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Entanglement is an essential quantum resource for the acceleration of information processing as well as for sophisticated quantum communication protocols. Quantum information networks are expected to convey information from one place to another by using entangled light beams. We demonstrated the generation of entanglement among three bright beams of light, all of different wavelengths (532.251, 1062.102, and 1066.915 nanometers). We also observed disentanglement for finite channel losses, the continuous variable counterpart to entanglement sudden death.

Subtle quantum properties offer exciting new prospects in optical communications. For example, quantum entanglement enables the secure exchange of cryptographic keys(1) and the distribution of quantum information by teleportation(2,3). Entangled bright beams of light are increasingly appealing for such tasks, because they enable the use of well-established classical communications techniques(4). However, quantum resources are fragile and are subject to decoherence by interaction with the environment. The unavoidable losses in the communication channel can lead to a complete destruction of entanglement(5-8), limiting the application of these states to quantum-communication protocols. We investigate the conditions under which this phenomenon takes place for the simplest case of two light beams, and analyse characteristics of states which are robust against losses. Our study sheds new light on the intriguing properties of quantum entanglement and how they may be harnessed for future applications

We revisit the problem of quantum state reconstruction of light beams from the photocurrent quantum noise. As is well-known, but often overlooked, two longitudinal field modes contribute to each spectral component of the photocurrent (sideband modes). We show that spectral homodyne detection is intrinsically incapable of providing all the information needed for the full reconstruction of the two-mode spectral quantum state. Such a limitation is overcome by the technique of resonator detection. A detailed theoretical description and comparison of both methods is presented, as well as an experiment to measure the six-mode quantum state of pump-signal-idler beams of an optical parametric oscillator above the oscillation threshold.Quantum optics employing continuous variables of the electromagnetic field is a mature and well-developed subject, with applications ranging from high resolution measurements [1], to manipulation and storage of quantum information [2][3][4], and quantum metrology [5]. Among its advantages are the use of techniques adapted from the classical communications community, which employ the spectral analysis of light [6]. Quantum features that play a role in these applications include quadrature squeezing [7], quantum correlations [8] and entanglement [9].In order to harness the advantages offered by quantum properties of light to improve high resolution measurements or quantum information protocols, it is often necessary to obtain full knowledge of the system's quantummechanical state. Techniques for complete quantumstate characterization have been a part of the quantum optics toolbox for 20 years [10,11]. However, when combining these techniques with the spectral analysis of measured signals [12], care must be exercised: it has been known for a long time that two (sideband) modes must be considered when measuring quantum noise (and correlation) spectra of a single beam of light. In many situations, an effective single-mode description can be applied but this is not always true.In a previous paper [13], we show experimentally that indeed two different light states could lead to the same homodyne detection signals, whereas they could be unambiguously discriminated by resonator detection. In the present paper, our purpose is to give a detailed and consistent description of spectral reconstruction of quantum states of light. For the sake of completeness, in part of the paper we review concepts that are already known (although sometimes neglected). This helps make clear the shortcomings of the most widely used detection technique, (spectral) homodyne detection (HD), as well as the demonstration that an alternative technique, res- * mmartine@if.usp.br onator detection (RD) [14,15], does not suffer from the same limitations.Information about the quantum state is retrieved from photodetection, which yields a photocurrent continuously varying in time. Interferometric techniques, usually involving a reference field (a Local Oscillator -LO), enable the acquisition of phase sensitive information, thus allowi...

Spectral homodyne detection, a widely used technique for measuring quantum properties of light beams, cannot retrieve all the information needed to reconstruct the quantum state of spectral field modes. We show that full quantum state reconstruction can be achieved with the alternative measurement technique of resonator detection. We experimentally demonstrate this difference by engineering a quantum state with features that go undetected by homodyne detection but are clearly revealed by resonator detection.