Two
superconducting devices have been coaxed into a special,
interdependent state that mimics the unusual interactions
sometimes seen in pairs of atoms, according to a team
of physicists at the National Institute of Standards
and Technology (NIST) and University of California,
Santa Barbara (UCSB). The experiments, performed at
the NIST laboratory in Boulder, Colo., are an important
step toward the possible use of "artificial atoms"
made with superconducting materials for storing and
processing data in an ultrapowerful quantum computer
of the future.
The work, reported in the Feb.
25 issue of the journal Science*, demonstrates that
it is possible to measure the quantum properties of
two interconnected artificial atoms at virtually the
same time. Until now, superconducting qubitsquantum
counterparts of the 1s and 0s used in today's computershave
been measured one at a time to avoid unwanted effects
on neighboring qubits. The advance shows that the
properties of artificial atoms can be coordinated
in a way that is consistent with a quantum phenomenon
called "entanglement" observed in real atoms.
Entanglement is the "quantum magic" allowing
the construction of logic gates in a quantum computer,
a means of ensuring that the value of one qubit can
be determined by the value of another in a predictable
way.
"This opens the door to
performing simple logic operations using artificial
atoms, an important step toward possibly building
superconducting quantum computers," says John
Martinis, who began the superconducting quantum computing
effort at NIST and is now on the physics faculty at
UCSB.
"Whether or not quantum
computing becomes practical, this work is producing
new ways to design, control and measure the quantum
world of electrical systems," says Ray Simmonds,
a NIST physicist and a coauthor of the Science paper.
"We have already detected previously unknown,
individual nanoscale quantum systems that have never
before been directly observed, a discovery that may
lead to unanticipated advances in nanotechnology."
If they can be built, quantum
computersrelying on the rules of quantum mechanics,
nature's instruction book for the smallest particles
of mattersomeday might be used for applications
such as fast and efficient code breaking, optimizing
complex systems such as airline schedules, much faster
database searching and solving of complex mathematical
problems, and even the development of novel products
such as fraudproof digital signatures.
Superconducting circuits are
one of a number of possible technologies for storing
and processing data in quantum computers that are
being investigated for producing qubits at NIST, UCSB
and elsewhere around the world. Research using real
atoms as qubits has advanced more rapidly thus far,
but superconducting circuits offer the advantage of
being easily manufactured, easily connected to each
other, easily connected to existing integrated circuit
technology, and mass producible using semiconductor
fabrication techniques. A single superconducting qubit
is about the width of a human hair. Two qubits can
be fabricated on a single silicon microchip, which
sits in a shielded box about 1 cubic inch in size.
The work reported in Science
creates qubits from superconducting circuit elements
called Josephson junctions. These devices consist
of two superconducting pieces of metal separated by
a thin insulating region with the special property
of being able to support a "super flow"
of current. Scientists have used Josephson junctions
for more than 40 years to manipulate and measure electrical
currents and voltages very precisely. The experiment
creates artificial atoms using currents that are 1
billion times weaker than the current needed to power
a 60watt light bulb. Using Josephson junctions, scientists
can create wave patterns in electrical currents that
oscillate back and forth billions of times per second,
mimicking the natural oscillations between quantum
states in atoms. And, as in a real atom, the quantum
states of a superconducting junction can be manipulated
to represent a 1, a 0, or even both at once.
As described in the paper,
the team of scientists measured the state of a superconducting
qubit by applying a voltage pulse lasting 5 nanoseconds,
and detecting a change in magnetic field through a
simple transformer coil incorporated in the qubit.
To detect the tiny variations in the magnetic field
they use a superconducting quantum interference device
(SQUID). If a signal is detected, the qubit is in
the 1 (or excited) state; if no signal is detected
the qubit is in the 0 state.
Through very precise timing,
the team also was able to measure the two qubits simultaneously.
This was key to avoid unwanted measurement crosstalk
that destroys quantum information. The scientists
were able to witness a pattern of quantum oscillations
that is consistent with the entanglement needed for
producing quantum logic gates.
NIST research on Josephson
junctionbased quantum computing, now led by Ray Simmonds,
is part of NIST's Quantum Information Program (http://qubit.nist.gov/index.html),
a coordinated effort to build the first prototype
quantum logic processor consisting of approximately
10 or more qubits. John Martinis' research group within
the UCSB Center for Spintronics and Quantum Computation,
a part of the California Nanosystems Institute (CNSI)
(http://www.cnsi.ucsb.edu/about/about.html), is primarily
focused on building a quantum computer based on Josephson
junction quantum bits.
The work reported in Science
uses qubits made of Josephson junctions, in which
a thin layer of nonconducting material is sandwiched
between two pieces of superconducting metal. At very
low temperatures, electrons within a superconductor
pair up to form a "superfluid" that flows
with no resistance and travels in a single, uniform
wave pattern. The uniform electronpair wave patterns
leak into the insulating middle of the "sandwich,"
where their wave properties overlap and interfere
with each other so that a superfluid can flow through
the insulator. The current flows back and forth through
the junction somewhat like a ball rolling back and
forth inside a curved bowl. The energy in these oscillations
can only be stored in discrete amounts or quanta.
In a Josephson junction qubit,
the 0 and 1 states can be thought of as the two lowestfrequency
oscillations of the currents flowing back and forth
through the junction. The speed of these oscillations
is typically billions of times per second. This behavior
is similar to the way an atom's electrons oscillate
naturally around its nucleus, forming discrete quantum
states, hence the term "artificial atom."
The qubit also can be thought
of as a child's swing rocking back and forth between
its extreme forward and back positions. However, unlike
an ordinary swing, a Josephson qubit can be in an
unusual quantum state called a "superposition"
in which it is oscillating at two different frequencies
at once, in a state that is both 1 and 0 at the same
time.
When two Josephson junctions
are connected through a standard capacitor, the application
of a small a.c. voltage pulse to the first qubit can
cause the two qubits to oscillate between two combined
states. In one combined state, the first qubit is
excited (1) while the second is not (0); later in
time the first qubit is fully relaxed (0) while the
second one is fully excited (1). They oscillate between
these extremes like two children on a swing set moving
back and forth at the same speed, but in opposite
directions. These oscillations occur only if the differences
in energy between the 0 and 1 states are equal in
both qubits. This behavior is indicative of the two
qubits becoming entangled.
The work was supported in part by the Advanced Research
and Development Agency.
As a nonregulatory agency
of the U.S. Department of Commerce's Technology Administration,
NIST develops and promotes measurement, standards
and technology to enhance productivity, facilitate
trade and improve the quality of life.
*R. McDermott, R.W. Simmonds,
M. Steffen, K.B. Cooper, K. Cicak, K. Osborn, S. Oh,
D.P. Pappas, and J.M. Martinis, "Simultaneous
state measurement of coupled Josephson phase qubits,"
Science, Feb. 25, 2005.
