# Quantum Information Science

## Quantum Information Science

### Theoretical Research on Information Processing

Based on Principles of Quantum Mechanics

By putting information on photons, atoms, etc, and controlling them precisely and individually, various information processing schemes that are currently impossible will become possible. Quantum information science is expected to be an important field in information processing in the near future. Our mission is to find innovative theoretical principles of quantum information science that make the impossible possible.

### Interdisciplinary Field of Information Science and Quantum Physics

Quantum information science is an interdisciplinary field that combines information science and quantum physics. Cooperation between software and hardware researchers is critical. NTT undertakes strategic research on quantum information science at the NTT Communication Science Labs. (CSL) and NTT Basic Research Labs. (BRL), which promote software and hardware research, respectively.

*Our research has three phases: Elucidation of the Nature of Quantum bits, Development of Quantum Algor* and *Implementation of Quantum Operations.* The results of this research are fed back to the hardware research at the NTT Basic Research Labs., and their research contributes to our next research cycle.

### Elucidation of nature of quantum bits

One of the biggest differences between classical and quantum information is related to copying; quantum bits (qubits) cannot be copied perfectly, although classical bits can. On the other hand, we can imperfectly copy quantum bits. For practical purposes, we must know how precisely we can copy quantum bits. We faced the challenge and successfully derived an ultimate theoretical limit for their precise copying.

### Development of Quantum Algorithms

With the help of the unique properties of quantum bits, we can sometimes achieve significant efficiency in computing that cannot be reached with current computing principles. For instance, the leader election problem, a fundamental problem in distributed computing, is unsolvable on current computer networks under certain general conditions. But we showed a quantum algorithm that solves this problem on quantum computer networks.

### Implementation of Quantum Operations

Measurement-based quantum computation is a promising computational model for realizing a quantum computer. In this model, various kinds of measurements are used for implementing quantum operations. To enhance its feasibility, we must construct a procedure that implements quantum operations using small measurements. We devised a procedure that uses measurements smaller than those in the previous procedure.