The efficiency of spontaneous parametric down conversion is calculated and measured for several nonlinear
crystals and waveguides in single spatial mode regime. Efficiency of waveguide sources is found to
be far superior compared to the bulk crystal sources.
We describe a robust heralded single photon source based on parametric down conversion
of CW 532-nm light in a periodically polled KTP waveguide. Low required pump power (sub-mW),
reasonable operational temperature (43oC), high heralding efficiency (60%), and narrow spectral
width of the heralded photons (sub-nm) make it an ideal light source for long-distance quantum
We report our recent results in development of the secure fiber-optics communication system based upon quantum key
distribution (QKD). Emphasize is made on the limitation imposed by the state-of-the-art components crucial for the
system performance. We discuss the problem of the interferometer design and highlight the possible security loopholes
known. Together with single photon counting performance it places the main restriction on the distance range and the
secure key rate of the QKD system based upon the weak coherent pulses. Finally we describe the result of the first test
of the system using single photons produced by non-degenerate parametric down-conversion as a source.
Different quantum information schemes, such as eavesdropping in quantum cryptography, dictate the necessity of extracting information about pair conjugate observables from a single copy of a quantum system. Mathematically, quantum measurements are usually described by an uncertainty relation. The difference between the simultaneous measurement uncertainty relation form those known from the textbooks on quantum mechanics is the additional uncertainty associated with the measurement procedure itself in contrast to the state preparation uncertainties described by the Schroedinger-Robertson type uncertainty relations. We present here an overview of our approach based on the state estimation theory and maximum likelihood strategy. We make a theoretical analysis and an experimental verification of minimum-uncertainty product of the two-states quantum system simultaneous measurement based on partially entangled photon pairs.
Interference phenomena lead to a wealth of applications in many areas of physics. Entangled quantum states allow one to surpass the classical measurement sensitivity or resolution in polarimetry, interferometry, and imaging. In this paper we shall review, in some depth, polarization properties of quantized two-mode electromagnetic fields and show how interference and quantum entanglement lead to new phenomena. We shall also briefly discuss subwavelength quantum lithography.
The results of photon flux noise measurements in a quantum well semiconductor laser with 1.3 micrometer wavelength and antireflection coating on the front surface are presented. The results are compared with the theory based on quantum mechanical Langevin-Heisenberg equations.
Spectra of some ridge-waveguide lasers grown by metal-organic chemical vapor deposition (MOCVD) undergo a reversible transformation at a certain value of drive current -- usually from 5 to 10 thresholds. When the current is increased past this point, the spectrum abruptly widens and its amplitude drops correspondingly. In the widened spectrum a structure with period equal to longitudinal mode separation can be seen. We call this effect `spectral collapse.' The effect seems to be typical for ridge-waveguide lasers with ternary active regions and independent of active region strain. Data on `collapse' in both cw and pulsed modes at different temperatures suggest its connection with active region overheating. The intensity noise versus current dependence for some of the lasers reveals two peaks, one near the threshold and the other near the `spectral collapse' point. This led us to a suggestion that the `collapse' can be explained by nonlinear mode interaction. Some LPE-grown InGaAsP/GaAs lasers of similar design also exhibit spectral collapse while other samples from the same wafers do not, which may be evidence of a competition between nonlinear effects that cause spectral collapse and continuous widening of the spectrum with current due to spinodal decomposition in quaternary active region.
The investigations of correlation between intensity noise of semiconductor laser and junction voltage noise were made. The laser was high-power quantum well separate confinement heterostructure (SCH) InGaAsP/GaAs laser with (lambda) equals 0.8 micrometers . The full negative correlation near the threshold between photon flux noise and junction voltage noise in constant-current regime was obtained. It was shown that at room temperature the laser exhibited constant-current regime due to base resistance. The theoretical treatment of correlation coefficient versus pump current was proposed and the role of optical losses has been revealed. The comparison of theoretical and experimental behavior near the threshold were made.