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Tracking a randomly varying optical phase is a key task in metrology, with applications in optical communication. The best precision for optical-phase tracking has until now been limited by the quantum vacuum fluctuations of coherent light. Here, we surpass this coherent-state limit by using a continuous-wave beam in a phase-squeezed quantum state. Unlike in previous squeezing-enhanced metrology, restricted to phases with very small variation, the best tracking precision (for a fixed light intensity) is achieved for a finite degree of squeezing because of Heisenberg's uncertainty principle. By optimizing the squeezing, we track the phase with a mean square error 15 ± 4% below the coherent-state limit.

The properties of quantum states have led to the development of new technologies, ranging from quantum information to quantum metrology 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 . A recent field of research to emerge is quantum imaging, which aims to overcome the limits of classical imaging by making use of the spatial properties of quantum states of light 13 , 14 , 15 , 16 , 17 , 18 . In particular, quantum correlations between twin beams represent a fundamental resource for these studies 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 . One of the most interesting proposed schemes takes advantage of the spatial quantum correlations between parametric down-conversion light beams to realize sub-shot-noise imaging of weak absorbing objects 14 , leading ideally to noise-free imaging. Here, we present the first experimental realization of this scheme, showing its potential to achieve a larger signal-to-noise ratio than classical imaging methods. This work represents the starting point for this quantum technology, which we anticipate will have applications when there is a requirement for low-photon-flux illumination (for example for use with biological samples). Sub-shot-noise imaging using spatial quantum correlations between parametric down-conversion light beams is demonstrated. The scheme exhibits a larger signal-to-noise ratio than is possible through classical imaging methods.

Use of nanomechanical resonators has the potential to offer microwave amplification with the minimum possible added noise, namely that due to quantum fluctuations. Noise-free amplification of weak radio signals In order to compensate for energy losses, the radio signals used in telecommunications and detection technologies require occasional electrical amplification. For specific applications, sensitive amplifiers have been demonstrated that operate near the quantum limit — where the only noise added is due to fundamental quantum fluctuations. This paper describes a new concept for amplifying weak electrical signals close to this fundamental limit, using a nanomechanical resonator. The system uses a resonator irradiated with microwave light of a frequency tuned so that it sets the resonator in motion with tiny vibrations; these amplify the signal. In this proof-of-principle study, signal amplification of 25 decibels is demonstrated, with only 20 fundamental noise quanta added. This mechanical amplification approach has the attraction that it is conceptually simple and could feasibly be used in integrated electrical circuits. The sensitive measurement of electrical signals is at the heart of modern technology. According to the principles of quantum mechanics, any detector or amplifier necessarily adds a certain amount of noise to the signal, equal to at least the noise added by quantum fluctuations 1 , 2 . This quantum limit of added noise has nearly been reached in superconducting devices that take advantage of nonlinearities in Josephson junctions 3 , 4 . Here we introduce the concept of the amplification of microwave signals using mechanical oscillation, which seems likely to enable quantum-limited operation. We drive a nanomechanical resonator with a radiation pressure force 5 , 6 , 7 , and provide an experimental demonstration and an analytical description of how a signal input to a microwave cavity induces coherent stimulated emission and, consequently, signal amplification. This generic scheme, which is based on two linear oscillators, has the advantage of being conceptually and practically simpler than the Josephson junction devices. In our device, we achieve signal amplification of 25 decibels with the addition of 20 quanta of noise, which is consistent with the expected amount of added noise. The generality of the model allows for realization in other physical systems as well, and we anticipate that near-quantum-limited mechanical microwave amplification will soon be feasible in various applications involving integrated electrical circuits.

