# Quantum Optical Engineering (Kitano Lab)

Both "light" and "matter" (such as atoms and molecules) have quantum properties, and show quantum "superposition" and "entangled" states that cannot be explained by classical physics theory. If we can find a way to exploit these purely quantum states, it would be possible to realize functions that had previously been inconceivable. A typical example of such an application would be quantum measurements as well as quantum computers and quantum information and communications, which have been attracting great interest in recent years. In addition, the exploration of quantum properties has substantial value in itself, and research aimed at gaining a basic understanding of the essence of quantum mechanics is also vitally important.

In addition, if quantum phenomena are utilized effectively, it may be possible to conduct unimaginably high-precision measurements. The extremely high precision of one such application—atomic clocks, whose operation is based on the interaction between atoms and electromagnetic waves—is utilized in real-world, day-to-day applications such as GPS devices. We are pursuing research that explores further improving this precision, by using laser light to attempt to create an atomic clock that remains accurate to within one second over vast cosmological spans of time. We are also studying the development of an optical frequency synthesizer that references this kind of super-stable laser frequency to freely generate laser light and microwaves of different wavelengths with ultra-high precision.

As described above, our lab studies quantum phenomena in terms of both pure research and applications development, and both experimentally and theoretically (including computer simulations).

## Academic Staff

### Masao KITANO

Professor (Graduate School of Engineering)

**Research Interests**

**Contacts**

TEL: +81-75-383-2321

FAX: +81-75-383-2324

E-mail: kitano＠kuee.kyoto-u.ac.jp

### Kazuhiko SUGIYAMA

Associate Professor (Graduate School of Engineering)

**Research Interests**

Quantum Electronics, Quantum Measurement

Ultra-high-precision measurement using quantum phenomena, including

- Laser (optical) atomic clocks and quantum computers with single or array of ions.
- Optical and microwave frequency synthesizers with ultra-fast mode-locked lasers.

**Classes**

- Electronics
- Quantum Measurement (Grad. School)

**Contacts**

Kyoto Univ. -Katsura, A1-124

TEL: +81-75-383-2322

FAX: +81-75-383-2324

E-mail: sugiyama kuee.kyoto-u.ac.jp

### Toshihiro NAKANISHI

Assistant Professor (Graduate School of Engineering)

**Research Interests**

**Contacts**

TEL: +81-75-383-2323

FAX: +81-75-383-2324

## Introduction to R&D Topics

### Single or string of ions in traps

A single ion confined by an electric field within a small region with an ultra-high vacuum can be maintained at rest by laser cooling. Since the single ion is isolated from its surroundings and exhibits the essential characteristics of an ion, it is possible to determine the frequency of the light the ion absorbs with extremely high accuracy. By stabilizing the frequency of a laser using this as a reference frequency, we are doing research aimed at developing an extremely accurate atomic clock that operates in the visible region. We have also commenced research aimed at developing a quantum computer using a small number of ions that have been cooled down to crystallize into a string.

### Optical frequency synthesizers

Ultra-short pulsed lasers known as "mode-locked lasers" can emit light pulses at time intervals of extremely high accuracy. If a Fourier transform is applied to the frequency axis of the signal, the result is equivalent to an aggregate of a large number of continuous-wave lasers oscillating at different frequencies at a fixed frequency interval. This waveform is known as an optical-frequency comb due to its shape, and it can be used like the graduations of a ruler for measuring optical frequency. Furthermore, if these light pulses are propagated along a special optical fiber known as a photonic crystal fiber, the spectral bandwidth of the optical frequency comb expands to more than one octave. Using an optical frequency comb having a spectral bandwidth of more than one octave it is possible to correlate the frequency of each mode (visible region) of the comb with the comb's mode interval (microwave region), thereby enabling the frequencies of visible light and microwaves to be directly compared. Our research in this area is focused on optical frequency measurement systems based on GPS clocks, optical frequency synthesizers that generate laser outputs of arbitrary wavelength, and the development of stable, accurate microwave sources of accurate division of laser frequencies of which stability are superior to those of masers. We are particularly focused on applications such as the development of small atomic clocks (laser atomic clock) utilizing semiconductor lasers.

### Simulation of wave propagation using metamaterials/electronic circuits

The response of materials that exist in the natural world to magnetic fields is extremely small, and the relative permeability of natural materials relative to microwave or light is effectively considered to be 1. However, the magnetic response of artificially created structural materials, known as "metamaterials," can be varied very significantly. Metamaterials can exhibit peculiar characteristics, such as negative refractive indices, whereby electromagnetic wave is bent in a "<" shape at the interface of the material. Our lab is fabricating metamaterials that have magnetic resonance, and we are investigating their characteristics.

In addition, we are studying methods for simulating a variety of wave propagation phenomena by using electronic circuits.

### Photon-pair formation and its applications

It is known that according to quantum mechanics, a correlation (known as "entanglement") exists even between remote systems. This matter of quantum correlation once puzzled the minds of many scientists, and was known as the "Einstein, Podolsky, and Rosen paradox". Later, however, it was proved that it is not actually a paradox (although it is still counterintuitive), but rather a phenomenon that must actually exist. The proof involved two photons having a quantum correlation known as a "photon pair." Currently, this quantum correlation is being used as a device in a variety of trials aimed at practical applications such as the development of new techniques for information processing and optical measurement. Our lab is pursuing research into the generation of high-efficiency photon pairs using parametric down-conversion (three-wave mixing) of quasi-phase matched crystal and into the generation of photon pairs using four-wave mixing in photonic crystal fibers, as well as on two-photon absorption using the generated photon pairs.