Quantum non-demolition measurements are designed to circumvent the limitations imposed by Heisenberg's uncertainty principle when performing repeated measurements of quantum states. Recent progress in quantum optics has enabled the experimental realization of quantum non-demolition measurements of the photon¯ux of a light beam. This achievement bears on fundamental issues about the ultimate sensitivity of measurements, and may open the way for applications such as noise-free information tapping in optical telecommunications.It is written at the very heart of quantum mechanics that a precise measurement in the microscopic world is not possible without the introduction of a perturbation or`back action' inherent to the very fact of measurement. This principle, which has been known since the 1930s 1 , can be directly related to Heisenberg's well known uncertainty relations. Using the fact that the quantum formalism describes physical quantities as non-commuting operators (that is, as mathematical objects A, B such that AB Þ BA), the Heisenberg inequalities state that the product of the dispersions of (the`uncertainty' in) A and B has a lower bound: DADB > 1 2 jhAB 2 BAij. Therefore, for non-commuting operators, a very precise measurement of A, resulting in a very small dispersion DA, will be associated with a large value of DB. Although this does not restrict directly the precision in the measurement of A itself, the large¯uctuations induced in B may eventually couple back to A, which will then also be perturbed. This`measurement back action' has far-reaching consequences from a practical point of view, because it may prevent the retrieval of the initial result in a series of repeated measurements. In response to this problem, Braginsky, Thorne, Unruh, Caves and others introduced in the 1970s the concept of``quantum non-demolition'' (QND) measurement 2±8 , in which a measurement strategy is chosen that evades the undesirable effect of back action. The key issue is to devise measurement schemes in which the back-action noise is kept entirely within unwanted observables, without being coupled back onto the quantity of interest. This quantity then remains uncontaminated by the measurement process, allowing repeated measurements to be performed with arbitrary high accuracy.Originally, QND ideas dealt with mechanical oscillators designed for detecting gravitational waves 9,10 , so-called Weber bars 11,12 . But these devices appeared to be limited by technical dif®culties, and QND moved on to other ®elds. In the mid-1980s, the emerging ®eld of quantum optics 13±15 seemed to be particularly well suited for implementing QND measurements 16±18 . The main reason for this was that the technical quality of optical sources and detectors was good enough for sensitivity at the quantum noise level to be achieved. Moreover, we show below that to manipulate the quantum noise of a light beam requires techniques of nonlinear optics that have also developed rapidly in recent years. The ®rst achievements of experimental quantum optics wer...
We experimentally demonstrate that a type-II pulsed optical parametric amplifier operated in a phaseinsensitive configuration works as a near-perfect classical optical amplifier whose noise figure approaches 3 dB at high gains. We further demonstrate that, when operated in a phase-sensitive configuration, this amplifier works as a quantum-optical amplifier whose noise figure goes below 3 dB and approaches 0 dB at high gains. The noise figure of 1.45 + 0.2 dB, measured for a gain of 9 dB, is clearly in the quantum regime.
We demonstrate significant enhancement of second-order nonlinear interactions in a one-dimensional semiconductor Bragg mirror operating as a photonic band gap structure. The enhancement comes from a simultaneous availability of a high density of states, thanks to high field localization, and the improvement of effective coherent length near the photonic band edge.
We show that Coherent Population Oscillations effect allows to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an Erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits to achieve a group velocity as low as 2.7 m/s with a transmission of 40 %. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40 % to 90 %, and the light group velocity from 2.7 m/s to 100 m/s. [5,6,7,8]. In addition to the strangeness of producing light propagating at speeds as low as few m/s, Slow Light Propagation (SLP) is at the very heart of new fundamental and applied fields of research in nonlinear and quantum optics. From the nonlinear optical side, SLP allows to strongly enhance the lightmatter interaction time. Moreover, this interaction time can be continuously tuned to produce optical buffers and variable delay lines for optical networks. From the quantum optical point of view, SLP should allow, under specific conditions, classical and quantum properties of an electromagnetic field to be mapped into an atomic system [10]. The fundamental physical idea at the origin of SLP is the creation of a very narrow spectral hole in the homogeneous absorption profile. As stipulated by Kramers-Krönig relations, this narrow spectral hole is accompanied by a strong dispersion of the index of refraction inducing a low group velocity and an increase of the transmission. These two aspects are crucial in the choice of the atomic system and the coherent interaction inducing SLP.The first direct demonstration of SLP [1,2,3,4,5] was achieved via Electromagnetically Induced Transparency (EIT) [9]. It was originally implemented by applying a secondary control field to eliminate the linear absorption of a resonant probe field through an otherwise absorbing medium. The standard scheme for EIT is a three-level Λ system, where the probe field drives the system from one of the ground states and the control field from the second ground state.
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