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NIST physicist Ray Simmonds holds a protective
box containing “artificial atoms” that might be used in
quantum computers. Next to him is a cryogenic
refrigerator that cools the box to temperatures near
absolute zero.
© Geoffrey Wheeler
For a
high-resolution version of this image, contact Gail Porter.
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Boulder,
Colo. -- 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 ultra-powerful 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
qubits—quantum counterparts of the 1s and 0s used in today’s
computers—have 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 co-author 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.”
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Optical micrograph showing an "artificial atom"
made with a superconducting circuit. The red arrow
points to the heart of the qubit -- the Josephson
junction device that might be used in a future quantum
computer to represent a 1, 0, or both values at once.
Credit: Ray Simmonds/NIST
Click on the
image to open a high-resolution version.
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If they
can be built, quantum computers—relying on the rules of
quantum mechanics, nature’s instruction book for the smallest
particles of matter—someday 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 fraud-proof
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 60-watt 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 junction-based 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
was supported in part by the Advanced Research and Development
Agency.
As a
non-regulatory 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.
Background on Superconducting Qubits
The work
reported in Science uses qubits made of Josephson
junctions, in which a thin layer of non-conducting 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
electron-pair 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 lowest-frequency 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.
*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.
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