Quantum chips advance

By Eric Smalley, Technology Research News

Today's rudimentary quantum computer prototypes come in many unusual forms, including laser beams, liquids and sets of single atoms.

Many researchers, however, are trying to make quantum computers that look more like their electronic predecessors. A promising avenue is superconducting circuits, and several research teams have used the technology to form the particle-based qubits that quantum computers use to manipulate the 1s and 0s of computing.

Researchers from the Institute of Physical and Chemical Research (Riken) in Japan and the State University of New York at Stony Brook have entangled a pair of electronic qubits in an integrated circuit. The work is a milestone on the road to chip-based, mind-bogglingly fast quantum computers.

The device "is the first solid-state electronic circuit that is capable of creating entanglement, the most important property required for an efficient quantum computer," said Jaw-Shen Tsai, a research fellow at NEC Fundamental Research Laboratories in Japan and head of the Macroscopic Quantum Coherence Laboratory at Riken.

Entanglement is a weird quantum phenomenon in which two or more particles like atoms or electrons become linked, changing in lockstep regardless of the distance between them. This property is key to the immense power of quantum computing because it allows a quantum computer to check every possible answer to a problem at once.

Classical computers, in contrast, must check each possible answer one at a time. A full-scale quantum computer could solve problems like cracking strong encryption codes that are beyond the reach of even the most powerful possible classical computers.

Quantum computers use opposite states of a particle to represent the 1s and 0s of digital information. An electron, for example, can spin in one of two directions, up or down, similar to a top spinning clockwise or counterclockwise.

When an atom or subatomic particle is isolated from its environment, it enters into superposition, which is a mixture of all possible states. An electron in superposition, for example, is spinning both up and down at the same time. This means that a qubit can represent both 1 and 0 at once, and a long enough string of qubits can represent every possible answer to a problem.

Two or more particles can become entangled when they are in superposition, and they stay entangled as long as they remain in superposition. This is the key to quantum computers' potential for phenomenal speed. A quantum computer can check every possible answer to a problem using a single series of operations across a set of entangled qubits.

The researchers' device is unusual because it can tapped these weird quantum traits on a larger-than-atomic scale.

The device consists of a pair of Josephson junction qubits connected to a capacitor, which briefly stores electric charges. Josephson junctions are tiny breaks in superconducting circuits. Electrons pair up to flow through a superconductor, and billions of these pairs form a single entity that behaves as one giant subatomic particle when the superconductor contains a Josephson junction.

When a Josephson junction circuit is connected to a reservoir of electron pairs, the number of pairs in the reservoir can be changed by exactly one, and this change can be reliably measured. The two states -- the original number of pairs and the original number plus one -- can represent 1 and 0.

And because the electron pairs behave as one entity, they can be in a superposition of the two states, which means they can serve as qubits. Josephson junction qubits are also much larger, and therefore easier to work with, than qubits that are individual particles.

The researchers tested the prototype by using an electric pulse to join the two qubits via the capacitor between them. When they measured the qubits' oscillation frequency they found that when the qubits were joined the oscillation pattern became more complex, a sign of quantum entanglement, said Tsai. "We have observed quantum oscillation in a two-qubit Josephson charge qubit system," he said.

If the researchers' results are confirmed, it would be the first demonstration of entanglement for macroscopic objects in solid-state devices, said Jens Siewert, a staff member of the Institute for Theoretical Physics at the University of Regensburg in Germany. "The [researchers] are careful enough not to claim that they have unambiguously obtained this result, but it is very likely that they did," he said.

Entangling solid-state charge qubits is of utmost importance not only for quantum computation but for understanding quantum mechanics in general, said Siewert. "It is the experiment quite a few people are trying to do at the moment," he said.

The researchers' next step is to make two-qubit logic gates from the Josephson junction qubits, said Tsai. It is likely to take 10 to 20 years before the research can be applied practically, he said.

Josephson junctions only work in temperatures close to absolute zero, so even if large Josephson junction quantum computers can be built they would likely be expensive, specialized systems.

Tsai's research colleagues were Oleg Astafiev of Riken, Yuri A. Pashkin of Riken and the Lebedev Physical Institute in Russia, Tsuyoshi Yamamoto and Yasunobu Nakamura of Riken and NEC Research, and Dima E. Averin of the State University of New York at Stony Brook. The work appeared in the February 20, 2003 issue of Nature. The research was funded by NEC and Riken.

Timeline:   10-20 years
Funding:   Corporate, Government
TRN Categories:   Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Quantum Oscillations in Two Coupled Charge Qubits," Nature, February 20, 2003


March 12/19, 2003

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