Marcelo Martinelli

Generation of bright two-color continuous variable entanglement.

We present the first measurement of squeezed-state entanglement between the twin beams produced in an optical parametric oscillator operating above threshold. In addition to the usual squeezing in the intensity difference between the twin beams, we have measured squeezing in the sum of phase quadratures. Our scheme enables us to measure such phase anticorrelations between fields of different frequencies. In the present measurements, wavelengths differ by approximately 1 nm. Entanglement is demonstrated according to the Duan et al. criterion [Phys. Rev. Lett. 84, 2722 (2000)] Delta2p- + Delta2q+ = 1.41(2) < 2. This experiment opens the way for new potential applications such as the transfer of quantum information between different parts of the electromagnetic spectrum.

Three-Color Entanglement

Entangling Rainbows Quantum mechanical entanglement is at the heart of quantum information processing. In the future, practical systems will contain a network of quantum components, possibly operating at different frequencies. Coelho et al. (p. 823, published online 17 September) present a technique that can entangle light beams of three different frequencies. The ability to swap entanglement between different light fields should prove useful in advanced quantum information protocols on systems comprising different operating frequencies. Three bright light beams of different colors can be entangled. 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.

Measurement of refractive nonlinearities in GaAs above bandgap energy

We present a technique for single-beam measurement of the optical nonlinearity in GaAs for photon energies above the bandgap. We measured the real and the imaginary parts of the nonlinear refractive index of a bulk crystal by using the change in reflection of dye laser pulses (10 ns, 538 nm). The values obtained, n(2) = (7.8 ? 0.6) x 10(-8) cm(2)/W and kappa(2) = (-2.8 ? 0.7) x 10(-8) cm(2)/W, are discussed.

Sensitivity-enhanced reflection Z-scan by oblique incidence of a polarized beam

We demonstrate a new experimental method which allows the measurement of the nonlinear optic index in absorptive media with great sensitivity. In this technique the reflection from a polarized Gaussian laser beam close to the Brewster angle is measured. A sensitivity enhancement factor of 30 with respect to other techniques is observed for an optical crystal, and higher values are possible to be obtained.

Experimental study of the spatial distribution of quantum correlations in a confocal optical parametric oscillator

We study experimentally the spatial distribution of quantum noise in the twin beams produced by a type-II optical parametric oscillator operating in a confocal cavity above threshold. The measured intensity correlations are at the same time below the standard quantum limit and not uniformly distributed inside the beams. We show that this feature is an unambiguous evidence for the multimode and nonclassical character of the quantum state generated by the device.

Entanglement in the above-threshold optical parametric oscillator

We investigate entanglement in the above-threshold optical parametric oscillator, both theoretically and experimentally, and discuss its potential applications to quantum information. The fluctuations measured in the subtraction of signal and idler amplitude quadratures are Δ2p−=0.50(1), or −3.01(9) dB, and in the sum of phase quadratures they are Δ2q+=0.73(1), or −1.37(6) dB. A detailed experimental study of the noise behavior as a function of pump power is presented, and the discrepancies with theory are discussed.

Experimental observation of three-color optical quantum correlations

Quantum correlations among bright pump, signal, and idler beams produced by an optical parametric oscillator, all with different frequencies, are experimentally demonstrated. We show that the degree of entanglement between signal and idler fields is improved by using information on pump fluctuations. To our knowledge this is the first observation of three-color optical quantum correlations.

Robustness of bipartite Gaussian entangled beams propagating in lossy channels

Quantum entanglement — used for quantum key distribution, communication and teleportation — is a fragile resource. Researchers investigate the conditions under which optical loss destroys entanglement, and report states that are particularly robust to such losses.

Beyond Spectral Homodyne Detection: Complete Quantum Measurement of Spectral Modes of Light

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.

Disentanglement in bipartite continuous-variable systems

Entanglement in bipartite continuous-variable systems is investigated in the presence of partial losses such as those introduced by a realistic quantum communication channel, e.g., by propagation in an optical fiber. We find that entanglement can vanish completely for partial losses, in a situation reminiscent of so-called entanglement sudden death. Even states with extreme squeezing may become separable after propagation in lossy channels. Having in mind the potential applications of such entangled light beams to optical communications, we investigate the conditions under which entanglement can survive for all partial losses. Different loss scenarios are examined, and we derive criteria to test the robustness of entangled states. These criteria are necessary and sufficient for Gaussian states. Our study provides a framework to investigate the robustness of continuous-variable entanglement in more complex multipartite systems.