Strong Quantum Correlations Research Articles

Matter-wave amplification and phase conjugation via stimulated dissociation of a molecular Bose-Einstein condensate

We propose a scheme for parametric amplification and phase conjugation of an atomic Bose-Einstein condensate (BEC) via stimulated dissociation of a BEC of molecular dimers consisting of bosonic atoms. This can potentially be realized via coherent Raman transitions or using a magnetic Feshbach resonance. We show that the interaction of a small incoming atomic BEC with a (stationary) molecular BEC can produce two counterpropagating atomic beams -- an amplified atomic BEC and its phase-conjugate or time-reversed replica. The two beams can possess strong quantum correlation in the relative particle number, with squeezed number-difference fluctuations.

Spatial quantum noise interferometry in expanding ultracold atom clouds

In a pioneering experiment, Hanbury Brown and Twiss (HBT) demonstrated that noise correlations could be used to probe the properties of a (bosonic) particle source through quantum statistics; the effect relies on quantum interference between possible detection paths for two indistinguishable particles. HBT correlations--together with their fermionic counterparts--find numerous applications, ranging from quantum optics to nuclear and elementary particle physics. Spatial HBT interferometry has been suggested as a means to probe hidden order in strongly correlated phases of ultracold atoms. Here we report such a measurement on the Mott insulator phase of a rubidium Bose gas as it is released from an optical lattice trap. We show that strong periodic quantum correlations exist between density fluctuations in the expanding atom cloud. These spatial correlations reflect the underlying ordering in the lattice, and find a natural interpretation in terms of a multiple-wave HBT interference effect. The method should provide a useful tool for identifying complex quantum phases of ultracold bosonic and fermionic atoms.

Parametric oscillation in semiconductor microcavities: nonlinear and quantum effects

AbstractWe report the observation of optical bistability in a semiconductor microcavity pumped at the “magic angle” above the parametric oscillation threshold. Experimental evidence is given in the form of a hysteresis cycle of the nonlinear emission as a function of the pump intensity. A model analogous to those developped for triply resonant Optical Parametric Oscillators (OPO) is in good agreement with the experimental findings. In the second part, we study the potential application of the parametric oscillation regime to quantum optics. The quantum correlations between the emitted “signal” and “idler” beams are computed. We show that very strong quantum correlations between the signal and idler polaritons can be achieved. However the quantum effects in the outgoing light fields are strongly reduced due to the large mismatch in the coupling of the signal and idler polaritons to the external photons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Twin polaritons in semiconductor microcavities

The quantum correlations between the beams generated by polariton pair scattering in a semiconductor microcavity above the parametric oscillation threshold are computed analytically. The influence of various parameters like the cavity-exciton detuning, the intensity mismatch between the signal and idler beams and the amount of spurious noise is analyzed. We show that very strong quantum correlations between the signal and idler polaritons can be achieved. The quantum effects on the outgoing light fields are strongly reduced due to the large mismatch in the coupling of the signal and idler polaritons to the external photons.

Fiber dispersion cancellation by all-optical quantum signal processing based on quantum correlations.

A quantum dispersion canceller is proposed that uses all-optical quantum signal processing based on strong quantum correlations in the signal and idler waves amplified by an optical parametric amplifier at the receiver end. It cancels the even-order dispersion and can regenerate an optical data packet without any ambiguity.

Coherent and Dissipative Spin Dynamics inN-Electron Systems

We investigate the ensemble averaged evolution of N-electron systems dynamically coupled to a statistical environment. The electrons are characterized by their spatial and by their spin properties. While the Hilbert space for single electrons is given by the tensor product of the Hilbert spaces associated with both properties, the corresponding Hilbert space for N-electron systems cannot be factorized. Consequently, quantum correlations between spatial and spin properties become extremely important. We assume that the evolution of the spin properties is controlled by spin-orbit interaction and that the spatial properties take the part of a bath held near some equilibrium. This description is appropriate for magnetic systems where the electronic states near the ground state correspond to different spin configurations, whereas electronic states with large excitation energies belong to different spatial-orbital configurations. In order to determine the coarse grained evolution of the spin properties, we have to know the evolution of the N-electron system over time intervals larger than the bath-correlation time. This is obtained from the first- and second-order contributions in the interaction picture. We show that, in spite of the strong quantum correlations between spin properties and spatial properties, the coarse grained statistical evolution of the electronic spin properties may be described by a set of coupled master equations.

Quantum information processing and precise optical measurement with entangled-photon pairs

Two photons in a pair generated in the nonlinear optical process of spontaneous parametric down-conversion are, in general, strongly quantum entangled. Accordingly, they contain extremely strong energy, time, polarization and momentum quantum correlations. This entanglement involves more than one quantum variable and has served as a powerful tool in fundamental studies of quantum theory. It is now playing a large role in the development of novel information processing techniques and new optical measurement technologies. Here we review some of these technologies and their origins.

