quantum bits go natural
Technology Research News
One of the first hurdles to developing
practical quantum computers is coming up with devices that let researchers
precisely control qubits, the individual atoms, electrons or photons that
are the basic building blocks of quantum computers.
usually involves minute, finely tuned and precisely aimed laser, microwave
or magnetic pulses. These control operations are very difficult for even
a single qubit and the task of controlling as few as 10 is daunting. Ultimately,
computers will require systems that can control at least several hundred
A team of researchers based at the University of California at Berkeley
has come up with an encoding scheme that sidesteps the problem. The trick
is making qubits out of the natural interactions of two or more particles
rather than changing the behavior of individual particles. The researchers
are proposing to fit computer logic to the natural actions of qubits,
rather than forcing qubits to do conventional logic operations.
There are two basic requirements quantum computers must satisfy in order
to perform all the binary
logic operations of
ordinary computers: controlling all possible quantum mechanical states
of each physical qubit and quantum mechanically linking two or more physical
qubits to form a Controlled-Not (CNOT) logic gate. A CNOT gate has a control
bit and a target bit. If the control bit is 1, it flips the target bit
from 0 to 1 or 1 to 0. If the control bit is zero, it leaves the target
These requirements, outlined in a 1995 paper, have become a sort of bible
of universal quantum computation, said Daniel Lidar, an assistant professor
of chemistry at the University of Toronto.
The trouble is, it's very difficult to make physical systems that can
satisfy these requirements, he said. "In... quantum dots, for example,
implementing the single-qubit operations can be very hard," he said.
In a quantum computer whose qubits are individual electrons trapped inside
quantum dots, which are microscopic specks of semiconductor material,
flipping a bit from a 0 to a 1 or a 1 to a 0 requires extreme accuracy,
said Lidar. "You need to apply a very, very local, microscopically accurate
magnetic field," he said.
This is where the Berkeley encoding scheme comes in. Rather than forcing
physical qubits to perform these difficult operations, the researchers
propose to use "what we call the natural talents of the physical system,"
said Lidar. "The paradigm shift that we're proposing is that you start
with whatever is natural for [a given] system," he said. "You investigate
whether the system as such is capable of implementing universal [quantum]
In a quantum dot system, one natural operation is switching information
between two neighboring quantum dots, said Lidar. "If you have two physical
qubits -- two electrons on two separate quantum dots -- [you can swap]
the wave functions of these two electrons. It's a lot easier to perform
than these single-qubit operations," he said.
The catch is that these natural operations don't translate directly to
the necessary quantum computing functions.
"If you want to just use the naturally available interactions in the system,
you'll have to play some tricks," said Lidar. "You're going to have to
represent your logical zeros and ones in terms of some entangled combinations
of [quantum mechanical] states of these physical qubits," he said.
And at some level the quantum computer still has to perform the same operations
that implement universal quantum computation. "Again you use single qubit
gates and a CNOT, but these single-qubit gates no longer operate on the
physical... qubits, rather they operate on the encoded qubits," said Lidar.
The downside to encoding is that quantum computers will need at least
twice as many physical qubits as non-encoded systems require.
"The trade-off in encoding is that you're using a number of physical qubits
in order to encode one logical qubit," said Lidar. "Whether the net balance
is positive, that's something that's going to depend on a particular implementation,"
he said. "What is easier to do, engineer a difficult operation or give
access to more physical qubits?"
The encoding scheme grew out of research on decoherence-free subspaces,
which protect qubits from decoherence. Decoherence occurs when energy
from the environment knocks a physical qubit out of its quantum mechanical
state. Limiting decoherence is one of the principal challenges to developing
practical quantum computers.
The encoding research shows how this idea can be extended to general notions
of fault-tolerant quantum computation, said Seth Loyd, an associate professor
of mechanical engineering at the Massachusetts Institute of Technology.
"This scheme is potentially useful, [but] whether this or any other scheme
will lead to large-scale quantum computation remains to be seen," he said.
In order to test the encoding scheme, researchers will need to build prototype
solid-state quantum computers with between two and four qubits and test
them first using the standard paradigm, said Lidar. That probably won't
happen for another five years, he said.
Many researchers say that it will be at least two decades before practical
quantum computers are developed.
Lidar's research colleagues were Dave Bacon, Julia Kempe and K. Birgitta
Whaley of the University of California at Berkeley and David P. DiVincenzo
of IBM Research. The research was funded by the Army Research Office,
the National Security Agency, and the Advanced Research and Development
Timeline: >20 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Encoded Universality
in Physical Implementations of a Quantum Computer," posted to the Los
Alamos National Laboratory e-Print archive Feb. 27, 2001 ;
Technical paper "Quantum computation," Science, October 13, 1995
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