Quantum Computer

Demonstration of Coupling-controlled Qubits and Future Outlook

Maintaining a quantum superposition state for a long time was said to be difficult in the case of solid-state devices since their interaction with the outside world is considerable. However, on successfully performing an experiment that maintained-though only for a few nanoseconds-a quantum superposition state, we became convinced that we could expect more possibilities from solid-state devices. We decided as our next research target to find out how we could maintain a quantum superposition state for an even longer time. Also, with the aim of building a practical quantum computer in the future, we took up the challenge of developing a mechanism for controlling this quantum superposition state as needed. The result of these efforts was "NEC, JST, and RIKEN Successfully Demonstrate World's First Controllably Coupled Qubits - Newly developed circuit technology enables execution of quantum algorithm -" announced in May 2007.--->Press release

This research result has the following two features:

1)The time for holding a quantum state was dramatically extended

Adopting a "flux qubit" as a quantum bit and devising appropriate fabrication conditions and operating-bias conditions, we extended the time for holding a quantum state by more than two orders of magnitude compared to the "charge qubit" that we had been experimenting with earlier.

2)Control of coupling between two qubits

We investigated new schemes for both the coupling mechanism and coupling control method. For the former, we adopted magnetic coupling using inductance, and for the latter, we devised an original principal that achieves coupling between two qubits through the use of a third qubit.

This new qubit arranged for coupling purposes (qubit 2 in Fig.2) has the same structure as the other two qubits (qubits 1 and 2) but has the role of turning coupling ON and OFF. Specifically, the act of irradiating or not irradiating qubit 3 with a microwave enables the strength of this coupling to be controlled as desired. This parameter was therefore prepared in a manner completely different from that of the other two qubits. In the ordinary state in which no microwave is applied to this coupling-controlling qubit that acts as a "non-linear transformer circuit," coupling between qubits 1 and 2 is essentially OFF. In this OFF state, irradiating qubits 1 and 2 each with resonating microwave pulses makes each of these qubits act independently as a quantum bit gate. On the other hand, applying a microwave pulse of a specific frequency to the coupling-controlling qubit turns coupling between qubits 1 and 2 ON thereby performing 2-qubit gate control. Achieving a coupling circuit by another qubit in this way means that the physical configuration of a quantum computer circuit can be achieved by repeating quantum bits in certain arrangements. Theoretically speaking, scalable circuits that connect a number of quantum bits can be constructed.

Figure 3 shows the results of an experiment performed to demonstrate this concept. The results indicated by the blue line correspond to the transition between the state in which both qubits 1 and 2 are 0 (state 00) and the state in which both are 1 (state 11) due to the coupling by the newly devised coupling-controlling qubit. For example, if pulse application is adjusted to a duration corresponding to the star in the figure, a logical operation called a doubly coupled NOT (DCNOT) gate between qubits 1 and 2 can be achieved. Combining this 2-qubit gate with 1-qubit gates that control individual quantum bits enables an all-purpose quantum-computer gate to be configured that could be used to perform any logical operation.

We have also demonstrated a simple three-step quantum operation protocol as shown by the inserted figure in Fig. 3. These steps consist of two 1-qubit gates and one 2-qubit gate. The red line shows the results of this protocol. The periods of the red and blue lines differ by a factor of two reflecting the quantum-mechanical characteristics of quantum bits. Here, quantum states could be controlled as expected supporting the idea that quantum bits can be controlled correctly.

Theoretically, it is estimated that at least 100 qubits would be needed to construct a quantum computer that can perform complex computations faster than a supercomputer. While there are still many issues to resolve in this research, achieving a quantum computer would mean computing power greater than that of a supercomputer in a low-energy, compact configuration the size of a sugar cube. Future issues that must be addressed include maintaining the quantum superposition state for a longer time, preparing effective algorithms to achieve efficient computations, and determining what kind of computations are most suitable for quantum computers.

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