Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
A noiseless quantum optical memory at room temperature
Patrick M. Ledingham, K. T. Kaczmarek, Benjamin Brecht, Amir Feizpour, Guillaume Thekkadath, Sarah E. Thomas, J. H. D. Munns, D. J. Saunders, Joshua Nunn, Ian A. Walmsley
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.
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 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.
Future quantum photonic networks require coherent optical memories, preferably operating at room temperature, for synchronizing quantum sources and gates of probabilistic nature. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise. Here we demonstrate a fast ladder memory (FLAME) mapping the optical field onto the superposition between electronic orbitals of rubidium vapor. Employing a ladder level-system of orbital transitions with nearly degenerate frequencies simultaneously enables high bandwidth, low noise, and long memory lifetime. We store and retrieve 1.7-ns-long pulses, containing 0.5 photons on average, and observe short-time external efficiency of 25%, memory lifetime (1/e) of 86 ns, and below 10−4 added noise photons. Consequently, coupling this memory to a probabilistic source would enhance the on-demand photon generation probability by a factor of 12, the highest number yet reported for a noise-free, room-temperature memory. This paves the way towards the controlled production of large quantum states of light from probabilistic photon sources.
Quantum memories are key components in quantum information networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their long storage times and experimental simplicity. Operation with the Raman protocol enables high time-bandwidth products, which allows for multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing. This makes the Raman memory a promising tool, whose broad spectral bandwidth facilitates direct interfacing with other photonic primitives, such as single photon sources. Here, such a light-matter interface is implemented in a warm caesium vapour. Firstly, we study the storage of polarisation-encoded information in the memory. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which is a key technological capability. Prior to storage, single photons are produced in a spontaneous parametric down conversion source, whose bespoke design spectrally tailors the photons to the memory acceptance line. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of 0.3 in the memory output. Nevertheless, a clear influence of the input's quantum nature is observed in the retrieved light by measuring signal's photon statistics. Finally, the memory noise processes are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory.
Future quantum photonic networks require coherent optical memories for synchronizing quantum sources and gates of probabilistic nature. Room temperature operation is also desirable for ease of scaling up. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise due to spontaneous four-wave-mixing. Here we demonstrate a new scheme for storing photons at room temperature, the fast ladder memory (FLAME). In this scheme, stimulated two-photon absorption is used instead of the previously used stimulated Raman scattering. As here the competing spontaneous processes would require spontaneous absorption of an optical photon, rather than emission, the noise is greatly suppressed. Furthermore, high external efficiency can be achieved as the control is well separated in frequency from the signal, and could be filtered out using highly efficient interference filters. We run the protocol in rubidium vapour, both on and off single-photon resonance, demonstrating a ratio of 50 between storage time and signal pulse width, an external total efficiency of over 25%, and only 2.3 × 10-4noise photons per extracted signal photon. This paves the way towards the efficient synchronization of probabilistic gates and sources at room temperature, and the controlled production of large quantum states of light.
Future quantum photonic networks require coherent optical memories for synchronizing quantum sources and gates of probabilistic nature. Room temperature operation is also desirable for ease of scaling up. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise due to spontaneous four-wave-mixing. Here we demonstrate a new scheme for storing photons at room temperature, the fast ladder memory (FLAME). In this scheme, stimulated two-photon absorption is used instead of the previously used stimulated Raman scattering. As here the competing spontaneous processes would require spontaneous absorption of an optical photon, rather than emission, the noise is greatly suppressed. Furthermore, high external efficiency can be achieved as the control is well separated in frequency from the signal, and could be filtered out using highly efficient interference filters. We run the protocol in rubidium vapour, both on and off single-photon resonance, demonstrating a ratio of 50 between storage time and signal pulse width, an external total efficiency of over 25%, and only 2.3 × 10−4noise photons per extracted signal photon. This paves the way towards the efficient synchronization of probabilistic gates and sources at room temperature, and the controlled production of large quantum states of light.
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.