Takuya Hirano

Quadrature-squeezed light generated via parametric amplification is an essential resource for continuous-variable quantum communication (CVQC) technology, such as quantum teleportation and dense coding [1], and parametric amplification itself is also important for optical communication technology, e.g. wavelength shifting and optical sampling. Therefore, the development of efficient parametric process is one of the key issues for future information technology. In telecommunication band, observation of 6.4 dB squeezed light using cw light and periodically poled KTiOPO4 bulk type crystal was recently reported by M. Mehmet et al.[2], and 4.1 dB of pulsed squeezing in periodically poled MgOLiNbO3 waveguide (PPLN-WG) was reported by Y. Eto et al. [3]. In the latter experiment, squeezing level was mainly limited by optical losses including quantum efficiency of photodiodes. © 2011 IEEE.

WO Patent 2,009,025,195

We report on a measurement of entanglement in the time domain. The entanglement is examined by the Duan-Simon criteria. We obtained Duan-Simon criteria of Delta(2)(x(1) - x(2)) + Delta(2)(p(1) + p(2)) = 1.4 < 2 without optimal gain and Delta(2)(x(1) - x(2)) + Delta(2)(p(1) + p(2)) = 1.2 < 2 with optimal gain, respectively. This opens the way to realize both efficient and truly causal EPR paradox.

We report on the generation of quadrature entangled light pulses at the telecommunication wavelength of 1.535 mu m by combining two squeezed pulses. The quadrature phase amplitudes of the entangled light pulses are measured by using two time-domain pulsed homodyne detectors, which yield one pair of measurement values by single-shot measurement for each pulse. The entanglement is confirmed with a sufficient criterion <Delta[(x) over cap (a) + (x) over cap (b)](2)> + <Delta[(p) over cap (a) - (p) over cap (b)](2)> = 0.64 < 1.

We report a pulsed homodyne detector that operates in the time domain and can measure individual pulses at a repetition rate of 76 MHz. The individuality of the measurement is interrogated by correlation coefficient between adjacent pulses. The homodyne detector is composed of a low-noise high-speed operational amplifier. We observed 2.3 dB of squeezing in both the time and frequency domains.

We experimentally observed the time dependence of the spin populations of spin-2 87Rb Bose-Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the stretched states |F=2,m (F) =+2, and |F=2,m (F) =-2 with several varieties of population imbalances. No spin-exchange collisions were observed in a weak magnetic field of 45 mG. The atom loss rate depended on the observed relative population of spin-states. We calculated the loss rate due to two-body inelastic collisions with the population imbalance using an experimentally estimated rate of 17.0(+/- 1.9)x10-14 cm3ss-1supercript stop under the population balance. The calculations were in good agreement with the measurements. Our results show that the dependence of inelastic collisions on spin channels plays an important role in the time-evolution of spin populations.

WO Patent App. PCT/JP2008/064,436

US Patent 7,305,091

US Patent App. 11/725,158

US Patent App. 11/725,158

We proposed a practical implement of continuous-variables quantum cryptography and measured quadrature phase amplitude of weak light fundamental to that. Experimental results with a 10km optical fiber satisfy a necessary security condition against realistic attack.

We propose efficient-phase-encoding schemes for quantum cryptography using coherent states and postselection. This encoding increases the detection efficiency by a factor of 2, at most. The protocol is robust against loss due to the postselection.

We report generation of pulsed squeezed light using a periodically poled LiNbO3 waveguide. The observed fluctuation is -2.7dB below the shot-noise level at the average pump power of 2mW due to optical confinement effect in a waveguide and high peak power of pulsed light.

Two-component Bose-Einstein condensates of 87Rb in |F = 2,m F=-2〈 and |1,-1〈 spin states were created in an optical trap. The evolution of the mixed condensates in the trap was studied.

We observe phase sensitive amplification in a quasi-phase matched LiNbO3 waveguide pumped by the second harmonics of 5-nsec duration pulses at 1535nm.

