We show that weak measurement can be used to "amplify" optical nonlinearities at the singlephoton level, such that the effect of one properly post-selected photon on a classical beam may be as large as that of many un-post-selected photons. We find that "weak-value amplification" offers a marked improvement in the signal-to-noise ratio in the presence of technical noise with long correlation times. Unlike previous weak-measurement experiments, our proposed scheme has no classical equivalent.An interaction between two independent photons could be used to serve as a "quantum logic gate," enabling the development of optical quantum computers [1][2][3], as well as opening up an essentially new field of quantum nonlinear optics [4]. Typical optical nonlinearities are many orders of magnitude too weak to create a π phase shift as required in initial proposals, but more recently it was realized that any phase shift large enough to be measured on a single shot could be leveraged into a quantum logic gate [5]. Much recent work has shown that atomic coherence effects [6][7][8][9] and nonlinearities in microstructured fiber [10,11] can generate greatly enhanced Kerr nonlinearities. While even a very small phase shift can be made larger than the quantum (shot) noise, by using a sufficiently intense probe, present experiments are limited by technical rather than quantum noise and difficult to carry out even with much averaging. For example, in Ref.[11], a phase shift of 10 −7 rad was measured by averaging over 3 × 10 9 classical pulses with singlephoton-level intensities. To date, no one has yet been able to observe the cross-Kerr effect induced by a single propagating photon on a second optical beam [12]. In this Letter, we show that using weak-value amplification (WVA) [13][14][15], a single photon can be made to "act like" many photons, and it is possible to amplify a cross-Kerr phase shift to an observable value, much larger than the intrinsic magnitude of the single-photon-level nonlinearity. In so doing, we also demonstrate quantitatively how WVA may improve the signal-to-noise ratio (SNR) in appropriate regimes, a result of broad general applicability to quantum metrology.Weak measurement is an exciting new approach to understanding quantum systems from a time-symmetric perspective, obtaining information from both their preparation and subsequent post-selection [16]. In the past several years, it has been widely studied to address foundational questions in quantum mechanics [17], as well as for its potential application to ultrasensitive measurements [14,15,18,19]. If a quantum system is coupled only weakly to a probe, then very little information may be obtained from a single measurement, and in compensation, this measurement disturbs the sys-

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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.

N00N states -maximally path-entangled states of N photons -exhibit spatial interference patterns sharper than any classical interference pattern. This is known as super-resolution. However, even with perfectly efficient number-resolving detectors, the detection efficiency of all previously demonstrated methods to measure such interference decreases exponentially with the number of photons in the N00N state, often leading to the conclusion that N00N states are unsuitable for spatial measurements. Here, we create spatial super-resolution fringes with two-, three-, and fourphoton N00N states, and demonstrate a scalable implementation of the so-called "optical centroid measurement" which provides an in-principle perfect detection efficiency. Moreover, we compare the N00N-state interference to the corresponding classical super-resolution interference. Although both provide the same increase in spatial frequency, the visibility of the classical fringes decreases exponentially with the number of detected photons, while the visibility of our experimentally measured N00N-state super-resolution fringes remains approximately constant with N. Our implementation of the optical centroid measurement is a scalable method to measure high photon-number quantum interference, an essential step forward for quantum-enhanced measurements, overcoming what was believed to be a fundamental challenge to quantum metrology.Many essential techniques in modern science and technology, from precise position sensing to high-resolution imaging to nanolithography, rely on the creation and detection of the finest possible spatial interference fringes using light. Classically, all such measurements face a fundamental barrier related to the "diffraction limit," which is determined by the wavelength of the light [1], but quantum entanglement can be used to surpass this limit by making the spatial interference fringes sharper (a result referred to as super-resolution) [2,3]. In particular, the N-photon entangled "N00N" state can display an interference pattern N times finer than that of classical light [4,5]. However, N00N states suffer from a weakness that has made their advantage controversial: the probability of all N photons arriving at the same place, and thus the detection efficiency, decreases exponentially with N [6,7]. Here we implement the optical centroid measurement (OCM) proposed by Tsang [8] to completely overcome this problem. A proof-of-principle experiment confirming the underlying concept of the OCM was recently performed [9], but, being limited to only two photons and two 'movable' detectors, it could not probe the scaling properties nor demonstrate the efficiency gain of the OCM. In our experiment, using an array of 11 fixed detectors, we measure two-, three-, and four-photon spatial fringes, and find that their visibility does not degrade with the number of entangled photons, clearly displaying the enhanced efficiency and scalability of the OCM. The visibility of an unentangled OCM, on the other hand, decays exponentially. In doing...

Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. / state. We find [46]Γ/ɣ = 1/285 and the ratio of the coupling constants is b = 0.72 . AbstractWe show that cold Rydberg gases enable an efficient six-wave mixing process where terahertz or microwave fields are coherently converted into optical fields and vice versa. This process is made possible by the long lifetime of Rydberg states, the strong coupling of millimeter waves to Rydberg transitions and by a quantum interference effect related to electromagnetically induced transparency. Our frequency conversion scheme applies to a broad spectrum of millimeter waves due to the abundance of transitions within the Rydberg manifold, and we discuss two possible implementations based on focussed terahertz beams and millimeter wave fields confined by a waveguide, respectively. We analyse a realistic example for the interconversion of terahertz and optical fields in rubidium atoms and find that the conversion efficiency can in principle exceed 90%.

We demonstrate a platform for implementing quantum walks that overcomes many of the barriers associated with photonic implementations. We use coupled fiber-optic cavities to implement time-bin encoded walks in an integrated system. We show that this platform can achieve very low losses combined with high-fidelity operation, enabling an unprecedented large number of steps in a passive system, as required for scenarios with multiple walkers. Furthermore the platform is reconfigurable, enabling variation of the coin, and readily extends to multidimensional lattices. We demonstrate variation of the coin bias experimentally for three different values. INTRODUCTIONQuantum walks are the quantum counterparts to random walks. A quantum walker moves by superpositions of possible paths, resulting in a probability amplitude for being observed at a particular position [1][2][3]. Optical multiport interferometers provide an attractive implementation for quantum walks, since optical fields naturally exhibit coherence between pathways and allow multi-walker scenarios using multiple single photons. However, this approach is susceptible to losses, which limit the achievable scale before being overtaken by noise, and which abrogate many of the advantages implied for applications in quantum information processing [4][5][6][7][8][9][10][11][12]. This challenge motivates the development of low-loss, modular, guided-wave interferometer networks for quantum walks.Quantum walks with multiple interacting walkers have been shown to realize universal quantum computation [13]. For the case of multiple non-interacting walkers, quantum walks are also thought to have quantum computational power, as shown by the boson sampling problem [14]. Quantum walks with a single walker can implement quantum computation, though this requires an exponentially large graph [15,16]. A key feature of quantum walks as information processors is that they do not require time-dependent feedforward control. Quantum walks, both discrete and continuous, have been realised using cold atoms [17,18], single optically trapped atoms [19], trapped ions [20][21][22], and photons. Optical systems have been used to implement quantum walks using bulk optics [23,24], photonic chips [25][26][27][28], fiber optics [29], and hybrid bulkfiber optic approaches [30][31][32].In most experimental implementations, the quantum walk takes place over spatial locations arranged in a lattice. The physical size of the lattice then determines the maximum size of the walk. This limitation can be avoided by use of optical cavities, in which case the number of physical elements required is independent of the size of the walk. This approach was first formulated for frequency-encoded quantum walks [33]. From an experimental perspective, a further key advance was the development of optical cavities that implement time-encoded walks [29,30]. In this case, the walker's location is represented by the time at which a pulse completes a round trip of a cavity [29,30]. In practice, the achievable scale of t...