Photograph of current-biased Josephson junction qubit

Quantum computing is a radically new approach to computing. A large part of the scientific community is very excited about this possible application of very fundamental physics.

In a conventional computer, information is often stored as an electrical charge on a tiny capacitor. The presence or absence of charge indicates a value of 1 or 0. In a quantum computer, information is stored as a wave function of a quantum bit, or qubit. Theorists project enormous increases in computing power if quantum computing can be made practical.

Many very different quantum systems are being considered worldwide for possible use in quantum computing. At NIST, staff in the Physics Laboratory are pursuing single ions and neutral ions. In EEEL, we are considering superconducting integrated circuits. The potential advantage of our approach is that we are using lithographic fabrication, which can be scaled from a single first qubit to an entire integrated circuit, necessary for any actual quantum computer.

Whether or not quantum computing becomes practical, our work will produce new knowledge of electrical measurements in systems with extremely low energy levels. This is the direction all computing is taking. With the current focus on massively integrated nanoscale devices, energy differences between different states in a computer are reduced because of the ever smaller device sizes, proving the important practical result of lower power dissipation.

In future systems exceptional care must be taken to eliminate both external noise and even self-generated noise. Low temperature operation is essential. While the ultimate success of quantum computing is still unknown, the measurement techniques that we will develop will find application in much advanced electronic technology.

Technical Background

Our first demonstration of a qubit uses a current- biased Josephson junction operated at about 20 millikelvin (mK). Our qubit is based on a current-biased Josephson junction. The energy levels are shown in the following diagram:

Potential energy of current-biased Josephson junction showing ground and first excited state wavefunctions as a function of junction phase difference.

Using a microwave source we can control the population of the first state above the ground state. Reading out that state is accomplished by applying a second microwave signal to move the system into the highest state (not shown). Adjusting the bias permits the system to tunnel through the right energy barrier into the Josephson voltage state, which is easily detectable using conventional instrumentation.

This very specialized circuit is fabricated in our versatile facility, described elsewhere in this publication.

Accomplishments

• Qubit Demonstrated – One important test of potential qubits is the observation of stimulated transitions between states, called Rabi oscillations. These are demonstrated in the figure on the next page. These and other data reveal a coherence time of about 10 ns. Calculations suggest that times as long as about 1 µs should be possible, which is long enough for experimental quantum communication.

Rabi oscillations demonstrated in plot of measured probability of occupation p1 of first state as function of microwave excitation (P01)1/2.

Our results demonstrate an operating qubit. The next steps in our work, in 2003, are to find ways to increase the coherence time. Once that is complete it will be necessary in 2004 to develop methods for coupling two or more qubits into a logic gate. Following this step we will have to demonstrate quantum computing, beginning with very simple circuits and learning techniques for increasing the complexity toward circuits, which can perform meaningful computation.

We are vigorously pursuing this fast-moving field of research and invite interested readers to check our web site for our latest results.

Recent Publications

J.M. Martinis, S. Nam, J. Aumentado, K.M. Lang, and C. Urbina, “Decoherence of a superconducting qubit from bias noise,” submitted to Physical Review B (in press).

K.M. Lang, S. Nam, J. Aumentado, C. Urbina, and J.M. Martinis, “Banishing quasiparticles from Josephson-junction qubits: why and how to do it,” Proc., 2002 Applied Superconductivity Conference, 4-9 August 2002, Houston, TX (2002), IEEE Trans. Appl. Supercon. (in press).

J.M. Martinis, S. Nam, J. Aumentado, and C. Urbina, “Rabi oscillations in a large Josephson-junction qubit,” Phys. Rev. Lett. 89(11), pp. 117901 1-4 (9 Sept. 2002).