K. T. Kaczmarek, Patrick M. Ledingham, Benjamin Brecht, Sarah E. Thomas, Guillaume Thekkadath, O. Lazo-Arjona, J. H. D. Munns, Eilon Poem, Amir Feizpour, D. J. Saunders, J. Nunn, Ian A. Walmsley
Optical quantum memories are devices that store and recall quantum light and are vital to the realization of future photonic quantum networks. To date, much effort has been put into improving storage times and efficiencies of such devices to enable long-distance communications. However, less attention has been devoted to building quantum memories which add zero noise to the output. Even small additional noise can render the memory classical by destroying the fragile quantum signatures of the stored light. Therefore, noise performance is a critical parameter for all quantum memories. Here we introduce an intrinsically noise-free quantum memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We demonstrate successful storage of GHz-bandwidth heralded single photons in a warm atomic vapor with no added noise, confirmed by the unaltered photon-number statistics upon recall. Our ORCA memory meets the stringent noise requirements for quantum memories while combining high-speed and room-temperature operation with technical simplicity, and therefore is immediately applicable to low-latency quantum networks.
Optical quantum memories are devices that store and recall quantum light and are vital to the realization of future photonic quantum networks. To date, much effort has been put into improving storage times and efficiencies of such devices to enable long-distance communications. However, less attention has been devoted to building quantum memories which add zero noise to the output. Even small additional noise can render the memory classical by destroying the fragile quantum signatures of the stored light. Therefore, noise performance is a critical parameter for all quantum memories. Here we introduce an intrinsically noise-free quantum memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We demonstrate successful storage of GHz-bandwidth heralded single photons in a warm atomic vapor with no added noise, confirmed by the unaltered photon-number statistics upon recall. Our ORCA memory meets the stringent noise requirements for quantum memories while combining high-speed and room-temperature operation with technical simplicity, and therefore is immediately applicable to low-latency quantum networks.
Light is an ideal information carrier for quantum networks: its quantum properties are not degraded by noise in ambient conditions and it has a large information capacity owing to its high bandwidth. However, quantum technologies based on photonic networks have been hampered by the exponentially poor scaling of the underlying probabilistic quantum operations. Quantum optical memories, devices that store, manipulate, and release on- demand quantum light, have been identified as an indispensable network component, because they facilitate scalability. Noise-free operation of the memory is critical, since even small additional noise can render the memory classical by destroying the quantum character of the light. Here I introduce a new broadband quantum memory protocol - the off-resonant cas- caded absorption (ORCA) memory - that is inherently noise-free and operates in ambient conditions. I model ORCA theoretically, after which I characterise the classical perfor- mance of a proof-of-concept implementation in warm caesium vapour, using weak near- infrared coherent states. In order to verify quantum operation of the ORCA memory, I interface it with a broadband heralded single-photon source. I measure and compare the quantum statistics of the stored and retrieved light, observing for the first time their full preservation in a room-temperature atomic quantum memory. Finally, I make headway into integrating the ORCA vapour memory by investigating an alkali-filled hollow-core fibre platform. I use light-induced atomic desorption (LIAD) to demonstrate record- breaking alkali vapour densities in fibre, a prerequisite for efficient memory operation. Because of its technical simplicity and integrability, its high bandwidth and its low noise, ORCA provides a viable route towards next generation, photonic quantum network technologies.
Quantum optical memories are devices that store quantum states of light, which can allow for the active synchronization of probabilistic events within large-scale quantum networks. Recent work on quantum memories have seen impressive quantum operation, albeit still suffering from noise on the output mode of the device. Here we demonstrate a noise-free quantum memory for light based on the off-resonant cascaded absorption of photons in a warm vapour of caesium atoms. The memory is characterized by measuring a noise floor of 8 × 10−6 photons per pulse. We demonstrate genuine quantum operation by storing and recalling on-demand heralded single photons with a heralded second-order autocorrelation function of g(2) = 0.028 ± 0.009.
The coherent storage, buffering and retrieval of photons in a quantum memory enables the scalable creation of photonic entangled states via linear optics and repeat-until success, unlocking applications in quantum communications and photonic quantum computing. Quantum memories based on off-resonant cascaded absorption (ORCA) in atomic vapors allow this storage to be broadband, noise free, and high efficiency. Here, we implement a cavity-enhanced ORCA memory with reduced footprint and reduced power requirements compared to conventional single-pass schemes. By combining a strong magnetic field with polarization control, we maintain a Doppler-free interaction and eliminate the need for optical pumping. Our design establishes the feasibility of large arrays of ultracompact, low-power, near-unit efficiency, noiseless quantum memories running at GHz bandwidth, without the need for atom trapping or cryogenics.
Summary form only given. A quantum optical memory (QM) is a device that can store and release quantum states of light on demand. Such a device is capable of synchronising probabilistic events, for example, locally synchronising non-deterministic photon sources for the generation of multi-photon states, or successful quantum gate operations within a quantum computational architecture [1], as well as for globally synchronising the generation of entanglement over long distances within the context of a quantum repeater [2]. Desirable attributes for a QM to be useful for these computational and communicational tasks include high end-to-end transmission (including storage and retrieval efficiency), large storage-time-bandwidth product, room temperature operation for scalability and, of utmost importance, noise free performance for true quantum operation.Here we present a new QM protocol that addresses the above issues, the quantum ladder memory (QLAD). This protocol is based on a two-photon `ladder' transition between the 6S1/2 ground state and 6D5/2 excited state of a caesium (Cs) ensemble at room temperature (see Fig. 1a). We characterise the memory with weak coherent states, storing GHz-band pulses with η=22% storage and retrieval efficiency and a characteristic storage time of τ=5.3ns. We measure the noise at the output to be 8×10-6 photons per pulse, giving rise to a noise-to-efficiency ratio of 3.6×10-5, three orders of magnitude better than any memory to date. We generate GHz-band heralded single photons from a spontaneous parametric down conversion source that are matched to the QLAD, coincidence traces seen in Fig. 1b. A heralded second-order autocorrelation function of g(2)=0.02+0.005 is measured for the input photon, and true quantum operation is confirmed by storing and retrieving this photon with g(2)=0.03+0.01 for a 3.5ns storage time. This result is the lowest g(2) on the output of an on-demand memory to date, and represents a significant step toward large-scale quantum technologies.
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.