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OCTOBER 2007 Volume 2 Number 10 Qubits switch on and off HIGHLIGHT OF THE MONTH Tuning into quantum computers RESEARCH HIGHLIGHTS Superconducting cement Molecules switching position Laser light switches molecular bonds Halting the infl ammation overload A better way to make a muscle? Observing single cells on the move New scaff old supporting our molecular understanding of psychiatric disorders Wrapping up a cell biology riddle Enzymes lead the way Surviving separation FRONTLINE Towards the creation of Astro Boy’s brain ROUNDUP RIKEN leads project to build world’s fastest supercomputer HISTORY OF RIKEN Opening to the outside world www.rikenresearch.riken.jp © 2007 RIKEN cover1.indd 3 07.10.2 5:00:33 PM HIGHLIGHT OF THE MONTH VOL. 2 | NUMBER 10 | OCTOBER 2007 Tuning into quantum computers Improved device design enables the tunable interaction between the elements of a quantum computer a qubit 1 qubit 3 qubit 2 b Science For fast and efficient operation of quantum computers, control of the interaction between their components, the 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 132 allows tunable control over the qubits of a superconducting quantum computer. 1μm Their study represents a large step towards the realization of large-scale quantum computers with controllable Figure 1: Tunable superconducting qubits. a) Schematic depiction of the system where the qubits are formed by superconducting loops interrupted by tunnel junctions (red stripes). Qubits 1 and 2 are controlled by microwave qubit interactions. pulses (blue arrows). The tunable interaction mediated via a passive coupler (qubit 3) is switched on only when the coupler is irradiated with microwaves at a particular frequency (green arrow). Tunable qubits play an important role Quantum computers are different in applications such as the simulation of quantum-mechanical systems. b) Photograph of the fabricated device. Compared with conventional computers, quantum computers hold great promise as a much faster alternative for solving this, the atoms are suspended in vacuum magnetic fields within these rings then certain mathematical problems, such as and lined up by complex electromagnetic are able to generate electrical currents the simulation of quantum-mechanical fields. Although larger systems of qubits across these Josephson junctions. systems. Another example is the composed of atoms have been successfully When exposed to the right magnetic factorization of integer numbers into demonstrated, such schemes remain fields, a stable situation is reached that the products of prime numbers—an relatively cumbersome. On the other balances the current flowing clockwise operation that would require a huge effort hand, solid-state based systems offer and anti-clockwise through the rings. by conventional computers and would desirable ease of fabrication—echoing This renders the rings insensitive to take considerably longer to complete. the success of conventional silicon-based small variations in the magnetic field The interaction between the qubits computing—and are therefore intensively and optimizes their use as qubits. Of gives quantum computers their unique pursued worldwide. importance to the performance of attributes, and it would take a significantly A representative type of qubit for solid- quantum-computing tasks is the quantum- larger number of bits from conventional state based quantum computers is formed mechanical coupling of neighboring rings. computers to even simulate the operation by loops of superconducting rings made of a quantum computer. from aluminum (Fig. 1). At certain points Controlling coupling between qubits There are a number of possible designs the rings are made insulating through While the interaction between the qubits for quantum computers. One example uses oxidation of the aluminum, which creates of a quantum computer is important, good atoms cooled to very low temperatures. For so-called Josephson junctions. Tiny control over this coupling is essential. 