Broadband quantum memories hold great promise as multiplexing elements in future photonic quantum information protocols. Alkali vapour Raman memories combine high-bandwidth storage, on-demand read-out, and operation at room temperature without collisional fluorescence noise. However, previous implementations have required large control pulse energies and suffered from fourwave mixing noise. Here we present a Raman memory where the storage interaction is enhanced by a low-finesse birefringent cavity tuned into simultaneous resonance with the signal and control fields, dramatically reducing the energy required to drive the memory. By engineering anti-resonance for the anti-Stokes field, we also suppress the four-wave mixing noise and report the lowest unconditional noise floor yet achieved in a Raman-type warm vapour memory, (15 ± 2) × 10 −3 photons per pulse, with a total efficiency of (9.5 ± 0.5)%. Quantum information technologies such as quantum key distribution and random number generators are beginning to transition into the commercial sphere, where key requirements are the ability to function at high speed, and in non-laboratory settings. The high carrier frequency of optical signals enables photonic quantum devices to operate noise-free at room temperature whilst offering GHz-THz operational bandwidths. However, direct photon-photon interactions are prohibitively weak, which has held back the development of photonic quantum processors. One solution to this problem has been to use probabilistic measurement-induced non-linearities [1], but the probability of success decreases exponentially with system size, limiting the state of the art to < 10 photons [2]. Further scaling photonic devices requires a multiplexing strategy. Quantum memories capable of storing photons and releasing them on-demand provide the ability to temporally multiplex a repeat-until-success architecture to achieve a freely scalable photonic quantum information platform operable at room temperature.There are many types of quantum memory for light under development: electromagnetically induced transparency [3], the full atomic-frequency comb protocol [4,5], gradient-echo memories [6,7], and the far-offresonant Raman memory [8,9]. Each of these protocols have advantages and challenges, see Ref.[10] for a recent review. A helpful metric for temporal multiplexing is the time-bandwidth product B = τ δ, with δ the acceptance bandwidth and τ the memory lifetime. B is the maximum number of time-bins over which a memory can synchronise an input signal. Time-bandwidth products of B > 1000 enable a dramatic enhancement in the multiphoton rate from parametric photon sources [11], as required to utilise multiphoton interference for computational speed-up (e.g. boson sampling [12]). Large timebandwidth products have been achieved with cold atom memories [13,14], cryogenic rare earth ion memories [15] and room-temperature Raman memories [8,16]. In this paper we present an implementation of an alkali-vapour Raman memory, operating at 75• C, and show the lo...

Optical quantum memories are devices that store and recall quantum light and are vital to the realisation 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 vapour with no added noise; confirmed by the unaltered photon number statistics upon recall. Our ORCA memory meets the stringent noise-requirements for quantum memories whilst combining high-speed and room-temperature operation with technical simplicity, and therefore is immediately applicable to low-latency quantum networks.

Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on-demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of Λ-type atoms. However at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.

It is proposed that the ground-state manifold of the neutral nitrogen-vacancy center in diamond could be used as a quantum two-level system in a solid-state-based implementation of a broadband, noise-free quantum optical memory. The proposal is based on the same-spin Λ-type three-level system created between the two E orbital ground states and the A1 orbital excited state of the center, and the cross-linear polarization selection rules obtained with the application of transverse electric field or uniaxial stress. Possible decay and decoherence mechanisms of this system are discussed, and it is shown that high-efficiency, noise-free storage of photons as short as a few tens of picoseconds for at least a few nanoseconds could be possible at low temperature.

Quantum memories are essential for large-scale quantum information networks. Along with high efficiency, storage lifetime and optical bandwidth, it is critical that the memory add negligible noise to the recalled signal. A common source of noise in optical quantum memories is spontaneous four-wave mixing. We develop and implement a technically simple scheme to suppress this noise mechanism by means of quantum interference. Using this scheme with a Raman memory in warm atomic vapour we demonstrate over an order of magnitude improvement in noise performance. Furthermore we demonstrate a method to quantify the remaining noise contributions and present a route to enable further noise suppression. Our scheme opens the way to quantum demonstrations using a broadband memory, significantly advancing the search for scalable quantum photonic networks.

