Takahiro Serikawa

Creation and measurement of broadband squeezed vacuum from a ring optical parametric oscillator

We report a 65 MHz-bandwidth triangular-shaped optical parametric oscillator (OPO) for squeezed vacuum generation at 860 nm. The triangle structure of our OPO enables the round-trip length to reach 45 mm as a ring cavity, which provides a counter circulating optical path available for introducing a probe beam or generating another squeezed vacuum. Hence our OPO is suitable for the applications in high-speed quantum information processing where two or more squeezed vacua form a complicated interferometer, like continuous-variable quantum teleportation. With a homemade, broadband and low-loss homodyne detector, a direct measurement shows 8.4 dB of squeezing at 3 MHz and also 2.4 dB of squeezing at 100 MHz.

500MHz resonant photodetector for high-quantum-effciency, low-noise homodyne measurement.

We design and demonstrate a resonant-type differential photodetector for a low-noise quantum homodyne measurement at 500 MHz optical sideband with 17 MHz of bandwidth. By using a microwave monolithic amplifier and a discrete voltage buffer circuit, a low-noise voltage amplifier is realized and applied to our detector. 12 dB of signal-to-noise ratio of the shot noise to the electric noise is obtained with 5 mW of a continuous-wave local oscillator. We analyze the frequency response and the noise characteristics of a resonant photodetector, and the theoretical model agrees with the shot noise measurement.

Optical quantum information processing and storage

Here we report our recent experimental progresses in optical quantum information processing. In particular, the following topics are included. First, we extend the heralding scheme to multi-mode states and demonstrate heralded creation of qutrit states. Next, we demonstrate storage of single-photon states and synchronized release of them. Then, we demonstrate real-time acquisition of quadrature values of heralded states by making use of an exponentially rising shape of wave-packets. Finally, we demonstrate cluster states in an arbitrarily long chain in the longitudinal direction.

Quantum information processing with a travelling wave of light

We exploit quantum information processing on a traveling wave of light, expecting emancipation from thermal noise, easy coupling to fiber communication, and potentially high operation speed. Although optical memories are technically challenging, we have an alternative approach to apply multi-step operations on traveling light, that is, continuous-variable one-way computation. So far our achievement includes generation of a one-million-mode entangled chain in time-domain, mode engineering of nonlinear resource states, and real-time nonlinear feedforward. Although they are implemented with free space optics, we are also investigating photonic integration and performed quantum teleportation with a passive liner waveguide chip as a demonstration of entangling, measurement, and feedforward. We also suggest a loop-based architecture as another model of continuous-variable computing.

Continuous-variable quantum optical experiments in the time domain using squeezed states and heralded non-Gaussian states

Continuous-variable quantum information processing with optical field quadrature amplitudes is advantageous in deterministic creation of Gaussian entanglement. On the other hand, non-Gaussian state preparation and operation are currently limited, but heralding schemes potentially overcome this difficulty. Here, we summarize our recent progress in continuous-variable quantum optical experiments. In particular, we have recently succeeded in creation of ultra-large-scale cluster-type entanglement with full inseparability, multiplexed in the time domain; storage and on-demand release of heralded single-photon states, which is applied to synchronization of two heralded single-photon states; real-time quadrature measurements regarding non-Gaussian single-photon states with exponentially rising wavepackets; squeezing with relatively broader bandwidth by using triangle optical parametric oscillator.