1 | RESEARCH www.rikenresearch.riken.jp © 2007 RIKEN VOL. 2 | NUMBER 10 | OCTOBER 2007 HIGHLIGHT OF THE MONTH Science Large-scale quantum computation These experiments are an important quantum measurement demonstration of the principle, yet further improvements are needed. For example, longer lifetimes of the quantum states would allow for better and more complex quantum operations. These longer lifetimes can be achieved by improving the stability Signal (arb. unit) Signal (arb. of the quantum states towards, for example, fluctuations in the external magnetic field. This example of microelectronics technology by the team shows that large- classical measurement scale integration requires precise control of each individual element. Indeed, 0 2π 4π 6π Nakamura points out that the team “hopes to demonstrate that we can precisely control and Operation angle characterize the quantum state of a large-scale Figure 2: Example of the detection of a computer hacker. The modifications made by the hacker to the state of qubits artificial quantum system.” Eventually, such cannot be detected classically but lead to an altered output in quantum circuits. improvements could lead to the first large- scale quantum computer. “Tunable coupling, in particular coupling the large energy difference. Therefore, the 1. Niskanen, A. O., Harrabi, K., Yoshihara, F., which can be switched off completely, is ‘coupler qubit’ merely mediates the coupling Nakamura, Y., Lloyd, S. & Tsai, J. S. Quantum highly desirable for implementing large- between the ‘active’ qubits. The microwave coherent tunable coupling of superconducting scale quantum computing,” comments radiation is fed to the qubits via an on-chip qubits. Science 316, 723–726 (2007). Yasunobu Nakamura from the RIKEN transmission line, which is readily scalable to About the researcher team. Turning off the coupling prevents a higher number of qubits. undesirable interaction between the qubits. Yasunobu Nakamura was born in Osaka, Otherwise, as Nakamura points out, “the Detecting hackers Japan, in 1968. He graduated from the qubits would interact with each other all the To experimentally apply the new design Faculty of Engineering, University of Tokyo, time and even a simple decomposition of concept, the researchers implemented a in 1990, and obtained an MSc in 1992 from the signal would be difficult.” Cumbersome simple quantum computing protocol to the Superconductivity Research Course of the same university. Immediately after that, architectures would be required to determine whether the system could detect he joined NEC corporation, in Tsukuba, Japan, compensate for such undesired side effects. a computer hacker attempting to modify where he started his career in mesoscopic Therefore, the team’s demonstration of the system. With a classical computer, such physics. Since the late 1990s he has been tunable coupling between the qubits of such infiltrations are impossible to detect. working on superconducting devices for a superconducting quantum computer, as If the hacker altered the quantum state of quantum information processing. Nakamura reported in the journal Science1, represents the coupled qubits by using the same tunable also spent one year as a guest researcher an important step towards the realization of interaction described above, it would lead to at TU Delft in Delft, the Netherlands, from complex quantum computers. a periodic modulation of the quantum state 2001 to 2002. Now he is a research fellow at In this approach, tunable coupling is based with a periodicity of 4π inherent to such the Nano Electronics Research Laboratories on two qubits that are linked via a third and quantum-mechanical manipulation. However, of NEC Corporation as well as a researcher at passive qubit that mediates the interaction classical measurements cannot detect the the RIKEN Frontier Research System. between the active qubits (Fig. 1a). subtle change and results in observation of a Importantly, the internal energy scale of 2π periodicity. each of the three qubits is different. While The test measurements performed by the two active qubits have only marginally the researchers are shown in Figure 2. In an different energy spectra, the mediating experiment simulating a classical computer, passive qubit has a larger energy scale. the 2π periodicity is seen (blue curve) and Microwave radiation at selected energies they could not distinguish whether the hacker then provides deliberate and controllable had modified the state or not. However, the coupling between the energy states of the quantum measurement (red curve) clearly active qubits, whereas the energetic states of reflects the doubled 4π period, which clearly the passive qubit remain unaffected due to indicates the presence of the hacker. www.rikenresearch.riken.jp RESEARCH | 2 © 2007 RIKEN RESEARCH HIGHLIGHTS VOL. 2 | NUMBER 10 | OCTOBER 2007 Superconducting cement Researchers find superconductivity in a material typically used as cement The metal oxide known as 12CaO∙7Al2O3 (C12A7) is often used to make aluminous Letters Nano cements. Researchers from RIKEN’s Discovery Research Institute in Wako Al and the Tokyo Institute of Technology have now found superconductivity in the metallic version of C12A7. The crystal structure of C12A7 consists of a number of cages formed by calcium, aluminum and oxygen atoms (Fig. 1). O Typically,