Quantum electronics: noise abatement The processing of the single-quantum-level signals produced by current nanoscale solid-state devices such as qubits and nanomechanical resonators would require the development of very sensitive active circuits, such as amplifiers or frequency up- and down-converters that could attain the ultimate performances limited by the laws of quantum mechanics, while remaining of practical use. Bergeal et al . now demonstrate a phase-preserving, superconducting parametric amplifier with ultra-low noise properties, following theoretical principles recently presented in Nature Physics ( http://go.nature.com/F7lwR2 ). Based on a Josephson ring modulator, the new device can operate within a factor of three of the quantum limit. Possible applications include quantum analog signal processing such as the production of entangled microwave signal pairs. Recent progress in solid-state quantum information processing has stimulated the search for amplifiers and frequency converters with quantum-limited performance in the microwave range. Here, a phase-preserving, superconducting parametric amplifier with ultra-low-noise properties has been experimentally realized. Recent progress in solid-state quantum information processing 1 has stimulated the search for amplifiers and frequency converters with quantum-limited performance in the microwave range. Depending on the gain applied to the quadratures of a single spatial and temporal mode of the electromagnetic field, linear amplifiers can be classified into two categories (phase sensitive and phase preserving) with fundamentally different noise properties 2 . Phase-sensitive amplifiers use squeezing to reduce the quantum noise, but are useful only in cases in which a reference phase is attached to the signal, such as in homodyne detection. A phase-preserving amplifier would be preferable in many applications, but such devices have not been available until now. Here we experimentally realize a proposal 3 for an intrinsically phase-preserving, superconducting parametric amplifier of non-degenerate type. It is based on a Josephson ring modulator, which consists of four Josephson junctions in a Wheatstone bridge configuration. The device symmetry greatly enhances the purity of the amplification process and simplifies both its operation and its analysis. The measured characteristics of the amplifier in terms of gain and bandwidth are in good agreement with analytical predictions. Using a newly developed noise source, we show that the upper bound on the total system noise of our device under real operating conditions is three times the quantum limit. We foresee applications in the area of quantum analog signal processing, such as quantum non-demolition single-shot readout of qubits 4 , quantum feedback 5 and the production of entangled microwave signal pairs 6 .

The quantum light–matter interaction between a superconducting artificial atom and squeezed vacuum reduces the transverse radiative decay rate of the atom by a factor of two, allowing the corresponding coherence time, T 2 , to exceed the ordinary vacuum decay limit, 2 T 1 . Squeezing the decay of an artificial atom The radiative decay time of an atom and other physical effects are set by vacuum fluctuations of the electromagnetic environment, and these quantum fluctuations also set fundamental limits on the sensitivity of measurements of these phenomena. Entanglement between photons can produce correlations that result in a reduction of these fluctuations below the vacuum level, or 'squeezed states'. The authors report a twofold reduction of the transverse radiative decay rate of a superconducting artificial atom coupled to a continuum squeezed vacuum. These results confirm a prediction of quantum optics and should enable new studies of the quantum light–matter interaction. Quantum fluctuations of the electromagnetic vacuum are responsible for physical effects such as the Casimir force and the radiative decay of atoms, and set fundamental limits on the sensitivity of measurements. Entanglement between photons can produce correlations that result in a reduction of these fluctuations below the ordinary vacuum level, allowing measurements that surpass the standard quantum limit in sensitivity 1 , 2 , 3 , 4 , 5 . The effects of such ‘squeezed states’ of light on matter were first considered in a prediction 6 of the radiative decay rates of atoms in squeezed vacuum. Despite efforts to demonstrate such effects in experiments with natural atoms 7 , 8 , 9 , a direct quantitative observation of this prediction has remained elusive. Here we report a twofold reduction of the transverse radiative decay rate of a superconducting artificial atom coupled to continuum squeezed vacuum. The artificial atom is effectively a two-level system formed by the strong interaction between a superconducting circuit and a microwave-frequency cavity. A Josephson parametric amplifier is used to generate quadrature-squeezed electromagnetic vacuum. The observed twofold reduction in the decay rate of the atom allows the transverse coherence time, T 2 , to exceed the ordinary vacuum decay limit, 2 T 1 . We demonstrate that the measured radiative decay dynamics can be used to reconstruct the Wigner distribution of the itinerant squeezed state. Our results confirm a canonical prediction 6 of quantum optics and should enable new studies of the quantum light–matter interaction.

