Electron teams make bigger qubits

By Eric Smalley , Technology Research News

One of the biggest challenges in building quantum computers is making quantum bits that are small enough to have the requisite quantum behavior, yet large enough to be reliably controlled by electronic circuits.

Quantum bits, or qubits, use traits of particles like electrons or photons to represent the 1s and 0s of computing. An electron can serve as a qubit because it is oriented in one of two directions, spin up and spin down.

Researchers from the University of Basel in Switzerland and the University of Pittsburgh have come up with a candidate qubit made from groups of electrons rather than harder-to-control single electrons.

The researchers have shown that as long as a spin cluster is made up of odd numbers of electrons it can behave like a single electron, according to the Florian Meier, a researcher at the University of Basel.

The method can potentially produce qubits that are relatively easy to control.

Spin clusters are groups of electrons that are close enough to each other that their spins are aligned. In cases where spin alignment is antiferromagnetic, meaning the magnetic orientations alternate from one electron to the next, spins from an even number of electrons cancel each other out and for odd numbers of electrons there is a net spin equivalent to the spin of one electron.

Electron spins are promising candidates for qubits because they can be built into computer chips, they are relatively well insulated from environmental disturbances like electronic noise and heat, and existing techniques allow electron-spin qubits to be controlled by magnetic and electric fields.

In practice, however, controlling magnetic and electric fields at the scale of individual electrons is extremely challenging, said Meier. The researchers' method eases the burden by widening the focus to a set of electrons rather than just one. "The conditions on local control of electric and magnetic fields are substantially relaxed," said Meier. "For quantum computing with electron spins in quantum dots, magnetic and electric fields need not be controlled on the length scale of 50 nanometers, but only on typical scales of 250 nanometers."

The placement of the spins and the size of the cluster can also vary considerably, he said.

Quantum computers gain their power from the weird traits of particles like electrons. When an electron is isolated from its environment, it enters into superposition, which is some mix of spin up and spin down. This allows a long enough string of qubits to represent every possible answer to a problem. The power of a quantum computer comes from being able to check all of the possible answers using a single set of operations instead of having to checking them one by one as is done by classical computers.

Quantum computers based on spin cluster qubits would work the same way as quantum computers made of single-spin qubits, said Meier. "Although the cluster is composed of many spins, with respect to its magnetic properties the large cluster behaves very similarly to a single electron spin," he said.

The researchers have shown theoretically that spin cluster quantum computers can use the same techniques for initialization, gate operation, error correction and readout as quantum computers that use single electron spins.

Spin-cluster-qubits can be made using any of a wide range of artificial magnetic molecules that have been synthesized during the past decade, said Meier.

Though such spin cluster hardware would be smaller than quantum dots, which are microscopic bits of semiconductor material used to trap electrons for some quantum computing schemes, they are easier to produce, he said. "Nature provides identical copies of these systems."

The researchers' next step is to form one-and two-qubit quantum gates using spin cluster qubits, said Meier. The main challenge in making practical spin cluster qubits is developing a method for measuring the tiny magnetic orientations produced by single-electron spins, he said. Practical, general-purpose quantum computers are 20 years away, according to Meier.

Meier's research colleagues were Jeremy Levy from the University of Pittsburgh and Daniel Loss from the University of Basel. The work appeared in the January 31, 2003 issue of Physical Review Letters. The research was funded by the University of Basel, the University of Pittsburgh, the European Union, the Defense Advanced Research Projects Agency (DARPA) and the Swiss National Science Foundation.

Timeline:   20 years
Funding:   Government, University
TRN Categories:  Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical papers, "Quantum Computing with Antiferromagnetic Spin Clusters," posted on the physics archive at arxiv.org/abs/cond-mat/0304296, and "Quantum Computing with Spin Cluster Qubits," Physical Review Letters, January 31, 2003.


September 10/17, 2003

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