University of Wisconsin–Madison

One of the remarkable recent discoveries in information science is that quantum mechanics can lead to efficient solutions for problems that are intractable on conventional classical computers. The realization of a practical quantum computer will require the development of fundamentally new quantum hardware: circuits whose operation is intrinsically quantum mechanical, designed for the high-fidelity generation and preservation of delicate entangled states. The University of Wisconsin-Madison has emerged as a world leader in the area of quantum devices, with three cutting edge experimental efforts focused on different quantum device implementations. The Wisconsin Institute for Quantum Information (WIQI) brings together experimental and theoretical research from across campus in the following areas:

Superconducting Qubits
Superconducting integrated circuits incorporating Josephson junctions act as “artificial atoms” with energy level separations in the microwave range. Realization of improved large-scale superconducting quantum circuits will require a thorough understanding of qubit dephasing, along with development of technologies for scalable qubit measurement and control. The image shows a near-quantum limited linear amplifier based on the SLUG for scalable superconducting qubit readout. See “High Fidelity Qubit Readout with the Superconducting Low-Inductance Undulatory Galvanometer Microwave Amplifier”, D. Hover, S. Zhu, T. Thorbeck, G. J. Ribeill, D. Sank, J. Kelly, R. Barends, J. M. Martinis, and R. McDermott, Appl. Phys. Lett. 104, 152601 (2014). [Journal Article]
Semiconductor Qubits
The hybrid quantum dot qubit is formed from three electrons in a Si/SiGe double quantum dot. The differentiating feature of the hybrid quantum dot qubit is its high speed (typical gate times are between 100 ps and 5 ns) combined with the simplicity of being built from only two quantum dots. The image shown here is a labelled SEM micrograph of the surface gates of a double quantum dot device with an integrated charger sensing quantum point contact (QPC). Image reproduced with permission from “Quantum control and process tomography of a semiconductor quantum dot hybrid qubit.” Dohun Kim, Zhan Shi, C. B. Simmons, D. R. Ward, J. R. Prance, Teck Seng Koh, John King Gamble, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, Nature 511, 70 (2014). [Journal Article | arXiv]
Neutral Atom Qubits
Qubits encoded in hyperfine states of neutral atoms are a promising approach for scalable implementations of quantum information processing. These atomic qubits are all identical, they exhibit long coherence times, and they can be entangled using long range Rydberg blockade interactions. The figure shows a fluorescence image of 49 atomic qubits with which high fidelity single qubit gates have been demonstrated with low crosstalk between neighboring qubits. This is the largest number of individually controllable qubits for which quantum gate operations have been characterized to date. For more details see T. Xia, M. Lichtman, K. Maller, A. W. Carr, M. J. Piotrowicz, L. Isenhower, and M. Saffman, “Randomized benchmarking of single qubit gates in a 2D array of neutral atom qubits”, Phys. Rev. Lett. 114, 100503 (2015) [Journal Article] and Physics synopsis.

WIQI also includes a theory group working on modeling of devices and materials for quantum information processing.

In addition, WIQI supports a growing effort in hybrid quantum implementations that combine the best features of disparate quantum technologies. [Journal Article]