How physics became cool It is ten years since the exotic form of matter known as a Bose–Einstein condensate was first created. It was the birth of ultra-low-temperature physics, and practitioners gathered last month in Banff, Canada, to celebrate and discuss the latest news, as Karen Fox reports. And this week a new development that could have a major impact in the field is announced. In the 1950s, Hanbury Brown and Twiss showed that it is possible to measure angular sizes of astronomical radio sources from correlations of signal intensities in independent detectors. ‘HBT interferometry’ later became a key technique in quantum optics, and now it has been harnessed to identify a quantum phase of ultracold bosonic atoms. In a pioneering experiment 1 , Hanbury Brown and Twiss (HBT) demonstrated that noise correlations could be used to probe the properties of a (bosonic) particle source through quantum statistics; the effect relies on quantum interference between possible detection paths for two indistinguishable particles. HBT correlations—together with their fermionic counterparts 2 , 3 , 4 —find numerous applications, ranging from quantum optics 5 to nuclear and elementary particle physics 6 . Spatial HBT interferometry has been suggested 7 as a means to probe hidden order in strongly correlated phases of ultracold atoms. Here we report such a measurement on the Mott insulator 8 , 9 , 10 phase of a rubidium Bose gas as it is released from an optical lattice trap. We show that strong periodic quantum correlations exist between density fluctuations in the expanding atom cloud. These spatial correlations reflect the underlying ordering in the lattice, and find a natural interpretation in terms of a multiple-wave HBT interference effect. The method should provide a useful tool for identifying complex quantum phases of ultracold bosonic and fermionic atoms 11 , 12 , 13 , 14 , 15 .

We investigate plasma oscillations in long electromagnetically coupled superconducting nanowires. We demonstrate that in the presence of inter-wire coupling plasma modes in each of the wires get split into two “new” modes propagating with different velocities across the system. These plasma modes form an effective dissipative quantum environment interacting with electrons inside both wires and causing a number of significant implications for the low-temperature behavior of the systems under consideration.

The Casimir effect between type-II superconducting plates in the coexisting phase of a superconducting phase and a normal phase is investigated. The dependence of the optical conductivity of the superconducting plates on the external magnetic field is described in terms of the penetration depth of the incident electromagnetic field, and the permittivity along the imaginary axis is represented by a linear combination of the permittivities for the plasma model and Drude models. The characteristic frequency in each model is determined using the force parameters for the motion of the magnetic field vortices. The Casimir force between parallel YBCO plates in the mixed state is calculated, and the dependence on the applied magnetic field and temperature is considered.

This work present a comprehensive discussion of the noise properties of mode-locked lasers, with an emphasis on the effect of quantum noise in passively mode-locked solid-state lasers. Of special interest is the timing jitter, which is coupled to noise in various other pulse parameters. The study is based on analytical results and on numerical tools as described in part one of this study. It results in useful guidelines for the comparison and optimization of different kinds of lasers concerning timing jitter.

Techniques that use quantum interference effects are being actively investigated to manipulate the optical properties of quantum systems 1 . One such example is electromagnetically induced transparency, a quantum effect that permits the propagation of light pulses through an otherwise opaque medium 2 , 3 , 4 , 5 . Here we report an experimental demonstration of electromagnetically induced transparency in an ultracold gas of sodium atoms, in which the optical pulses propagate at twenty million times slower than the speed of light in a vacuum. The gas is cooled to nanokelvin temperatures by laser and evaporative cooling 6 , 7 , 8 , 9 , 10 . The quantum interference controlling the optical properties of the medium is set up by a ‘coupling’ laser beam propagating at a right angle to the pulsed ‘probe’ beam. At nanokelvin temperatures, the variation of refractive index with probe frequency can be made very steep. In conjunction with the high atomic density, this results in the exceptionally low light speeds observed. By cooling the cloud below the transition temperature for Bose–Einstein condensation 11 , 12 , 13 (causing a macroscopic population of alkali atoms in the quantum ground state of the confining potential), we observe even lower pulse propagation velocities (17?m?s −1 ) owing to the increased atom density. We report an inferred nonlinear refractive index of 0.18?cm 2 ?W −1 and find that the system shows exceptionally large optical nonlinearities, which are of potential fundamental and technological interest for quantum optics.