Improved device design enables the tunable interaction between the elements of a quantum computer

For fast and efficient operation of quantum computers, control of the interaction between their components, quantum bits (qubits), is necessary. Now, researchers from RIKEN’s Frontier Research System, Wako, the Japan Science and Technology Institute, NEC Corporation and the US Massachusetts Institute of Technology have developed an architecture that allows tunable control over the qubits of a superconducting quantum computer.

Compared with conventional computers, quantum computers hold great promise as a much faster alternative for solving certain mathematical problems, such as the simulation of quantum-mechanical systems. The quantum physical interaction between qubits gives quantum computers their unique attributes. There are a number of possible designs for quantum computers, for example, one uses atoms cooled to very low temperatures. Another possibility that is much easier to realize uses qubits formed by loops of superconducting rings.

Indeed, the team’s demonstration of tunable coupling between the qubits of such a superconducting quantum computer, as reported in the journal Science1, represents a large step towards the realization of complex quantum computers. “Tunable coupling, in particular coupling which can be switched off completely, is highly desirable for implementing large-scale quantum computing,” comments Yasunobu Nakamura from the RIKEN team. Turning off the coupling prevents undesirable interaction between the qubits. Otherwise, cumbersome architectures would be required to compensate for side effects.

The team’s concept of tunable coupling is based on two qubits that are linked via a third passive qubit that mediates the interaction between the active qubits (Fig. 1). However, the internal energy scale of each of the three qubits is different and microwave irradiation at selected energies provides deliberate and controllable coupling between the active qubits. Importantly, the ‘coupler qubit’, with a much higher transition energy, always stays in its ground state and mediates the coupling between the ‘active’ qubits.

To test the new design concept, the researchers implemented a simple quantum computing protocol to determine whether the system could detect a computer hacker attempting to modify the system. Impossible to detect with a classical computer, the team’s quantum computer successfully detected the modified quantum states of the qubits.

While these experiments are an important demonstration of the principle, Nakamura points out that further technical improvements are needed such as “achieving a longer decoherence time and read-out of individual qubits.” With these improvements, the realization of more complex qubit arrays would become feasible, possibly leading to the first large-scale quantum computer.

Reference

1. Niskanen, A. O., Harrabi, K, Yoshihara, F., Nakamura, Y., Lloyd, S. & Tsai, J. S. Quantum coherent tunable coupling of superconducting qubits. Science 316, 723–726 (2007).