Takuya Hirano

Quantum cryptography using pulsed homodyne detection

We report an experimental quantum key distribution that utilizes pulsed homodyne detection, instead of photon counting, to detect weak pulses of coherent light. Although our scheme inherently has a finite error rate, homodyne detection allows high-efficiency detection and quantum state measurement of the transmitted light using only conventional devices at room temperature. Our prototype system works at 1.55 {mu}m wavelength and the quantum channel is a 1-km standard optical fiber. The probability distribution of the measured electric-field amplitude has a Gaussian shape. The effect of experimental imperfections such as optical loss and detector noise can be parametrized by the variance and the mean value of the Gaussian distribution.

Practical limitation for continuous-variable quantum cryptography using coherent States.

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 Gaussian 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.

Efficient-phase-encoding protocols for continuous-variable quantum key distribution using coherent states and postselection

We propose efficient-phase-encoding protocols for continuous-variable quantum key distribution using coherent states and postselection. By these phase encodings, the probability of basis mismatch is reduced and total efficiency is increased. We also propose mixed-state protocols by omitting a part of classical communication steps in the efficient-phase-encoding protocols. The omission implies a reduction of information to an eavesdropper and possibly enhances the security of the protocols. We investigate the security of the protocols against individual beam splitting attack.

Security of quantum cryptography using balanced homodyne detection

In this paper we investigate the security of a quantum cryptographic scheme which utilizes balanced homodyne detection and weak coherent pulses (WCP). The performance of the system is mainly characterized by the intensity of the WCP and postselected threshold. Two of the simplest intercept and resend eavesdropping attacks are analyzed. The secure key gain for a given loss is also discussed in terms of the pulse intensity and threshold.

Wide-band suppression of photon-number fluctuations in a high-speed light-emitting diode driven by a constant-current source

We demonstrate experimental results on the generation of sub-Poissonian photon fluxes emanating from an AlGaAs light-emitting diode, which manifest a wide-band (0–100 MHz) noise suppression below the standard quantum limit level despite low current density (∼38 A/cm2) operation at room temperature. The experimental noise power spectrum is well fitted in terms of the theoretical curve estimated with the quantum mechanical Langevin equations.

Pulse-resolved measurement of quadrature phase amplitudes of squeezed pulse trains at a repetition rate of 76 MHz

We report pulsed homodyne detection of squeezed pulses at a repetition rate of 76 MHz. Measurement of individual pulses, which were interrogated by the correlation coefficient between adjacent pulses, was realized. A homodyne detector was constructed using a low-noise, high-speed operational amplifier; it had a usable bandwidth of 250 MHz. We observed 2.3 dB of squeezing in both the time and frequency domains.

Spin-dependent inelastic collisions in spin-2 Bose-Einstein condensates

We studied spin-dependent two-body inelastic collisions in

Observation of quadrature squeezing in a χ (2) nonlinear waveguide using a temporally shaped local oscillator pulse

F=2

3 dB Wideband squeezing in photon number fluctuations from a light emitting diode

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