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Quantum Computer

Basic principles of quantum computers and differences with existing computers

OK, let's move on to the main topic of this report. In this section, I will introduce the basic principles behind quantum computers.

In existing computers, all information is expressed in terms of 0s and 1s, and the entity that carries such information is called a "bit." Physically speaking, a bit of information can be expressed, for example, by the on/off states of a transistor, the presence/absence of charge in a memory cell, the magnetization direction of a magnetic body on a hard disk, etc. A bit can be in either a 0 or 1 state at any one moment in time.

A quantum computer, on the other hand, uses a quantum bit or "qubit" instead of a bit. A qubit also makes use of two states (0 and 1) to hold information, but in contrast to a bit, it also makes use of a "superposition state" that I mentioned earlier. In this state, a qubit can take on the properties of 0 and 1 simultaneously at any one moment. Accordingly, two qubits in this superposition state can express the four values of 00, 01, 10, and 11 all at one time and N such qubits can express the 2N values from 00…00 to 11…11 all at once as shown in the figure. In a quantum computer, using such a superposition state as input can lead to simultaneous parallel processing of 2N values that are being expressed simultaneously by N qubits. This makes it possible to perform high-speed computations at a level significantly higher than that of existing computers. Roughly speaking, if an existing computer must perform a total of 2N processes one at a time with respect to the values from 00…00 to 11…11, a quantum computer can operate on all of those values in a superposition state at one time. Multiple qubits may also enter an "entangled state," which is another type of state particular to quantum mechanics. The entangled state also plays a major role in quantum computing, and I will explain it a bit later. Furthermore, to derive the results of parallel computations in a quantum computer, a computation algorithm is needed in addition to preparing qubits. In this article, however, I would like to focus on qubits, a major achievement of our research at NEC.

Difference between Classical Bit and Quantum Bit

We may now ask "What is the best way of creating a quantum computer or giving a qubit form?" Let me first point out that almost all existing computers consist of semiconductor devices made up of field-effect transistors on silicon. How can a quantum computer be configured? At the present stage of quantum-computer research, the following means of configuring qubits are considered as candidates for the components of a quantum computer.

  1. Using a solid-state device:
    There are two well-known qubits of this type. The first is a qubit achieved by a superconducting circuit using a Josephson junction that creates a weak bond between two superconductors. The second is a qubit achieved by a semiconductor quantum dot, which is a structure from 10 to several hundred nanometers in size for confining an electron. There is also a method that uses the nuclear spin of phosphorus impurities in silicon as a qubit.
  2. Using an ion:
    This method uses two electron orbits of an ion (charged atom) trapped within an electromagnetic field in a vacuum to form a qubit (ion trap method).
  3. Using a molecule:
    This method uses the up and down directions of the nuclear spins of molecules in a liquid to form a qubit. Here, the expression "nuclear spin" indicates that the atomic nucleus moves in a manner similar to the rotation of the earth. Technology for controlling and measuring this spin is commonly called nuclear magnetic resonance (NMR). In 2001, IBM announced the successful development of a 7-qubit system using this method.
  4. Using a photon:
    One method of this type uses the interaction between an atom and photon in a resonator, and another uses optical devices such as a beam splitter, mirror, etc.

Each of these approaches has advantages and disadvantages, but the first one outlined above, which uses a solid-state device (to which semiconductor LSI manufacturing technology can be applied), is generally considered to be essential for achieving greater integration that is required for a practical quantum computer in the future. NEC, together in collaboration with RIKEN, is focusing our R&D efforts on a "qubit using a Josephson junction," one of the methods using a solid-state device.

There are actually two types of qubits formed by a Josephson junction. One is a "charge qubit" that creates superposition states by manipulating the number of Cooper-pairs in a superconducting electrode. The other is a "flux qubit" achieved by manipulating the number of fluxes that arise due to the flow of superconducting current in a superconducting loop. While NEC is researching both of these qubit types, the qubit research that Yasunobu Nakamura will introduce in the following section deals with the formation of charge qubits.