Katsuyuki Kasai

Investigation of the photon-number statistics of twin beams by direct detection

We present the results of an experiment in which we observed photon-number statistics of twin beams emerging from a nondegenerate optical parametric oscillator. We generated the photocurrent for recording by detecting the light and mixing it with a standard electrical oscillator. The measured photocurrent variances exhibited a quantum correlation of as much as -4.9 dB between signal and idler, whereas their photon number distributions were super-Poissonian. We also obtained the difference photon-number distribution.

Generation of twin beams from an optical parametric oscillator pumped by a frequency-doubled diode laser

Quantum-correlated twin beams were generated from a triply resonant optical parametric oscillator with an a-cut KTP crystal pumped by a frequency-doubled diode laser. A total output of 5.1 mW was obtained in the classical-nonclassical light-conversion system driven by a 50-mW diode laser at 1080 nm. A quantum-noise reduction of 4.3 dB (63%) in the intensity difference between the twin beams was successfully observed at the detection frequency of 3 MHz.

Conditional transfer of quantum correlation in the intensity of twin beams

A conditional protocol of transferring quantum correlations in a continuous variable regime was experimentally demonstrated. The quantum correlation in two pairs of twin beams, each characterized by intensity-difference squeezing of 7.0{+-}0.3 dB, was transferred to two initially independent idler beams. The quantum-correlation transfer resulted in intensity-difference squeezing of 4.0{+-}0.2 dB between two idler beams. The dependence of the preparation probability and transfer fidelity on the selection bandwidth was also studied.

Quantum channel using photon number correlated twin beams

We report quantum communications channel using photon number correlated twin beams. The twin beams are generated from a nondegenerate optical parametric oscillator, and the photon number difference is used to encode the signal. The bit error rate of our system will be 0.067 by using the twin beams comparing with 0.217 by using the coherent state as the signal carrier.

Generation of 22.8 mW single-frequency green light by frequency doubling of a 50-mW diode laser

We report frequency doubling of an extended-cavity diode laser in an a-cut KTP crystal. Continuous-wave, single-mode green light at 540 nm with a power of 22.8 mW was generated from an input of 44.2 mW by type II non-critical phase matching. Stable operation with intensity fluctuations smaller than 1 % peak-to-peak for more than two hours was achieved by employing resonant optical feedback. The optical feedback also reduced the fundamental-wave linewidth to the upper limit of 41 kHz.

Experimental investigation of the intensity fluctuation joint probability and conditional distributions of the twin-beam quantum state

We give the intensity fluctuation joint probability of the twin-beam quantum state, which was generated with an optical parametric oscillator operating above threshold. Then we present what to our knowledge is the first measurement of the intensity fluctuation conditional probability distributions of twin beams. The measured inference variance of twin beams 0.62+/-0.02, which is less than the standard quantum limit of unity, indicates inference with a precision better than that of separable states. The measured photocurrent variance exhibits a quantum correlation of as much as -4.9+/-0.2 dB between the signal and the idler.

Single-beam noise characteristics of quantum-correlated twin beams

We investigated the intensity noise spectra of a single beam of a pump-enhanced continuous-wave optical parametric oscillator (OPO), which was used to generate quantum-correlated twin beams, as a function of the pump power. With a triply (pump-, signal-, and idler-) resonant cavity, the oscillation threshold of our OPO was 8.5±1.3 mW and the measured slope conversion efficiency was 0.72±0.02. Twin beams with a power of 240 mW were generated at a pump power of 350 mW. The relaxation oscillation frequencies, which depend on the pump power, were observed when the pump power of the OPO was 12.5–28 mW. The experimental results confirm the predicted increase in OPO relaxation frequency with pump power. We experimentally inferred squeezing of the single-beam intensity, for the first time to our knowledge, by exploiting the nature of quantum noise that is dependent on loss.

Bacteriorhodopsin-based bipolar photosensor for biomimetic sensing

Bacteriorhodopsin (bR) is a promising biomaterial for several applications. Optical excitation of bR at an electrode-electrolyte interface generates differential photocurrents while an incident light is turned on and off. This unique functional response is similar to that seen in retinal neurons. The bR-based bipolar photosensor consists of the bR dip-coated thin films patterned on two ITO plates and the electrolyte solution. This bipolar photocell will function as a biomimetic photoreceptor cell. The bipolar structure, due to the photocurrent being generated in alignment with the cathodic direction, makes the excitatory and inhibitory regions possible. This scheme shows our bipolar cell can act as a basic unit of edge detection and forms the artificial visual receptive field.

Generation of two-mode bright squeezed light using a noise-suppressed amplified diode laser

We present the generation of nonclassical state using an amplified diode laser as a light source. The intensity noise of an amplified diode laser was significantly suppressed and reached the shot noise limit at 15 MHz using both a filter cavity and resonant optical feedback. Frequency doubling efficiency of 66% and up to 120 mW output power of green has been achieved in cw second-harmonic generation from 1080 nm to 540 nm. Bright two-mode amplitude-squeezed state was generated from a type-II nondegenerate optical parametric amplifier pumped by generated green light. The measured noise reduction is 2.1+/-0.2 dB below the shot-noise level.

Classical and quantum properties of optical parametric amplifier/deamplifier

Abstract The classical gain and the variances of coupled, signal and idler modes of output from continuous nondegenerate optical parametric amplifier are theoretically analyzed under parametric amplification and deamplification conditions. The correlation between the signal and idler modes is also discussed. The results provide design references for nonclassical light generation systems.