Direct experimental test of non-separability and other quantum techniques using continuous variables of light

We present schemes for the generation and evaluation of continuous variable entanglement of bright optical beams and give a brief overview of a variety of optical techniques and quantum communication applications on this basis. A new entanglement-based quantum interferometry scheme with bright beams is suggested. The performance of the presented schemes is independent of the relative interference phase which is advantageous for quantum communication applications. Received 1st August 2001     

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. Received 5 February 2001 and Received in final form 11 April 2001     

Triggered single photons and entangled photons from a quantum dot microcavity

Current quantum cryptography systems are limited by the attenuated coherent pulses they use as light sources: a security loophole is opened up by the possibility of multiple-photon pulses. By replacing the source with a single-photon emitter, transmission rates of secure information can be improved. We have investigated the use of single self-assembled InAs/GaAs quantum dots as such single-photon sources, and have seen a tenfold reduction in the multi-photon probability as compared to Poissonian pulses. An extension of our experiment should also allow for the generation of triggered, polarization-entangled photon pairs. The utility of these light sources is currently limited by the low efficiency with which photons are collected. However, by fabricating an optical microcavity containing a single quantum dot, the spontaneous emission rate into a single mode can be enhanced. Using this method, we have seen 78% coupling of single-dot radiation into a single cavity resonance. The enhanced spontaneous decay should also allow for higher photon pulse rates, up to about 3 GHz. Received 8 July 2001 and Received in final form 25 August 2001     

Continuous variables quantum switch teleportation using two-mode squeezed light

We propose a novel quantum switch teleportation with a continuous variable, which can teleport a quantum state to two different receivers alternatively, using a pair of two-mode squeezed lights as the quantum switching to manipulate the transmission route. In this scheme, the EPR entangled beams shared by sender and receivers are produced by mixing a pair of two-mode squeezed lights on one beamsplitter and separating them by using a polarizing beam splitter. The teleportation capability of this system is examined by the criteria proposed by Ralph and Lam [#!ralph!#] from a small signal quantum optical point of view. Received 20 May 2002 / Received in final form 29 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: zhangyun@crl.go.jp     

Effective quantum efficiency in the pulsed homodyne detection of a n-photon state

Homodyne detection can be used to perform measurements on various quantum states of the light, such as conditional single photon states produced by parametric fluorescence processes. In the pulsed regime, the time and frequency overlap between the single photon wave packet and the local oscillator field plays a crucial role. We show in this paper that this overlap can be characterized by an effective quantum efficiency, which is explicitly calculated in various situations of experimental interest. Received 27 July 2000 and Received in final form 29 November 2000     

What does an observed quantum system reveal to its observer?

The evolution of a quantum system under observation becomes retarded or even impeded. We review this “quantum Zeno effect” in the light of the criticism that has been raised upon a previous attempt to demonstrate it, of later reexaminations of both the projection postulate and the significance of the observations, and of the results of a recent experiment on an individual cold atom. Here, the micro-state of the quantum system gets unveiled with the observation, and the effect of measurement is no longer mixed up with dephasing the object's wave function by the reactive effect of the detection. A procedure is outlined that promises to provide, by observation, an upper limit for the delay of even an exponential decay. Received 11 January 2001 and Received in final form 28 February 2001     

Collective atomic spin squeezing and control

Using a quantum theory for an ensemble of two- or three-level atoms driven by electromagnetic fields in an optical cavity, we show that the various spins associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand self-spin squeezing when the input fields are coherent ones and the atomic ensemble exhibits a large non-linearity; on the other hand squeezing transfer when one of the incoming fields is squeezed. Received 14 August 2001 and Received in final form 7 November 2001     

Intensity noise of injected Nd:YVO 4 microchip lasers

The combined effects of the pump noise suppression and injection locking technique on the intensity noise of a diode pumped Nd:YVO₄ microchip laser are theoretically and experimentally investigated. Complete cancellation of the relaxation oscillation peak is experimentally achieved. Very good agreement between experimental results and theoretical predictions of a fully quantum model describing lasers with injected signal is found. Received 10 December 2001 and Received in final form 6 March 2002     

Dichromatic squeezing generation

We propose a novel scheme for the joint generation of two squeezed beams at arbitrary frequencies ω ₁ and ω ₂. The scheme consists of two successive steps, both involving nonlinear interactions in χ⁽²⁾ crystals. The dynamics of the setup is analyzed both quantum mechanically and classically within the parametric approximation. An experimental implementation involving the fundamental and the harmonics of a Nd:YAG laser pulse, and β-BaB ₂ O ₄ nonlinear crystals is suggested. Received 17 May 2000 and Received in final form 9 October 2000     

Quantum limits of cold damping with optomechanical coupling

Thermal noise of a mirror can be reduced by cold damping. The displacement is measured with a high-finesse cavity and controlled with the radiation pressure of a modulated light beam. We establish the general quantum limits of noise in cold damping mechanisms and we show that the optomechanical system allows to reach these limits. Displacement noise can be arbitrarily reduced in a narrow frequency band. In a wide-band analysis we show that thermal fluctuations are reduced as with classical damping whereas quantum zero-point fluctuations are left unchanged. The only limit of cold damping is then due to zero-point energy of the mirror. Received 1st August 2001 and Received in final form 12 October 2001     

Photon entanglement in the phase-space Q representation of quantum optics

Several examples of photon entanglement are studied in the Q representation of quantum optics. In particular, the entangled states produced in parametric downconversion are studied in detail, and we determine the conditions for the violation of Bell's inequality. Our approach shows that photon entanglement is related to the existence of correlations between the quantum fluctuations of the electromagnetic field associated to different modes. Received 10 August 2002 / Received in final form 7 November 2002 Published online 4 February 2003     

Quantum state of an injected TROPO above threshold: purity,Glauber function and photon number distribution

In this paper, we investigate several properties of the full signal-idler-pump mode quantum state generated by a triply resonant non-degenerate Optical Parametric Oscillator operating above threshold, with an injected wave on the signal and idler modes in order to lock the phase diffusion process. We determine and discuss the spectral purity of this state, which turns out not to be always equal to 1 even though the three interacting modes have been taken into account at the quantum level. We have seen that the purity is essentially dependent on the weak intensity of the injected light and on an asymmetry of the synchronization. We then derive the expression of its total three-mode Glauber P-function, and calculate the joint signal-idler photon number probability distribution and investigate their dependence on the injection.     

