of a minimum of two quantum memory units (QMems) for successful operation. Therefore a network composed of numerous nodes will ideally require scalable repeaters that contain QMems that are portable, robust and do not require a high amount of overhead. Here we investigate the performance of a commercial QMem based on a warm vapour of Rb atoms. Specifically, we characterize the coherence time, absolute efficiency, signal to noise ratio and bit error ratio of this device over a range of vapour temperatures that are readily controllable. For our system, we place these performance metrics into the same conventions as those established for current optical communication protocols. Development of QMem systems meeting these benchmarks will be a vital step for seamless integration of QMems systems into present-day communication architectures.

Journal reference
2024 IEEE International Conference on Quantum Computing and Engineering (QCE), 15 Sept. 2024 - 20 Sept. 2024, Montreal, QC, Canada, Volume 01, id.227
DOI
10.1109/QCE60285.2024.00227

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Quantum repeaters will be paramount to the success of the future quantum internet. Presently however, the losses incurred in fibre communications are unsustainable when fragile quantum states are transmitted over long distances. The most effective repeater nodes will require incorporation of a minimum of two quantum memory units (QMems) for successful operation. Therefore a network composed of numerous nodes will ideally require scalable repeaters that contain QMems that are portable, robust and do not require a high amount of overhead. Here we investigate the performance of a commercial QMem based on a warm vapour of Rb atoms. Specifically, we characterize the coherence time, absolute efficiency, signal to noise ratio and bit error ratio of this device over a range of vapour temperatures that are readily controllable. For our system, we place these performance metrics into the same conventions as those established for current optical communication protocols. Development of QMem systems meeting these benchmarks will be a vital step for seamless integration of QMems systems into present-day communication architectures.
In the fields of quantum computing and cryptography, methods to connect networks include the use of quantum repeaters. Challenges include problems of synchronization, where the Quantum Memory (QMem) comes into [play]. By storing photons and releasing them on demand, the issue of widespread synchronization equipment is replaced by the technical issues of optical storage. This thesis will summarize the classical performance of a Rb vapour based QMem, along with a description of the ancillary optics required to interface with such a system. We will see a system with a minimum coherence time of 130 μs130\, \mu s130μs, Bit error rate (BER) below 10−1010^{-10}10−10 for up to 100 μs100\, \mu s100μs of storage time, a Signal to Noise Ratio (SNR) of up to 17 dB, and a maximum efficiency of approximately 12\%. Last will be discussion of improvements to the apparatus and future works.
By harnessing aspects of quantum mechanics, communication and information processing could be radically transformed. Promising forms of quantum information technology include optical quantum cryptographic systems and computing using photons for quantum logic operations. As with current information processing systems, some form of memory will be required. Quantum repeaters, which are required for long distance quantum key distribution, require quantum optical memory as do deterministic logic gates for optical quantum computing. Here, we present results from a coherent optical memory based on warm rubidium vapour and show 87% efficient recall of light pulses, the highest efficiency measured to date for any coherent optical memory suitable for quantum information applications. We also show storage and recall of up to 20 pulses from our system. These results show that simple warm atomic vapour systems have clear potential as a platform for quantum memory. Efficient memory systems are vital for the development of quantum communications technologies. Hosseini and colleagues describe an optical memory based on warm rubidium vapour that achieves 87% pulse recall efficiency, illustrating the potential of warm atomic vapour systems for quantum memory.
The success of long-distance quantum communication will hinge on quantum repeater nodes equipped with robust and scalable quantum memories. Such memories must possess specific operating characteristics to make these nodes practical. An attractive option for these memory devices is an atomic warm vapor system operating under the conditions of electromagnetically induced transparency (EIT). These systems are easy to implement and require relatively low equipment overhead. A key metric in atomic-based quantum memory performance is optical depth, as it is closely tied to the strength of light-matter coupling. Here, we simulate and measure the optical depth-based performance of an EIT quantum memory using a warm atomic vapor cell. We then translate the simulated and measured optical depth and efficiency metrics into expected entanglement distribution rates. Our analysis evaluates the feasibility of these memories in optical quantum repeater nodes, which are essential for robust long-distance quantum communication.
The efficient storage and on-demand retrieval of quantum optical states that are compatible with the telecommunications C-band is a requirement for future terrestrial-based quantum optical networking. Spectrum in the C-band minimises optical fiber-propagation losses, and broad optical bandwidth facilitates high-speed networking protocols. Here we report on a telecommunication wavelength and bandwidth compatible quantum memory. Using the Off-Resonant Cascaded Absorption protocol in hot 87^{87}87Rb vapour, we demonstrate a total memory efficiency of 20.90(1) %20.90(1)\,\%20.90(1)% with a Doppler-limited storage time of 1.10(2) 1.10(2)\,1.10(2)ns. We characterise the memory performance with weak coherent states, demonstrating signal-to-noise ratios greater than unity for mean photon number inputs above 4.5(6)×10−64.5(6)\times10^{-6}4.5(6)×10−6 per pulse.
Single photons are the flying qubits of choice for distributing entanglement in a quantum internet. Quantum memories embedded in quantum repeaters are crucial to overcome transmission loss and enhance the rate of quantum communication. A multimode memory can further boost the channel capacity. However, benchmarking and building a practical quantum memory that simultaneously optimizes multiple performance metrics poses two key challenges. Here, we introduce quantum interconnect rate to comprehensively quantify quantum memories, and further demonstrate a high-performance quantum memory that simultaneously integrates three essential criteria at once: large multimode capacity, high efficiency, and high fidelity. Operating on 11-dimensional spatial modes, our memory achieves a uniform efficiency exceeding 80% and qubit storage fidelities above 99%, enabling the efficient storage of high-dimensional qudits. Based on these capabilities, we estimate a distribution of 3.56 bits of quantum information over a 1000-km repeater link in one minute, highlighting a practical pathway toward scalable quantum interconnects and quantum networks.
Quantum communication between two remote locations often involves remote parties sharing an entangled quantum state. At present, entanglement distribution is usually performed using photons transmitted through optical fibers. However, the absorption of light in the fiber limits the communication distances to less than 200 km, even for optimal photon telecom wavelengths. To increase this distance, the quantum repeater idea was proposed. In the quantum repeater architecture, one divides communication distance into segments of the order of the attenuation length of the photons and places quantum memory nodes at the intermediate locations. Since the photon loss between intermediate locations is low, it is possible then to establish entanglement between intermediate quantum memory nodes. Once entanglement between adjacent nodes is established, one can extend it over larger distances using entanglement swapping.\n\nThe long coherence time of a quantum memory is a crucial requirement for the quantum repeater protocol. It is obvious that the coherence time of a quantum memory should be much longer that the time it takes for light to travel between remote locations. For a communication distance l = 1000 km, the corresponding time is t = l/c = 3.3 ms. One can show that for a simple repeater protocol and realistic success probabilities of entanglement generation, the required coherence time should be on the order of many seconds, while for the more complicated protocols that involve multiplexing and several quantum memory cells per intermediate node, the required coherence time is on the order of milliseconds.\n\nIn this thesis, I describe a quantum memory based on an ensemble of rubidium atoms confined in a one-dimensional optical lattice. The use of the magnetically- insensitive clock transition and suppression of atomic motion allows us to increase coherence time of the quantum memory by two-orders of magnitude compared to previous work. I also propose a method for determining the Zeeman content of atomic samples. In addition, I demonstrate the observation of quantum evolution under continuous measurement. The long quantum memory lifetime demonstrated in this work opens the way for scalable processing of quantum information and long distance quantum communication.