BERKELEY,
CA — Quantum computers promise to solve
many difficult problems much faster than so-called
classical computers, and they will be essential for
certain calculations impossible by any other means.

While
a quantum computer could conceivably look very
different from its classical forebears, hardware
that draws on the experience of the classical past — meaning
silicon, in this case — would have significant technological
and manufacturing advantages.

That's
why Thomas Schenkel of Lawrence Berkeley Laboratory's
Accelerator and Fusion Research Division (AFRD)
and his collaborators are working to demonstrate
that a quantum logic system can work in silicon devices.

"Essentially the goal is to find out if quantum
computing is possible with donor electron-spin qubits
in silicon," says Schenkel, explaining that he and
his group propose to use the spin states of donor
atoms embedded in silicon as the fundamental components
of a quantum computer: its quantum bits, or "qubits." Unlike
a classical bit that codes for one state or another — a
1 or a 0 — a qubit encodes these states simultaneously,
holding them in superposition until measured.

An
often used if overly simple illustration of the
potential power of quantum computing is the tiny
three-bit register. Whereas a three-bit register
in a classical computer outputs just one of eight
discrete states (000, 001, 010 ... 111), three
entangled qubits represent a mixture of all these
states simultaneously. In general, the capacity
of a quantum computer's register is 2 raised to
the power of the number of qubits — in this case 2 3 — a
value that expands rapidly. A few dozen entangled
qubits could represent a difficult problem in a
vast computational space.

In a classical computer bits are routinely encoded
as distinguishable states of a physical system, for
example, the orientation of magnetic domains on a
hard drive or tape, or the number of electrons stored
in transistors on a flash drive. Classical calculations
are performed essentially one bit after another.
What makes qubits distinctive is that they are subject
to the peculiar laws of quantum mechanics, in particular
entanglement .

Entangled
systems with a limited number of permissible quantum
states are spookily "connected." Two electrons
prepared together, one with spin up and one with
spin down, remain entangled until a measurement is
performed on one of them; when the state of one is
measured (spin down, say), the state of the other
is instantly determined (spin up), no matter how
distant it may be. The same holds true for a dozen
entangled particles, or a hundred, or more.

With
quantum computing, says Schenkel, "The idea
is to find the solution by first acting on all the
entangled qubits in parallel through clever quantum
gate operations, and then to extract the solution
in measurements that simultaneously 'collapse' the
superpositions of the entire system to a series of
classical zeros and ones."

One
system for realizing a qubit is electron spin,
an intrinsic property of electrons that, given
an external magnetic field, forms an accessible
two-level system. In conventional silicon transistors,
group V elements like phosphorus, antimony, and
bismuth are widely used as donors — atoms having one more
valence electron than group IV silicon and thus useful
for adjusting its electronic properties (commonly
by causing the silicon to become n-type, or negatively
conducting). At low temperature these donors, having
one extra electron, represent natural quantum dots;
the spin state of the extra electron defines the
qubit.

But
the idea of donor electron-spin qubits in silicon
is just one of many different proposals for realizing
qubits, Schenkel says. Other approaches include schemes
involving superconducting tunnel junctions, quantum
dots, and neutral atoms in optical lattices. He says, "Ion-trap
systems are currently in the lead, with amazing demonstrations
of qubit control, with up to eight ions."

Ion-trap
systems electromagnetically suspend ionized atoms
in free space and use laser beams to alter and
measure their spin states, a tricky procedure.
But in the long run, quantum computers based on
silicon could be manufactured with familiar materials
and methods — maybe even mass produced — and could
prove easier to scale up.

#### One qubit at a time

Demonstrating
a working single-qubit device is the first step
to proving that quantum computing can work in silicon.
Schenkel and his colleagues are developing a field-effect
transistor made of isotopically enriched silicon,
in which the flow of current through the device
is sensitive to the spin state of a single donor
atom — a "single-spin readout" device.