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Artist's rendition of the NIST
superconducting quantum computing cable.
Illustration by: Michael Kemper
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BOULDER, Colo.— Physicists at the National Institute of
Standards and Technology (NIST) have transferred information
between two “artificial atoms” by way of electronic vibrations on a
microfabricated aluminum cable, demonstrating a new
component for potential ultra-powerful quantum computers of the
future. The setup resembles a miniature version of a cable-television
transmission line, but with some powerful added
features, including superconducting circuits with zero electrical
resistance, and multi-tasking data bits that obey the unusual
rules of quantum physics.
The resonant cable might someday be used in quantum
computers, which would rely on quantum behavior to carry out
certain functions, such as code-breaking and database searches, exponentially faster than today’s
most powerful computers. Moreover, the superconducting components in the NIST demonstration
offer the possibility of being easier to manufacture and scale up to a practical size than many
competing candidates, such as individual atoms, for storing and transporting data in quantum
computers.
Unlike traditional electronic devices, which store information in the form of digital bits that
each possess a value of either 0 or 1, each superconducting circuit acts as a quantum bit, or qubit,
which can hold values of 0 and 1 at the same time. Qubits in this “superposition” of both values
may allow many more calculations to be performed simultaneously than is possible with traditional
digital bits, offering the possibility of faster and more powerful computing devices. The resonant
section of cable shuttling the information between the two superconducting circuits is known to
engineers as a “quantum bus,” and it could transport data between two or more qubits.
The NIST work is featured on the cover of the Sept. 27 issue of Nature. The scientists
encoded information in one qubit, transferred this information as microwave energy to the resonant
section of cable for a short storage time of 10 nanoseconds, and then successfully shuttled the
information to a second qubit.
“We tested a new element for quantum information systems,” says NIST physicist Ray
Simmonds. “It’s really significant because it means we can couple more qubits together and
transfer information between them easily using one simple element.”
The NIST work, together with another letter in the same issue of Nature by a Yale
University group, is the first demonstration of a superconducting quantum bus. Whereas the NIST
scientists used the bus to store and transfer information between independent qubits, the Yale
group used it to enable an interaction of two qubits, creating a combined superposition state.
These three actions, demonstrated collectively by the two groups, are essential for performing the
basic functions needed in a superconductor-based quantum information processor of the future.
In addition to storing and transferring information, NIST’s resonant cable also offers a
means of “refreshing” superconducting qubits, which normally can maintain the same delicate
quantum state for only half a microsecond. Disturbances such as electric or magnetic noise in the
circuit can rapidly destroy a qubit’s superposition state. With design improvements, the NIST
technology might be used to repeatedly refresh the data and extend qubit lifetime more than 100-fold, sufficient to create a viable short-term quantum computer memory, Simmonds says. NIST’s
resonant cable might also be used to transfer quantum information between matter and light—microwave energy is a low-frequency form of light—and thus link quantum computers to ultrasecure
quantum communications systems.
If they can be built, quantum computers—harnessing the unusual rules of quantum
mechanics, the principles governing nature’s smallest particles—might be used for applications
such as fast and efficient code breaking, optimizing complex systems such as airline schedules,
making counterfeit-proof money, and solving complex mathematical problems. Quantum
information technology in general allows for custom-designed systems for fundamental tests of
quantum physics and as-yet-unknown futuristic applications.
A superconducting qubit is about the width of a human hair. NIST researchers fabricate two
qubits on a sapphire microchip, which sits in a shielded box about 8 cubic millimeters in size. The
resonant section of cable is 7 millimeters long, similar to the coaxial wiring used in cable television
but much thinner and flatter, zig-zagging around the 1.1 mm space between the two qubits. Like a
guitar string, the resonant cable can be stimulated so that it hums or “resonates” at a particular
tone or frequency in the microwave range. Quantum information is stored as energy in the form of
microwave particles or photons.
The NIST research was supported in part by the Disruptive Technology Office.
As a non-regulatory agency of the U.S. Department of Commerce, NIST promotes U.S.
innovation and industrial competitiveness by advancing measurement science, standards and
technology in ways that enhance economic security and improve our quality of life.
*M.A. Sillanpää, J.I. Park, and R.W. Simmonds. 2007. Coherent quantum state storage and transfer between two phase
qubits via a resonant cavity. Nature, Sept. 27.
BACKGROUND
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When NIST scientists repeatedly transferred quantum
information between two superconducting qubits via a
microfabricated resonant cable, the overall pattern of
results (left image) closely matched ideal theoretical
predictions (right image), confirming that the transfer
process generally proceeded as expected. The left image
is a plot of the interaction time (in nanoseconds) between
each qubit (A vs. B) and the resonant cable with colors
indicating the final state of qubit B being excited (red) or not
(blue).
Credit: NIST
View hi-resolution version |
The heart of each NIST superconducting qubit is a component known as a Josephson
junction. The junction is made of two superconducting pieces of metal separated by a thin
electrically insulating region with the special property of supporting a “super flow” of electrical
current traveling in a single, uniform wave pattern. The electrical wave patterns move, or oscillate,
back and forth through the junction billions of times per second, acting as an “artificial atom” that
mimics the natural oscillations or energy states in real atoms. The two lowest-energy oscillations of
these wave currents correspond to the 0 and 1 states of digital bits of information.
As described in Nature, the latest NIST experiments begin with the qubits and the cable
oscillating at different frequencies. By applying a microwave pulse of a particular frequency, power,
and time span, scientists place the first qubit A in a superposition of the 0 and 1 states. Then they
apply a voltage pulse of a particular size to
place qubit A briefly “on resonance,” at the
same frequency, with the resonant section
of cable, inducing an interaction between
the two devices. This transfers the
quantum information to the resonant
section of cable in the form of microwave
energy or photons. Then qubit A is tuned
away from the resonance frequency
(“detuned”) and qubit B is placed on
resonance with the cable to receive the
information. Finally, qubit B is also detuned
and both qubits are measured
simultaneously. The measurement causes
each qubit to choose either the 0 or the 1
state.
To read out this result, scientists
detect tiny changes in the magnetic field
produced by each qubit using a
superconducting quantum nterference
device (SQUID). They apply a quick current
pulse to the SQUID. A shift in the timing of a returning voltage pulse signals that the qubit is in the
1 (or excited) state; if no shift is detected then the qubit is in the 0 state. This process is repeated
many times to determine which outcomes have the highest probability.
NIST scientists stored and transferred quantum information through the resonant section of
cable repeatedly, millions of times, starting with qubit A in various different superposition states.
The overall pattern of results closely matched theoretical predictions, confirming that the qubits
maintained quantum superpositions throughout the transfer process and generally evolved as
expected. However, because of imperfections in qubit fabrication, measurements of individual
quantum states were imprecise, making it difficult to evaluate the quality of the quantum bus or the
transfer error rate. Scientists are working to improve the overall system performance through
developments in qubit materials, designs, and biasing electronics. In the future, complete
optimization of this quantum system should enable scientists to precisely quantify the error rate
associated with the quantum bus and, if needed, to develop methods for error correction.
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