RESEARCH

Here you can see some of our past and ongoing research projects.

Our projects are categorized as

Development of quantum optical devices.
Fundamental study of quantum physics.
Demonstration of new quantum-enhanced and -assisted technologies.

Those categories are not completely independent; for example, an advanced quantum device can be applied to new fundamental research, and vice versa.

Quantum optical devices

A single-photon state — a quantum state of light having one and only one photon — is a fundamental resource for quantum information technologies. Generation, manipulation, and detection of photons with high efficiency and precision are required for studying quantum physics and developing quantum technologies. Since a single photon has a quantum mechanically minimum brightness (energy), photons must be handled with very low loss as opposed to classical light (e.g., laser light). Moreover, photons need to be quantum mechanically pure and identical (indistinguishable) from each other. Indistinguishable single photons can exhibit quantum interference, fundamental in quantum optical gate operations.

We develop sources of photon pairs using spontaneous parametric downconversion (SPDC), where one photon in an optical pulse is split into a pair of photons. Using techniques of manipulation of entanglement (i.e., quantum correlation of a photon pair) in the SPDC process, we have demonstrated optimized sources for various quantum applications (e.g., a low-loss frequency un-entangled photon-pair source). To characterize SPDC sources with a high precision, we are developing spectroscopy techniques using quantum and classical optics.

SPDC sources are widely used in quantum optics experiments. However, the generation probability of a single photon pair in the SPDC process is intrinsically limited (< 25%) due to multi-pair generation. To overcome the inefficiencies and loss of photons, we study photon storage techniques. Our all-optical storage scheme enables us to store a single photon (produced with a low probability) and to release it at a desired time. We are also developing electro-optic and all-optical switching techniques to manipulate a state of single photons with high speed and low loss.

Fundamental studies using photons

The concept of a photon was important for understanding quantum phenomena such as black-body radiation and the photo-electric effect. Due to the stability in ordinary temperatures and matured (classical) optical technologies, photons are a useful tool to explore the quantum world.

We have tested the quantum uncertainty relation in the measurement of photons. By implementing a quantum indirect measurement apparatus using entanglement, a trade-off relation of quantum-mechanically defined measurement error and disturbance is investigated by the three-state method and the weak measurement method. We also study novel two-photon interference, which cannot be explained by the classical theory of light. We are currently investigating single-photon nonlinear optics, multi-photon interference, and measurement of joint observables for efficient measurement of quantum systems.

Quantum technologies

Photons are expected as an essential resource (qubits) for quantum communication, quantum computing, and quantum metrology. We have demonstrated the quantum-memory-assisted quantum key distribution, which has security against detector side-channel attacks and an enhanced secure key generation rate. We also develop high-efficiency sources and detectors of multi-photon entanglement to study quantum-enhanced optical measurements.