9 ] However, the ultra-weak nature of the dipole moment in TSCC referred to below may reduce the effects of such long-range dipole interactions to a narrow temperature region around the critical point. This would mean that more conventional classical or quantum critical behaviour would be expected over a wider range of temperatures and tuning parameters. TSCC is hydrogen bonded and was characterized in the past as order-disorder albeit with no direct supporting evidence. [ 16,17 ] Since the time of these studies TSCC has come to be a prototypical displacive ferroelectrics, [ 18 ] as demonstrated by the existence of an under-damped soft mode. This mode can be followed into the GHz frequency regime [ 19 ] from high-T values of ca 630 GHz = 21 cm −1 . Deuteration [ 20 ] produces little change in T C , implying that the transition is not controlled by the N-Cl-hydrogen bonds, a result compatible with the soft-mode mechanism operating in TSCC. [ 2 ] The Curie constant of pure TSCC is 27 K, the smallest known Curie constant of any known ferroelectric. By way of comparison, typical values for oxide perovskites such as BaTiO 3 are 50 000 K. This puts TSCC in the family of ultra-weak ferroelectrics. An unrelated form of weak ferroelectricity arises in certain multiferroics where small polarizations are induced by magneto-electric coupling. In pure TSCC, the Curie constants near T C = 130 K, taken as the inverse slopes of the inverse dielectric constants versus temperature above and below T C , differ by a factor of 2.0. This ratio is that predicted by the simplest mean-fi eld Landau theory of phase transitions, but in fact exists in almost no other known ferroelectric. The ratio between the slopes in other ferroelectrics is typically greater than two due to the coupling between lattice strain and polarization. [ 21 ] Under modest pressures pure TSCC becomes what has been termed antiferroelectric, [ 22 ] although no supporting X-ray or dielectric evidence has been given for that label. Four additional enigmatic anomalies have been detected at 42 K, 64 K, 185 K, and another at 283 K [ 3 ] at ambient pressures. The supposed phase transition found near 283 K was described as a modulated quasi-hexagonal phase. [ 23 ] Another possible phase transition was found near 185 K as observed in the relaxation time in proton nuclear magnetic resonance (NMR) spectroscopy. [ 24 ] An additional phase transition was found using specifi c heat at 64 K [ 25 ] and verifi ed by dielectric loss and ultrasonic spectroscopy measurements. [ 26 ] A fourth transition was found near 42 K in dielectric measurements, [ 27 ] and was later confi rmed by resonant ultrasonic spectroscopy (RUS) [ 19 ] and specifi c-heat measurements. At present these anomalies involve unknown changes in the structure.Some studies have reported the phase transition at 64 K (by either dielectric or specifi c-heat data), and some studies have Quantum phase transitions occur in the halide salts of trissarcosine. [ 1 ] Substitution of bromine or iodine for chlorine in (CH 3 NHCH...

Raman interactions in alkali vapours are used in applications such as atomic clocks, optical signal processing, generation of squeezed light and Raman quantum memories for temporal multiplexing. To achieve a strong interaction the alkali ensemble needs both a large optical depth and a high level of spin-polarisation. We implement a technique known as quenching using a molecular buffer gas which allows near-perfect spin-polarisation of over 99.5% in caesium vapour at high optical depths of up to 2 10 ; 5´a factor of 4 higher than can be achieved without quenching. We use this system to explore efficient light storage with high gain in a GHz bandwidth Raman memory. OPEN ACCESS RECEIVED

Nanostructures can be used for boosting the light outcoupling of color centers in diamond; however, the fiber coupling performance of these nanostructures is rarely investigated. Here, we use a finite element method for computing the emission from color centers in inverted nanocones and the overlap of this emission with the propagation mode in a single-mode fiber. Using different figures of merit, the inverted nanocone parameters are optimized to obtain maximal fiber coupling efficiency, free-space collection efficiency, or rate enhancement. The optimized inverted nanocone designs show promising results with 66% fiber coupling or 83% free-space coupling efficiency at the tin-vacancy center zero-phonon line wavelength of 619 nm. Moreover, when evaluated for broadband performance, the optimized designs show 55% and 76% for fiber coupling and free-space efficiencies, respectively, for collecting the full tin-vacancy emission spectrum at room temperature. An analysis of fabrication insensitivity indicates that these nanostructures are robust against imperfections. For maximum emission rate into a fiber mode, a design with a Purcell factor of 2.34 is identified. Finally, possible improvements offered by a hybrid inverted nanocone, formed by patterning into two different materials, are investigated and increase the achievable fiber coupling efficiency to 71%.