Electron teams make bigger qubits
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
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
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
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
Spin-cluster-qubits can be made using any of a wide range of artificial
magnetic molecules that have been synthesized during the past decade,
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
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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