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.

We introduce a filter using a noise-free quantum buffer with large optical bandwidth that can both filter temporal-spectral modes, as well as inter-convert them and change their frequency. We show that such quantum buffers optimally filter out temporal-spectral noise; producing identical single-photons from many distinguishable noisy single-photon sources with the minimum required reduction in brightness. We then experimentally demonstrate a noise-free quantum buffer in a warm atomic system that is well matched to quantum dots and can outperform all intensity (incoherent) filtering schemes for increasing indistinguishability. * dylan.saunders@physics.ox.ac.uk, ian.walmsley@physics.ox.ac.uk dominant mode residual modes arXiv:1902.07720v1 [quant-ph]

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 nondeterministic 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.Impressive realisations of QMs have materialised based on optical transitions in atomic systems [3][4][5][6][7]. However, several issues remain that prevent these devices from being used within large-scale networks. These include: elaborate cold atom [3,4] or cryogenic [5] experimental setups, complex preparation of the atomic system [5], additional loss through filtering required for noise suppression, and noise photons being induced in the same frequency, spatial or temporal mode as the output thus reducing the quality of the QM readout [3][4][5][6][7].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 6S 1/2 ground state and 6D 5/2 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. Fig. 1 (a) QLAD protocol in Cs level structure. (b) Coincidence histogram of storage and retrieval of heralded single photons. The blue (yellow) histogram represents the memory off (on), labelled Input (Output). (c) Log scale with Output and Noise (dark red histogram).

We show that the entanglement between two distant magnetic impurities, generated via electron scattering, can be easily modulated by controlling the magnitude of an applied external electric field. We assume that the two magnetic impurities are fixed and located on an one-dimensional quantum wire. A ballistic electron moving through the wire is scattered off by both

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