We experimentally studied the spin-dependent collision dynamics of Rb-87 spin-2 Bose-Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the \F=2,m(F)=0> state, and their time evolutions in the trap were measured in the presence of external magnetic field strengths ranging from 0.1 to 3.0 G. The atom loss rate due to inelastic two-body collisions was found to be 1.4(2)x10(-13) cm(3) s(-1). Spin mixing in the F=2 manifold developed dramatically for the first few tens of milliseconds, and the oscillations in the population distribution between different magnetic components were observed over a limited range of magnetic field strengths. The antiferromagnetic property of this system was deduced from the magnetic field dependence on the evolution of relative populations for each m(F) component.

In this Letter, first, we investigate the security of a continuous-variable quantum cryptographic scheme with a postselection process against individual beam splitting attack. It is shown that the scheme can be secure in the presence of the transmission loss owing to the postselection. Second, we provide a loss limit for continuous-variable quantum cryptography using coherent states taking into account excess Gausian noise on quadrature distribution. Since the excess noise is reduced by the loss mechanism, a realistic intercept-resend attack which makes a Gaussian mixture of coherent states gives a loss limit in the presence of any excess Gaussian noise.

US Patent 6,495,822

US Patent 6,495,822

US Patent 6,495,822

We report an experimental quantum cryptography using balanced homodyne detection over a 1km optical fiber. In this scheme we can measure the distribution of the quadrature amplitudes of the weak coherent pulse.

We have constructed a simple double magneto-optical trap system for a quick creation of a Bose-Einstein condensate. More than 10(9) atoms are continuously accumulated with a resonant cw laser within a few seconds.

The generation of photon-number squeezed states or sub-Poissonian light using semiconductor devices is especially attractive because of its simple experimental configuration, small energy consumption, and the possibility of large amplitude noise reduction by using high quantum efficiency devices. Recently, 3 dB squeezing at liquid nitrogen temperature and 80 MHz (3 dB roll-off bandwidth) squeezing at room temperature were reported by using a light-emitting diode. In this paper, we demonstrate over 200 MHz squeezing at 48 K.

We demonstrate experimentally and numerically that a laser system can be synchronized to any chaotic pulsation intrinsic to the system by modulating a system parameter. A sequence of pulses is prepared, the heights and interpulse intervals of which are artificially constructed from the correlation function of a prerecorded chaotic pulsation. When an on-off modulation signal obtained from this pulse sequence is applied to intracavity absorption, the laser reproduces the chaotic time sequence.

We review our experimental result on the generation of sub-Poissonian photon fluxes from high-speed light emitting diodes (LEDs) driven by high-impedance constant-current sources and present our new scheme on the basis of Stark-effect blockade of photon emissions. The experiments demonstrate a large noise-suppression of spontaneous photon stream, 3.1 dB below the standard quantum limit level at 80 K and increasing bandwidth (3-16 MHz) for the noise-suppression with increasing pump current (50 mu A-5 mA) at room temperature. The latter has been clearly interpreted in terms of the distinctive contribution of collective Coulomb-blockade for pump events and of recombination dynamics to the bandwidth. The basic physics behind the proposed scheme is illustrated, theoretically demonstrating, as a consequence of the coexistence of the Stark-effect-and Coulomb-blockade, 4.5 dB-noise-suppression to below the standard quantum limit level in a tailor-made diode driven by a constant voltage source. (C) 1997 Elsevier Science S.A.

We have investigated the intensity squeezing bandwidth of sub-Poissonian light generated by a high-speed light-emitting diode. The squeezing bandwidth of a constant-current-driven light-emitting diode is proportional to a driving current in a lower-current regime and approaches a constant in a higher-current regime. This dependence on driving current is well explained by the contributions of the thermionic emission time and the radiative lifetime to the squeezing bandwidth. (C) 1997 Optical Society of America.