Paolo Villoresi

Absolute-phase phenomena in photoionization with few-cycle laser pulses.

Currently, the shortest laser pulses that can be generated in the visible spectrum consist of fewer than two optical cycles (measured at the full-width at half-maximum of the pulses envelope). The time variation of the electric field in such a pulse depends on the phase of the carrier frequency with respect to the envelope—the absolute phase. Because intense laser–matter interactions generally depend on the electric field of the pulse, the absolute phase is important for a number of nonlinear processes. But clear evidence of absolute-phase effects has yet to be detected experimentally, largely because of the difficulty of stabilizing the absolute phase in powerful laser pulses. Here we use a technique that does not require phase stabilization to demonstrate experimentally the influence of the absolute phase of a short laser pulse on the emission of photoelectrons. Atoms are ionized by a short laser pulse, and the photoelectrons are recorded with two opposing detectors in a plane perpendicular to the laser beam. We detect an anticorrelation in the shot-to-shot analysis of the electron yield.

Few-optical-cycle pulses tunable from the visible to the mid-infrared by optical parametric amplifiers

Ultrafast optical parametric amplifiers (OPAs) can provide, under suitable conditions, ultra-broad gain bandwidths and can thus be used as effective tools for the generation of widely tunable few-optical-cycle light pulses. In this paper we review recent work on the development of ultra-broadband OPAs and experimentally demonstrate pulses with durations approaching the single-cycle limit and almost continuous tunability from the visible to the mid-IR.

Experimental verification of the feasibility of a quantum channel between space and Earth

Extending quantum communication to space environments would enable us to perform fundamental experiments on quantum physics as well as applications of quantum information at planetary and interplanetary scales. Here, we report on the first experimental study of the conditions for the implementation of the single-photon exchange between a satellite and an Earth-based station. We built an experiment that mimics a single photon source on a satellite, exploiting the telescope at the Matera Laser Ranging Observatory of the Italian Space Agency to detect the transmitted photons. Weak laser pulses, emitted by the ground-based station, are directed toward a satellite equipped with cube-corner retroreflectors. These reflect a small portion of the pulse, with an average of less- than-one photon per pulse directed to our receiver, as required for faint-pulse

Sub-two-cycle light pulses at 1.6 μm from an optical parametric amplifier

We generate ultrabroadband pulses, spanning the 1200-2100 nm wavelength range, from an 800 nm pumped optical parametric amplifier (OPA) working at degeneracy. We compress the microjoule-level energy pulses to nearly transform-limited 8.5 fs duration by an adaptive system employing a deformable mirror. To our knowledge, these are the shortest light pulses generated at 1.6 μm.

Feasibility of satellite quantum key distribution

In this paper, we present a novel analysis of the feasibility of quantum key distribution between a LEO satellite and a ground station. First of all, we study signal propagation through a turbulent atmosphere for uplinks and downlinks, discussing the contribution of beam spreading and beam wandering. Then we introduce a model for the background noise of the channel during night-time and day-time, calculating the signal-to-noise ratio for different configurations. We also discuss the expected error-rate due to imperfect polarization compensation in the channel. Finally, we calculate the expected key generation rate of a secure key for different configurations (uplink, downlink) and for different protocols (BB84 with and without decoy states, entanglement-based Ekert91 protocol).

Experimental Satellite Quantum Communications

Quantum communication (QC), namely, the faithful transmission of generic quantum states, is a key ingredient of quantum information science. Here we demonstrate QC with polarization encoding from space to ground by exploiting satellite corner cube retroreflectors as quantum transmitters in orbit and the Matera Laser Ranging Observatory of the Italian Space Agency in Matera, Italy, as a quantum receiver. The quantum bit error ratio (QBER) has been kept steadily low to a level suitable for several quantum information protocols, as the violation of Bell inequalities or quantum key distribution (QKD). Indeed, by taking data from different satellites, we demonstrate an average value of QBER=4.6% for a total link duration of 85 s. The mean photon number per pulse μ_{sat} leaving the satellites was estimated to be of the order of one. In addition, we propose a fully operational satellite QKD system by exploiting our communication scheme with orbiting retroreflectors equipped with a modulator, a very compact payload. Our scheme paves the way toward the implementation of a QC worldwide network leveraging existing receivers.

Cluster effects in high-order harmonics generated by ultrashort light pulses

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield.

Intense femtosecond extreme ultraviolet pulses by using a time-delay-compensated monochromator

Extreme-ultraviolet pulses, produced by high-order harmonic generation, have been spectrally selected by a time-delay-compensated grating monochromator. Temporal characterization of the harmonic pulses has been obtained using cross-correlation method: pulses as short as 8 fs, with high photon flux, have been measured at the output of the monochromator.

Compensation of optical path lengths in extreme-ultraviolet and soft-x-ray monochromators for ultrafast pulses

The extraction of the spectrum corresponding to a single extreme-ultraviolet ultrashort pulse embedded in an extended spectrum may alter the duration of the pulse itself. This is due to the spectral filtering of optics and the differences in the optical path of the rays caused by ordinary diffraction when a grating is used. The basic mechanism that leads to the latter effect is the difference of one wavelength of the path length of two rays diffracted at the first order by nearby grating grooves. A study of these effects and some possible solutions obtained from using a pair of diffraction gratings is presented. The aim of this study is the selection without dispersion of one or more high-order laser harmonics produced by a pulse lasting a few femtoseconds and interacting with a gas jet.

Optimization of high-order harmonic generation by adaptive control of a sub-10-fs pulse wave front.

We present a method for the optimization of high-order harmonic generation based on wave-front correction of the driving laser beam. The technique exploits wave-front adaptive control by means of a deformable mirror, governed by an optimization procedure.