Switcheroo. The presence of a pair of electrons in one superconducting aluminum sliver (Box 1) flips the state of electrons in a second (Box 2), thanks to a quantum link between the two.

New Logic for Quantum Device

A quantum computer built from silicon-style circuits just crept closer to reality, now that physicists have learned to use the quantum state of electrons to perform a basic logical operation.

Decades from now quantum computers may crunch exponentially more numbers than their present-day counterparts. Their power would come from the weird fact that a quantum bit, or qubit, can exist as a 0 and 1 simultaneously. The most basic logical operation involving two qubits is to have one qubit switch the state of the other--or not, depending on its own state. For example, if the state of the first qubit represented 1, the state of the second would flip; but if the state of the first qubit were 0, the second would stay the same.

Such gates have been crafted already from qubits made of atoms. Physicists would like to do the same with easier-to-manufacture "solid-state" devices. In one such design, a superconducting circuit can enter a state in which an aluminum sliver, called a Cooper pair box, simultaneously does and does not contain an extra pair of electrons. Japanese researchers reported in February that a sequence of voltage pulses could entangle two such qubits, meaning that their two quantum states--Cooper pair present or not--mixed together (ScienceNOW, 22 February). However, the simplified controls couldn't put the qubits in all possible states.

Now the same group has taken the next step. In their new device, if the control qubit lacks a Cooper pair, a voltage pulse to the second qubit switches its state. If the control has a Cooper pair, the second qubit feels this presence through the entanglement and the pulse's effect is blocked. The key was adding a second voltage source so they could manipulate each qubit separately, report Tsuyoshi Yamamoto of the NEC Fundamental Research Laboratories in Tsukuba and the Institute of Physical and Chemical Research in Wako, Japan, and colleagues in the 30 October issue of Nature.

"The beautiful experiment they have performed is the first demonstration of a QUANTUM gate operation with a solid-state circuit," says Michel Devoret of Yale University, who works on a different type of superconducting qubit. "This is a remarkable step for quantum physics in general." The next step, he says, is to improve the success rate of the switch from the current 60%, by entangling the solid-state qubits completely.

Related sites
Institute of Physical and Chemical Research qubit page
National Academy of Engineering journal report on solid-state qubits