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Sept. 26, 2007

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NIST
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Artist's rendition of the NIST superconducting quantum computing cable. Illustration by: Michael Kemper

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

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

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.

Created: 9/26/07
Last updated: 9/26/07
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