Abstract. The advanced interferometer network will herald a new era in observational astronomy. There is a very strong science case to go beyond the advanced detector network and build detectors that operate in a frequency range from 1 Hz-10 kHz, with sensitivity a factor ten better in amplitude. Such detectors will be able to probe a range of topics in nuclear physics, astronomy, cosmology and fundamental physics, providing insights into many unsolved problems in these areas.PACS numbers: 95.36.+x, 97.60.Lf, 98.62.Py, 04.80.Nn, 95.55.Ym, 97.60.Bw, 97.60.Jd
Around the globe several observatories are seeking the first direct detection of gravitational waves (GWs). These waves are predicted by Einstein's general theory of relativity and are generated, for example, by black-hole binary systems. Present GW detectors are Michelson-type kilometre-scale laser interferometers measuring the distance changes between mirrors suspended in vacuum. The sensitivity of these detectors at frequencies above several hundred hertz is limited by the vacuum (zero-point) fluctuations of the electromagnetic field. A quantum technology--the injection of squeezed light--offers a solution to this problem. Here we demonstrate the squeezed-light enhancement of GEO600, which will be the GW observatory operated by the LIGO Scientific Collaboration in its search for GWs for the next 3-4 years. GEO600 now operates with its best ever sensitivity, which proves the usefulness of quantum entanglement and the qualification of squeezed light as a key technology for future GW astronomy
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...
The distribution of entangled states of light over long distances is a major challenge in the field of quantum information. Optical losses, phase diffusion and mixing with thermal states lead to decoherence and destroy the non-classical states after some finite transmission-line length. Quantum repeater protocols 1,2 , which combine quantum memory 3 , entanglement distillation 4,5 and entanglement swapping 6 , were proposed to overcome this problem. Here we report on the experimental demonstration of entanglement distillation in the continuous-variable regime 7-9 . Entangled states were first disturbed by random phase fluctuations and then distilled and purified using interference on beam splitters and homodyne detection. Measurements of covariance matrices clearly indicate a regained strength of entanglement and purity of the distilled states. In contrast to previous demonstrations of entanglement distillation in the complementary discrete-variable regime 10,11 , our scheme achieved the actual preparation of the distilled states, which might therefore be used to improve the quality of downstream applications such as quantum teleportation 12 .Quantum information makes use of the special properties of quantum states to improve the quality of communication and information processing tasks. Generally, a quantized field can be described by the number operator or alternatively by two non-commuting position and momentum-like operators. The corresponding measurement results have either discrete or continuous spectra and form the basis of discrete-variable or continuous-variable quantum information, respectively. In the regime of continuous variables, entangled states of light can be deterministically generated in optical parametric amplifiers (OPAs), precisely manipulated with linear optics and measured with very high efficiency in balanced homodyne detectors. These entangled two-mode squeezed states show Gaussian probability distributions and were used for quantum teleportation 12 and entanglement swapping 13,14 . Entangled states of the collective spins of two atomic ensembles analogous to two-mode squeezed states have been generated 15 , storage of quantum states of light in an atomic memory has been demonstrated 3 and teleportation from light onto an atomic ensemble has been reported 16 . High-speed quantum cryptography with coherent light beams and homodyne detection has been demonstrated 17 . All these spectacular achievements reveal the great potential of this approach to quantum information processing.A missing piece in this toolbox has been a feasible protocol for entanglement distillation and purification. Entanglement distillation 4,5 extracts from several shared copies of weakly entangled mixed states a single copy of a highly entangled state using only local quantum operations and classical communication between the two parties sharing the states. This turned out to be a very challenging task for continuous-variable states, because it was proved that it is impossible to distil Gaussian entangled states by ...
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