What is Quantum Information?

Quantum information means information processing based on the rules of quantum mechanics. Things in the microscopic-world, such as atoms, electrons and photons, obey rules different from those of things that we are familiar with. In our everyday life, information is processed no matter how it is presented. We can calculate 1+1 in an identical manner using either apples or abacus beads. This is because the rule for expressing and operating the information is identical in either case. If the rule changed with the representation of the information, it would not be unusual for what is impossible with one form of information becoming possible with another. However, it is not interesting if such a wonderful rule cannot be realized. Quantum Information is realizable with a device made of microscopic materials and provides a new rule in information processing. Since its discovery, active research has been done on what kind of information processing we can do efficiently based on the rules of quantum mechanics and how we can fabricate an information processing device.
It is believed that quantum information will enable what the conventional information technology cannot realize. For example, a quantum computer can decrypt in a short period of time the code that cannot be decoded today’s most powerful computers. Quantum Cryptography also realizes a secure code that can never be eavesdropped on. The two statements seem inconsistent, but remember that I didn’t say the quantum computer can break any codes. An analysis based on quantum information tells us that the quantum cryptography beats the strongest code-breaker equipped with a quantum computer. The situation is different when we consider the game between code creators and breakers. Conventional information technology cannot deny the possibility that a previous loser might be the next winner, but quantum information can assert the result in principle. This is an advantage of quantum information.

 

What Kind of Research are You Doing?

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I am conducting research toward the realization of quantum information technology. The core of this research is divided into two aspects: the practical implementation of quantum cryptography (particularly quantum key distribution, QKD.) and basic research on new quantum information processing with the use of light.
In fact, QKD is already close to practical application. An experiment to verify its application by actually using an existing fiber optic network (Tokyo QKD Network) is being conducted in cooperation with national research institutions, companies and universities. Our group conducts research to certificate the secure cryptographic key that can be created on an actual device. Our goal is to fill a gap between theory and practice. Recently, we developed a measurement method of coherence of the light source, which is one of characteristics of the semiconductor laser used in QKD systems. At the same time, we clarify how the deviation of the actual coherence value from that assumed by theory affects the security of QKD. We have just been relieved to find that the observed gap level does not cause any problems with the practical application However, we also discovered that attention needs to be given to a higher-speed device in the future.
The devices for QKD systems were originally developed for optical fiber communication, which is mainstream for current long-distance communication. As shown in the photographs of the measurement system and QKD system, a number of components and instruments for optical fiber communication are used in the laboratory. We also study the components for QKD systems, such as the light source, modulators, avalanche photodiodes, a random number generator, and the key-distillation method, in order to improve the transmission distance and key generation speed.

Actual quantum key distribution system and its component (offered by NCT and NEC)

In comparison to quantum cryptography, there are many problems to be solved for the realization of a quantum computer. A gap between theory and practice appears again; quantum mechanical characteristics may be lost easily in a large system, if not handled carefully. A quantum computer is more complicated and requires more quantum circuit elements than a QKD system, and therefore it is difficult to realize.  I believe that it is necessary to not only improve fabrication and control technology on quantum devices, but also explore various possibilities of the operating principle for quantum computation. Our approach is to find applications that are enhanced by using quantum information technology in lieu of conventional information processing. We would like to find them with a small scale quantum circuit, and not necessarily target a quantum computer in one fell swoop. We are investigating new ideas about a control method of the quantum states and a connecting method between quantum circuit elements required for that purpose, and looking at its feasibility.

 

What Will be Your Next Goal?

We are planning to develop a new method that will significantly improve performance of QKD. The principle of secure communication is to create a situation where legitimate users have the advantage over eavesdroppers. It is necessary to create this type of situation with as few conditions as possible to improve the performance. Moreover, there is a possibility that quantum cryptography can be applied to communication other than fiber communication, for example, intersatellite optical communication. We are beginning basic investigation in that direction.

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In addition, we are also planning to conduct research on a quantum repeater to exceed the current limit of the transmission distance in QKD communication. Technology to realize the quantum repeaters is also useful for the development of a quantum computer. Therefore, we would like to expand the research to quantum information network using light in a broad sense, that is, a distributed quantum information processing combing quantum computers with quantum secure communication. We will work on development of quantum information processing technology that can be used even at home and our goal is to find more applications for quantum networks using light to create a bright future