The distinctive non-classical features of quantum physics were first discussed in the seminal paper 1 by A. Einstein, B. Podolsky and N. Rosen (EPR) in 1935. In his immediate response 2 , E. Schrödinger introduced the notion of entanglement, now seen as the essential resource in quantum information 3-5 as well as in quantum metrology [6][7][8] . Furthermore, he showed that at the core of the EPR argument is a phenomenon that he called steering. In contrast to entanglement and violations of Bell's inequalities, steering implies a direction between the parties involved. Recent theoretical works have precisely defined this property, but the question arose as to whether there are bipartite states showing steering only in one direction 9,10 . Here, we present an experimental realization of two entangled Gaussian modes of light that in fact shows the steering effect in one direction but not in the other. The generated one-way steering gives a new insight into quantum physics and may open a new field of applications in quantum information.The steering effect can be described by considering two remote observers, Alice and Bob, who share a bipartite quantum state. Their local systems are in a mixed state and therefore permit a decomposition into pure states. Schrödinger found that within quantum mechanics certain states do not allow such a decomposition locally. Depending on the observable Alice chooses to measure, Bob's local state is decomposed into incompatible mixtures of conditional states. So, if pure states were a local complete description of Bob's system, this would require some interaction from Alice to Bob. This is what Schrödinger named steering and Einstein later called the 'spooky action at a distance'. The first experimental demonstration of this effect was achieved by Ou et al.11 , and was followed by a great number of experiments [12][13][14][15] .Steering is strictly stronger than entanglement and strictly weaker than the violation of a Bell inequality; that is, steering does not imply the violation of any Bell inequality, while the violation of at least one Bell inequality immediately implies steering in both directions 16 , as shown in Fig. 1. In contrast to entanglement and Bell tests, Alice and Bob have certain roles in the steering scenario that are not interchangeable. This intrinsic asymmetry raises the question 9 of whether there are physical states certifying steering only in one direction for arbitrary observables. This one-way steering would lead to the peculiar situation that two experimenters measuring the same observables on their subsystems would describe the same shared state in qualitatively different ways. Whereas, in general, this question cannot as yet be answered, in the Gaussian regime (that is, for Gaussian state preparation and Gaussian measurements) the answer is yes. In a pioneering paper by H.-A. Bachor and co-workers, two-way steering with an asymmetry in the steering strengths was observed 17 . Their theoretical analysis proposes a possible extension of their set-up with a v...
Only a few years ago, it was realized that the zero-area Sagnac interferometer topology is able to perform quantum nondemolition measurements of position changes of a mechanical oscillator. Here, we experimentally show that such an interferometer can also be efficiently enhanced by squeezed light. We achieved a nonclassical sensitivity improvement of up to 8.2 dB, limited by optical loss inside our interferometer. Measurements performed directly on our squeezed-light laser output revealed squeezing of 12.7 dB. We show that the sensitivity of a squeezed-light enhanced Sagnac interferometer can surpass the standard quantum limit for a broad spectrum of signal frequencies without the need for filter cavities as required for Michelson interferometers. The Sagnac topology is therefore a powerful option for future gravitational-wave detectors, such as the Einstein Telescope, whose design is currently being studied.All currently operating interferometric gravitational- GEO [3] and TAMA [4]) are Michelson interferometers. Their purpose is to measure the position changes (displacements) of quasi free-falling mirrors thereby revealing changes of space-time curvature, i.e. gravitational waves. The current detectors are aiming for the first direct observation of gravitational waves. Future detectors will aim for establishing gravitational wave astronomy which will require a considerable increase of the detectors' displacement sensitivity. The Einstein Telescope (ET) [5] is an on-going European design study project for such a gravitational wave detector. An important issue for future detectors is a reduction of the quantum measurement noise (photon shotnoise) and quantum back-action noise (quantum fluctuations in the radiation pressure acting on the mirrors) for a given laser power. This way, the standard quantum limit (SQL) can eventually be surpassed allowing for quantum-non-demolition (QND) displacement measurements. Future detectors will most likely be operated at cryogenic temperatures in order to reduce thermally excited motions of the mirror surfaces and optical absorption will set an upper limit to the laser power inside the interferometer. For Michelson interferometers a nonclassical reduction (squeezing) of the quantum measurement noise can be achieved by injecting squeezed light [6][7][8][9]. Squeezing of back-action noise is also possible but turned out to be experimentally challenging because in Michelson interferometers the back-action noise depends on Fourier frequency. Long-baseline narrow-band filter cavities are required to compensate for the frequency dependence [10] which complicates the topology and introduces additional optical loss.
Secret communication over public channels is one of the central pillars of a modern information society. Using quantum key distribution this is achieved without relying on the hardness of mathematical problems, which might be compromised by improved algorithms or by future quantum computers. State-of-the-art quantum key distribution requires composable security against coherent attacks for a finite number of distributed quantum states as well as robustness against implementation side channels. Here we present an implementation of continuous-variable quantum key distribution satisfying these requirements. Our implementation is based on the distribution of continuous-variable Einstein–Podolsky–Rosen entangled light. It is one-sided device independent, which means the security of the generated key is independent of any memoryfree attacks on the remote detector. Since continuous-variable encoding is compatible with conventional optical communication technology, our work is a step towards practical implementations of quantum key distribution with state-of-the-art security based solely on telecom components.
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