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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. |
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 National Security Agency's
Advanced Research and Development Activity.
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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