Generation of Quantum-Correlated Twin Beams and Its Application

The signal and idler beams from a non-degenerate, above threshold, optical parametric oscillator have a strong quantum correlation, which are called twin beams. This correlation arises from the simultaneous generation of the signal and idler photons in the form of twin pairs of photons through the parametric down conversion, and the generated twin beams have exactly the same photon statistics. As a result, the fluctuations on the difference between the intensities of the twin beams are reduced with respect to the shot noise limit, that is, intensity-difference squeezing. We give a comprehensive overview of the generation of twin beams using the optical parametric oscillator, and the applications exploiting the quantum correlation of twin beams are presented.

Transient photon-number correlations of interacting solitons

The multimode quantum fluctuations of colliding solitons are investigated. Strong mutual quantum correlations appear in their intensity spectrum as the solitons approach each other in the collision. The leaving solitons erase the interpulse correlations. These transient correlations are induced by cross phase modulation, whereas the intrapulse correlations are due to self phase modulation. As a possible application a direct detection quantum-nondemolition (QND) measurement of the photon number is proposed. It shows a performance better than a collision induced phase-shift QND measurement.

De Broglie wavelength reduction for a multiphoton wave packet

An experiment is proposed that permits the observation of the reduced de Broglie wavelengths of two and four-photon wave packets using present technology. It is suggested to use a Mach-Zehnder setup and feed both input ports with light generated by a single non-degenerate down-conversion source. The strong quantum correlations of the light in conjunction with boson-enhancement at the input beam splitter allow to detect a two- and fourfold decrease in the observed de Broglie wavelength with perfect visibility. This allows a reduction of the observed de Broglie wavelength below the wavelength of the source.

Origin of gauge bosons from strong quantum correlations.

The existence of light [a massless U(1) gauge boson] is one of the unresolved mysteries in nature. We propose that light is originated from certain quantum orders in our vacuum. We construct quantum spin models on lattice to demonstrate that some quantum orders can give rise to light without breaking any symmetries and without any fine-tuning. Through our models, we show that the existence of light can simply be a phenomenon of quantum coherence in a system with many degrees of freedom. Massless gauge fluctuations appear commonly and naturally in strongly correlated quantum systems which originally contain no gauge fields.

Quantum Cascade of Photons in Semiconductor Quantum Dots

We have obtained pairs of correlated single photons from the emission cascade of an isolated InAs quantum dot. The cross-correlation function of the two photons in a pair exhibits the coexistence of asymmetric bunching and antibunching features, which is the signature for their sequential emission with a definite order. This observation opens the way to the use of semiconductor quantum dots as triggered sources of photon pairs with strong quantum correlations for quantum information applications.

Quantum structures in traveling-wave spontaneous parametric down-conversion

We analyze theoretically spatial structures appearing in the far diffraction zone of the electromagnetic field emitted in the cavityless parametric down-conversion. We investigate in detail the spatial correlation functions of intensity and demonstrate the existence of strong quantum correlations between the regions of the far field symmetrical with respect to the optical axis. Our simplified model allows us to obtain analytical results for some limiting cases. We demonstrate that in the limit of small diffraction and ideal quantum efficiency of photodetection the noise reduction in the photocurrent difference between symmetrical regions in the far diffraction field becomes complete at zero frequency of photocurrent fluctuations.

A quantum dot single-photon turnstile device.

Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Long-distance Bell-type tests using energy-time entangled photons

Long-distance Bell-type experiments are presented. The different experimental challenges and their solutions in order to maintain the strong quantum correlations between energy-time entangled photons over more than 10 km are reported and the results analyzed from the point of view of tests of fundamental physics as well as from the more applied side of quantum communication, especially quantum key distribution. Tests using more than one analyzer on each side are also presented.

Experimental investigation of intensity difference squeezing using Nd:YAP laser as pump source

A strong quantum correlation between twin-beams at wavelength was observed. The intensity difference noise of twin-beams, which is generated by a semimonolithic OPO cavity pumped by a frequency-doubled and stabilized Nd:YAP laser, is about 80% (7 dB) below the short noise limit at the measurement frequency of 1.5 MHz. The threshold of the optical parametric oscillator is about 20 mW. The parametric dependence of the quantum correlation is experimentally investigated. The experimental results agree with the theory well.

Filtering of two-photon quantum correlations by optical cavities: Cancellation of dispersive effects.

The spectral filtering of a field with strong quantum correlations is studied. The changes in the correlation characteristics of a field, produced by a down converter, passing through a strongly dispersive element like a Fabry-P\'erot cavity, are calculated. In the special case when the central frequency of the idler and signal photons coincides with the resonance frequency of the cavity, there is a cancellation of the dispersive effects of the cavity. Detailed numerical results for the two-photon joint counting rate are presented. The effects of the absorption in the medium on the two-photon correlations are also discussed.

Quasi-free states and a unified treatment of the quantum and thermal fluctuations

We show that the pure quasi-free state obtained from a mixed quasi-free state by the purification map is a state with strong quantum correlations between the initial modes and the auxiliary ones. This pure quasi-free state can be obtained from the Fock state by a Bogoliubov automorphism. An inverse map for the purification map is defined. It is shown that this map produces thermal fluctuations from quantum correlations.