Quantum key distribution using quantum-correlated photon sources

Quantum key exchanges using weak coherent (Poissonian) single-photon sources are open to attack by a variety of eavesdropping techniques. Quantum-correlated photon sources provide a means of flagging potentially insecure multiple-photon emissions and thus extending the secure quantum key channel capacity and the secure key distribution range. We present indicative photon-counting statistics for a fully correlated Poissonian multibeam photon source in which the transmitted beam is conditioned by photon number measurements on the remaining beams with non-ideal multiphoton counters. We show that significant rejection of insecure photon pulses from a twin-beam source cannot be obtained with a detector having a realistic quantum efficiency. However quantum-correlated (quadruplet or octuplet) multiplet photon sources conditioned by high efficiency multiphoton counters could provide large improvements in the secure channel capacity and the secure distribution range of high loss systems such as those using the low earth orbit satellite links proposed for global quantum key distribution. Received 14 July and Received in final form 20 November 2001     

Quantum imaging,quantum lithography and the uncertainty principle

One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Even though we still have questions in regard to fundamental issues of the entangled quantum systems, quantum entanglement has started to play important roles in practical applications. Quantum imaging is one of the hot topics. Quantum imaging has many interesting features which are useful for different applications. For example, quantum imaging can be nonlocal, which is useful for secure two-dimensional information transfer. Quantum imaging can reach a much higher spatial resolution comparing with classical imaging, even beyond the diffraction limit, which is useful for lithography and other microsystem fabrication technology. It is not a violation of the uncertainty principle, however, a quantum mechanical multi-particle phenomenon. Received 30 August 2002 / Received in final form 11 November 2002 Published online 4 February 2003     

Quantum state transfer from light beams to atomic ensembles

We show the equivalence between an ensemble of two-level atoms driven by a squeezed vacuum field, and a harmonic oscillator coupled to a squeezed field. We give the conditions for optimal squeezing transfer from the field to the atomic ensemble. We show that EPR-type correlations are created between the atomic ensemble and the incoming field. Received 23 January 2001     

Coherence properties of two trapped particles

We analyse the coherence properties of two particles trapped in a one-dimensional harmonic potential. This simple model allows us to derive analytic expressions for the first and second order coherence functions. We investigate their properties depending on the particle nature and the temperature of the quantum gas. We find that at zero temperature non-interacting bosons and fermions show very different correlations, while they coincide for higher temperatures. We observe atom bunching for bosons and atom anti-bunching for fermions. When the effect of s-wave scattering between bosons is taken into account, we find that the range of coherence is enhanced or reduced for repulsive or attractive potentials, respectively. Strongly repelling bosons become in some way more “fermion-like" and show anti-bunching. Their first order coherence function, however, differs from that for fermions. Received 19 September 2002 Published online 4 February 2003     

Room temperature stable single-photon source

We report on the realization of a stable solid state room temperature source for single photons. It is based on the fluorescence of a single nitrogen-vacancy (NV) color center in a diamond nanocrystal. Antibunching has been observed in the fluorescence light under both continuous and pulsed excitation. Our source delivers 2×10⁴ s^-1 single-photon pulses at an excitation repetition rate of 10 MHz. The number of two-photon pulses is reduced by a factor of five compared to strongly attenuated coherent sources. Received 1st August 2001 and Received in final form 2 October 2001     

Spontaneous emission of an excited two-level atom without both rotating-wave and Markovian approximation

The spontaneous emission of an excited atom is analyzed by quantum stochastic trajectory approach without both rotating-wave approximation and Markovian approximation. The atom finite size effect is also taken into account. We show by an example that the correction due to the counter-rotating wave term is rather small, even for the largest atomic number of real nuclei. Received 10 July 2002 / Received in final form 12 November 2002 Published online 4 February 2003     

Quantum effects in the interaction of the exciton with a leaky quasi-mode cavity

We have studied quantum effects in the interaction of the exciton with a leaky quasi-mode cavity field. When the exciton is initially prepared in a superposition state which exhibits holes in its photon-number distribution, whereas the cavity field initially is in the vacuum state, it is found that there exists an energy exchange between the exciton and the cavity field. The exciton and the cavity field may exhibit sub-Poissonian distributions and quadrature squeezings. It is shown that there does not exist a violation of the Cauchy-Schwartz inequality, which means that the correlation between the exciton and the cavity field is classical. Received 25 November 2000 and Received in final form 1st January 2001     

Shor and Preskill's and Mayers's security proof for the BB84 quantum key distribution protocol

We review two security proofs for the BB84 quantum key distribution protocol: Mayers's security proof and the more recent proof of Shor and Preskill. We focus on the basic principles and the intuition in Mayers's proof instead of technical details. We present a variation on Shor's and Preskill's proof which is convenient for purpose of comparison. We explain the connection between these two proofs. Received 14 July 2001