We propose and experimentally realize a new scheme for universal phase-insensitive optical amplification. The presented scheme relies only on linear optics and homodyne detection, thus circumventing the need for nonlinear interaction between a pump field and the signal field. The amplifier demonstrates near optimal quantum noise limited performance for a wide range of amplification factors.PACS numbers: 42.65.Yj, 42.50.Lc, Optical amplification is inevitably affected by fundamental quantum noise no matter whether it is phase sensitive or phase insensitive as stressed by Louisell et al. [1] and by Haus and Mullen [2]. The ultimate limits imposed by quantum mechanics on amplifiers was later concisely formulated by Caves [3] in fundamental theorems. This intrinsic noise, intimately linked with measurement theory and the no-cloning theorem, gives rise to many inextricable restrictions on the manipulations of quantum states. For example, microscopic quantum objects cannot be perfectly transformed, through amplification, into macroscopic objects for detailed inspection [4]: for phase insensitive amplification nonclassical features of quantum states, such as squeezing or oscillations in phase space, will be gradually washed out, and the signal to noise ratio of an information carrying quantum state will be reduced during the course of amplification. Despite these limitations, the universal phase insensitive amplifier is however rich of applications, in particular in optical communication. Amplifiers operating at the quantum noise limit are of particular importance for quantum communication where information is encoded in fragile quantum states, thus extremely vulnerable to noise.Numerous apparatuses accomplish, in principle, ideal phase insensitive amplification as, for instance, solid state laser amplifiers [5], parametric downconverters [6] and schemes based on four wave mixing processes [7]. However, to date phase-insensitive amplification at the quantum limit has been only partially demonstrated [8,9]: a number of difficulties are indeed involved in practice, especially for low gain applications. These difficulties mainly lie in the fact that the amplified field has to be efficiently coupled, mediated by a non linearity, to a pump field.Following a recent trend in quantum information science, where non linear media are efficiently replaced by linear optics [10], we show in this Letter that universal phase insensitive amplification can also be achieved using only linear optics and homodyne detection. The simplicity and the robustness of this original scheme enable us to achieve near quantum noise limited amplification of coherent states, even in the low gain regime.Let us first briefly summarize the basic formalism describing a phase-insensitive amplifier [3]. Because of the symmetry of such an amplifier, it can be described by the following input-output transformation:â out = √ Gâ in +N whereâ in(out) represent the input (output) annihilation bosonic operators, G is the power gain and N the operator associated wi...

We investigate the optimal trade-off between information gained about an unknown coherent state and the state disturbance caused by the measurement process. We propose several optical schemes that can enable this task, and we implement one of them, a scheme that relies on only linear optics and homodyne detection. Experimentally we reach near optimal performance, limited only by detection inefficiencies. In addition, we show that such a scheme can be used to enhance the transmission fidelity of a class of noisy channels.

A fundamental requirement for enabling fault-tolerant quantum information processing is an efficient quantum error-correcting code (QECC) that robustly protects the involved fragile quantum states from their environment [1][2][3][4][5][6][7][8][9][10]. Just as classical errorcorrecting codes are indispensible in today's information technologies, it is believed that QECC will play a similarly crucial role in tomorrow's quantum information systems. Here, we report on the first experimental demonstration of a quantum erasurecorrecting code that overcomes the devastating effect of photon losses. Whereas errors translate, in an information theoretic language, the noise affecting a transmission line, erasures correspond to the in-line probabilistic loss of photons. Our quantum code protects a four-mode entangled mesoscopic state of light against erasures, and its associated encoding and decoding operations only require linear optics and Gaussian resources. Since in-line attenuation is generally the strongest limitation to quantum communication, much more than noise, such an erasure-correcting code provides a new tool for establishing quantum optical coherence over longer distances. We investigate two approaches for circumventing in-line losses using this code, and demonstrate that both approaches exhibit transmission fidelities beyond what is possible by classical means.Quantum information protocols are inevitably affected by noise, which in turn produces errors in the extremely sensitive processed quantum information [1]. Thus, in order to gain the full advantage of quantum information processing, including long-distance quantum communication and fault-tolerant quantum computing [11][12][13], these errors must be efficiently corrected. This can be done by encoding the information in special quantum error-correcting codes, which introduce redundancy and thereby protect the fragile quantum information from environment-induced decoherence. Using such codes, the transmission errors can be diagnosed through so-called syndrome measurements, the results of which are used to correct the corrupted quantum information.Quantum error correcting codes (QECC) were first discovered for discrete variable qubit systems [2][3][4][5][6][7] and later extended to systems where information is encoded into observables with a continuous spectrum [8-10, 14, 15]. Only a few experimental implementations demonstrating quantum error correction have been carried out to date, e.g., in nuclear magnetic resonance systems [16], in an ion-trap system [17], and in a pure optical system [18,19]. All these works have reported on the correction of errors, which are the manifestation of line noise. However, it is very often the loss of photons in a transmission line (corresponding to erasures in an information theoretic language) that is the main obstacle to the survival of quantum coherence.Erasure-correcting codes have long been known in classical coding theory, and their quantum counterparts have also been theoretically developed. The experimental progr...

We investigate the properties of an atmospheric channel for free space quantum communication with continuous polarization variables. In our prepare-and-measure setup, coherent polarization states are transmitted through an atmospheric quantum channel of 100 m length on the flat roof of our institute's building. The signal states are measured by homodyne detection with the help of a local oscillator (LO) which propagates in the same spatial mode as the signal, orthogonally polarized to it. Thus the interference of signal and LO is excellent and atmospheric fluctuations are auto-compensated. The LO also acts as a spatial and spectral filter, which allows for unrestrained daylight operation. Important characteristics for our system are atmospheric channel influences that could cause polarization, intensity and position excess noise. Therefore we study these influences in detail. Our results indicate that the channel is suitable for our quantum communication system in most weather conditions.