A device composed of three qubits coupled in series. The device is made of an aluminum (Al) film and other components. The qubit in the center is used as a tunable coupler.
A device composed of three qubits coupled in series. The device is made of an aluminum (Al) film and other components. The qubit in the center is used as a tunable coupler.
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NEC Corp. announced that it has successfully demonstrated a system using multiple qubits coupled with one other by a tunable coupler that can be turned on/off for the first time in the world toward the practical application of quantum computers. The most remarkable feature of the system is that the coherence time, the qubit lifetime, is prevented from being reduced significantly at the time of coupling the qubits. The company claims that, in principle, the development makes it possible to build a system composed of several tens of qubits. NEC's latest achievement was reported in the May 4, 2007 issue of the US academic journal "Science."

NEC's system is composed of three flux qubits connected with each other. The Josephson junction device is used for these qubits. It is operated in an ultralow temperature at 20 mK. The qubit, measuring several mum on a side, is basically the same as the one announced by the company in 2004. The latest system, however, differs in that only two qubits on both ends are quantum operators while the one in the center is used either as a tunable coupler for the other two, or as a microwave receiver during the two-bit operation.

The initial state of each qubit can be set to "1" by applying a microwave with a specific wavelength and to "0" without the microwave. The tunable coupler is also turned on when such microwave is applied and turned off without it. Moreover, when the coupler is turned on, the system can perform the two-bit control in which the quantum state is transitioned from "00" to "11" and so on as a result of mutual induction phenomenon caused by selecting the specific wavelength of the microwave. The company has demonstrated the bit control of a double controlled NOT (DCNOT) gate.

Further, NEC has reportedly succeeded in operating the three-step bit control by using the latest system. Specifically, one of two qubits is bit-controlled firstly and then two-bit control is performed after turning on the coupling; finally, one-bit control is performed after turning off the coupling. "The result of output was just what we expected," says NEC.

Transition from "device" to "circuit" in quantum computer studies

NEC regards its latest achievement as what has elevated the level of quantum computer studies, whose focus was mainly on the development of various qubits, to the "circuit level" for the first time in the world. The company cites the following two points for the reasons: (1) The new coupler is "tunable," i.e. able to be turned on/off, so that either one- or two-bit control is possible by the same circuit configuration and (2) In case where more qubits are required, the circuit scale can be expanded only by increasing the number of qubits because the coupler itself is a qubit.

NEC stated that it has made a significant achievement in solving the problem in which the coherence time is seriously reduced in the "entanglement state" where multiple qubits are operated in cooperation with one another. "This is because, according to the architecture of our latest system which uses a tunable coupler, a qubit state that can be less influenced by the external noise may be selected during the bit operation," says Mr. Tsai, an NEC researcher, explaining the reason.

Even so, Tsai adds that a problem remains in which "the coherence time is reduced to 1/N when N qubits are coupled." Thus, in order to develop a quantum computer that has a performance superior than that of a supercomputer currently in use, "it is necessary to substantially increase the coherence time of 1-2 qubits," says NEC.

A system composed of 30-50 qubits is required to outperform a supercomputer. However, the coherence time of a single qubit using the Josephson junction device is only about 3 μs at the longest at present. Since the quantum oscillation which is equivalent to the "operating frequency" is around 20 MHz, a single qubit can only handle about 60 arithmetic operations. For example, when a system is made of 50 qubits, the coherence time of each qubit is reduced to 1/50, thereby failing to operate proper arithmetic processing. "We have not discovered the factor which determines the length of coherence time. We're certain that we can cope with the problem once we find it out," says Tsai.