F. De Martini

Experimental Realization of Teleporting an Unknown Pure Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels

We report on a quantum optical experimental implementation of teleportation of unknown pure quantum states. This realizes all of the nonlocal aspects of the original scheme proposed by Bennett et al. and is equivalent to it up to a local operation. We exhibit results for the teleportation of a linearly polarized state and of an elliptically polarized state. We show that the experimental results cannot be explained in terms of a classical channel alone. The Bell measurement in our experiment can distinguish between all four Bell states simultaneously allowing, in the ideal case, a 100% success rate of teleportation. [S0031-9007(97)05275-7]

Complete and deterministic discrimination of polarization Bell states assisted by momentum entanglement

A complete and deterministic Bell state measurement was realized by a simple linear optics experimental scheme which adopts two-photon polarization-momentum hyperentanglement. The scheme, which is based on the discrimination among the single photon Bell states of the hyperentangled state, requires the adoption of standard single photon detectors. The four polarization Bell states have been measured with average fidelity F=0.889 +/- 0.010 by using the linear momentum degree of freedom as the ancilla. The feasibility of the scheme has been characterized as a function of the purity of momentum entanglement.

Experimental realization of the quantum universal NOT gate

In classical computation, a ‘bit’ of information can be flipped (that is, changed in value from zero to one and vice versa) using a logical NOT gate; but the quantum analogue of this process is much more complicated. A quantum bit (qubit) can exist simultaneously in a superposition of two logical states with complex amplitudes, and it is impossible to find a universal transformation that would flip the original superposed state into a perpendicular state for all values of the amplitudes. But although perfect flipping of a qubit prepared in an arbitrary state (a universal NOT operation) is prohibited by the rules of quantum mechanics, there exists an optimal approximation to this procedure. Here we report the experimental realization of a universal quantum machine that performs the best possible approximation to the universal NOT transformation. The system adopted was an optical parametric amplifier of entangled photon states, which also enabled us to investigate universal quantum cloning.

Squeezed states in second-harmonic generation

We consider the process of second-harmonic generation with the nonlinear crystal placed in a resonant cavity. Both the fundamental and the second-harmonic mode are shown to exhibit squeezing. An analysis is made for the general values of the ratio between the damping constants of the two modes. We suggest that second-harmonic generation is the most suitable system to use for an experimental observation of the squeezing effect.

Spontaneous and stimulated emission in the thresholdless microlaser

High gain and virtual zero threshold have been recognized to be distinctive properties of the microlaser since its original proposal and realization by our laboratory in 1988. These properties are investigated both theoretically and experimentally. They are found to be determined by the synergy of several quantum-statistical processes that take place in the condition of extreme field confinement provided by the peculiar Casimir-type topology of the optical microcavity. The determination of the microcavity mode structure leads to a detailed study of the process of spontaneous emission (SpE), its merging with stimulated emission, and the consequent anomalous onset of the collective atomic behavior at low excitation levels. A microlaser excitation threshold of ~50 pJ has been determined experimentally with a molecular Oxazine microlaser excited by a femtosecond source. The relevance in atomic dynamics of the processes of SpE inhibition—enhancement, mode competition, fluorescence loss, interatomic transverse Bose correlations, and periodic excitation—is investigated both theoretically and experimentally. A discussion of the overall process in terms of a second-order phase transition in a nonequilibrium statistical problem is given. The extension of the microlaser dynamics to other quantum systems, such as the microscopic parametric oscillator, and to Raman and Compton scattering is considered.

High power subnanosecond pulse generation in Nd YAG lasers

Abstract We describe the application of the self-injection and cavity dumping techniques to a Nd YAG laser. Single or double, highly stabilized nanosecond pulses are obtained with large efficiency.

Separating the Classical and Quantum Information via Quantum Cloning

An application of quantum cloning to optimally interface a quantum system with a classical observer is presented; in particular, we describe a procedure to perform a minimal disturbance measurement on a single qubit by adopting a 1-->2 cloning machine followed by a generalized measurement on a single clone and the anticlone or on the two clones. Such a scheme can be applied to enhance the transmission fidelity over a lossy quantum channel.

Orientation of xanthene adsorbate molecules at dielectric interfaces

Abstract The polarization dependence of the second harmonic signal from monolayers of xanthene dyes allows us to develop a model for the orientation of the adsorbates and to locate the group responsible for adsorption. Possible extension of this method to other dyes of biological and medical interest is discussed.

Line profile of the Q01(1) vibrational resonance in H2 in the zone of dicke narrowing

Abstract A new method of high resolution non-linear spectroscopy based on the detection of the anti-Stokes intensity arising from a resonant four-photon coherent interaction is applied to the determination of the line profile of the Raman resonance Q 01 (1) in hydrogen gas. The real part as well as the imaginary part of the non-linear Raman susceptibility has been observed in a thermodynamical condition for the gas in which the Dicke narrowing process and the dephasing collision-broadening effect determine competitively the shape of the resonance line.

Contextual realization of the universal quantum cloning machine and of the universal-NOT gate by quantum-injected optical parametric amplification

A simultaneous, contextual experimental demonstration of the two processes of cloning an input qubit vertical bar {psi}> and of flipping it into the orthogonal qubit vertical bar {psi}{sup perpendicular>} is reported. The adopted experimental apparatus, a quantum-injected optical parametric amplifier is transformed simultaneously into a universal optimal quantum cloning machine and into a universal-NOT quantum-information gate. The two processes, indeed forbidden in their exact form for fundamental quantum limitations, were found to be universal and optimal, i.e., the measured fidelity of both processes F<1 was found close to the limit values evaluated by quantum theory. A contextual theoretical and experimental investigation of these processes, which may represent the basic difference between the classical and the quantum worlds, can reveal in a unifying manner the detailed structure of quantum information. It may also enlighten the yet little explored interconnections of fundamental axiomatic properties within the deep structure of quantum mechanics.