P. Nussenzveig

Classical analog of electromagnetically induced transparency

We present a classical analog of electromagnetically induced transparency (EIT). In a system of just two coupled harmonic oscillators subject to a harmonic driving force, we reproduce the phenomenology observed in EIT. We also describe a simple experiment with two linearly coupled RLC circuits which can be incorporated into an undergraduate laboratory.

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.

Direct production of tripartite pump-signal-idler entanglement in the above-threshold optical parametric oscillator

We calculate the quantum correlations existing among the three output fields (pump, signal, and idler) of a triply resonant nondegenerate optical parametric oscillator operating above threshold. By applying the standard criteria [P. van Loock and A. Furusawa, Phys. Rev. A 67, 052315 (2003)], we show that strong tripartite continuous-variable entanglement is present in this well-known and simple system. Furthermore, since the entanglement is generated directly from a nonlinear process, the three entangled fields can have very different frequencies, opening the way for multicolored quantum information networks.

Orbital angular momentum exchange in an optical parametric oscillator

We present a study of orbital angular momentum transfer from pump to down-converted beams in a type-II optical parametric oscillator. Cavity and anisotropy effects are investigated and demonstrated to play a central role in the transverse mode dynamics. While the idler beam can oscillate in a Laguerre-Gauss mode, the crystal birefringence induces an astigmatic effect in the signal beam that prevents the resonance of such a mode.

Noise spectroscopy of nonlinear magneto-optical resonances in Rb vapor

Nonlinear magneto-optical resonances occurring for near-zero magnetic field are studied in Rb vapor using light-noise spectroscopy. With a balanced detection polarimeter, we observe high contrast variations of the noise power (at fixed analysis frequency) carried by diode laser light resonant with the 5S{sub 1/2} (F=2){yields}5P{sub 1/2} (F=1) transition {sup 87}Rb and transmitted through a rubidium vapor cell as a function of magnetic field B. A symmetric resonance doublet of anticorrelated noise is observed for orthogonal polarizations around B=0 as a manifestation of ground-state coherence. We also observe sideband noise resonances when the magnetic field produces an atomic Larmor precession at a frequency corresponding to one-half of the analysis frequency. The resonances on the light fluctuations are the consequence of phase to amplitude noise conversion owing to nonlinear coherence effects in the response of the atomic medium to the fluctuating field. A theoretical model (derived from linearized Bloch equations) is presented that reproduces the main qualitative features of the experimental signals under simple assumptions.

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.

Super-Poissonian intensity fluctuations and correlations between pump and probe fields in Electromagnetically Induced Transparency

We measured intensity fluctuations of pump and probe beams after interaction with Rb atoms in electromagnetically induced transparency. Both fields presented super-Poissonian statistics and become correlated in intensity, in qualitative agreement with theoretical predictions. The measured correlation is a first step towards the observation of macroscopic entanglement in ElT.

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.