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.
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.
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.
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.
Abstract Coherent optical memories will likely play an important role in future quantum communication networks. Among the different platforms, memories based on ladder-type orbital transitions in atomic gasses offer high bandwidth (>100 MHz), continuous (on-demand) readout, and low-noise operation. Here we report on an upgraded setup of our previously-reported fast ladder memory, with improved efficiency and lifetime, and reduced noise. The upgrade employs a stronger control field, wider signal beam, reduced atomic density, higher optical depth, annular optical-pumping beam, and weak dressing of an auxiliary orbital to counteract residual Doppler-broadening. For a 2 ns-long pulse, we demonstrate 53% internal efficiency, 35% end-to-end efficiency, 3 × 10−5 noise photons per pulse, and a 1/e lifetime of 108 ns. This combination of performances is a record for continuous-readout memories.
Quantum memories are a key technology for distributing entanglement in large-scale quantum networks. They allow for coherent control and temporal reordering of single photons which is a fundamental prerequisite for quantum repeaters, local synchronization and buffering of quantum information. These applications require bright and pure sources of indistinguishable single photons, as well as long-lived, efficient, and high-bandwidth quantum memories with minimal noise. The demonstration of robust, scalable, multi-mode photonic interfaces between different types of quantum memories and single-photon sources remains an open experimental challenge. This work contributes to this endeavor by investigating a heterogeneous interface between a semiconductor quantum dot single-photon source and a room-temperature atomic vapor memory. The first part of this work presents the first room-temperature, ladder-type atomic vapor memory that operates on the Cs D1 line at 895\,nm. The performance of the memory is characterized using attenuated laser pulses at the single-photon level. The memory achieves an internal efficiency of ηint=15(1)% at a storage time of τs=17.4(2)\,ns. We derive a quantitative benchmark for the memory compatibility with quantum dot single-photon sources. The memory's high signal-to-noise ratio of 830(80), and its spectral acceptance window of W=560\,MHz promise compatibility with single photons from state-of-the-art InGaAs quantum dot devices. Furthermore, we present such a nano-integrated quantum dot device and characterize its optical properties. We measure a radiative lifetime of 1.3(1)\,ns and an inhomogeneously broadened linewidth of 5.1(1)\,GHz. The temperature tuning of the emission wavelength to the Cs D1 line allows coupling of the quantum dot to the memory in a joint experiment. The storage of single photons from a quantum dot source in a room-temperature atomic memory is demonstrated with on-demand retrieval. The variable storage time of up to τs=19.8\,ns and the memory's fractional delay of Df=14(1) present a significant advancement in the research field. We compare our results with other types of interfaces between single-photon sources and quantum memories and discuss the performance in the context of quantum network applications. This work marks an important step towards local buffering and synchronization of quantum information in heterogeneous quantum network nodes.