Prof. Shih-I Chu

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SQUID Quantum Computing and Quantum Information

Recently increasing worldwide research effort has been directed toward exploring systems and devices operating in the quantum regime. As the size of conventional devices is reduced to the submicron scale and even molecular scale, quantum effects become very prominent. The performance of "quantum devices" could move beyond incremental improvements and bring revolutionary developments in science and technology. For example, a quantum computer, if realized, can perform tasks exponentially faster than any conventional supercomputer. The basic principle of quantum computing such as quantum entanglement and quantum logic, have been recently demonstrated by several experiments, including NMR and trapped ions. However, the long-term prospect for NMR and trapped ion quantum computing may be not great since the number of spins and ions can be entangled will be limited to the order of 10. In terms of scalability, the solid-state systems such as quantum dots and SQUID (Superconducting Quantum Interference Device), is far better, since they can be designed, manufactured, and scaled up easily using modern integrated circuits fabrication technology. However, the solid-state devices can be subject to larger decoherence. Professor Chu has recently initiated the SQUID quantum computing initiative, in cooperation with experimental groups at the University of Kansas and Kansai Advanced Research Center in Japan. The initial goal is to pursue joint experimental and theoretical investigations of the characterization and control of decoherence in SQUIDs, macroscopic phenomena in SQUIDs, qubit-microwave interactions, optimization of quantum gate operation in solid state qubits, and quantum entanglement in SQUID qubits and multiple qubits, etc. Significant progress has been made in the last two years, particularly, in the first observation of coherent temporal Rabi oscillations of macroscopic quantum states in a Josephson Junction, and the determination of the lower limit of the phase decoherence time to be greater than 5 μs [see ref. 5 below]. This advancement greatly enhances the prospect of realizing quantum computation with microwave pulse-driven superconducting qubits in the future.

Representative Publications:

S. Han, Y. Yu, X. Chu, S. I. Chu, and Z. Wang, Time-resolved measurement of dissipation-induced decoherence in a Josephson junction, Science 293, 1457–1459 (2001) [BibTeX] [link] [pdf]
Z. Y. Zhou, S. I. Chu, and S. Y. Han, Quantum computing with superconducting devices: A three-level SQUID qubit, Phys. Rev. B 66, 054527 (2002) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Han, A scheme for protecting one-qubit information against erasure error, J. Opt. B: Quantum Semiclass. Opt. 4, 256–259 (2002) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Han, Error-prevention scheme with two pairs of qubits, Phys. Rev. A 66, 034301 (2002) [BibTeX] [link] [pdf]
Y. Yu, S. Han, X. Chu, S. I. Chu, and Z. Wang, Coherent temporal oscillations of macroscopic quantum states in a Josephson junction, Science 296, 889–892 (2002) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Y. Han, Error-prevention scheme for protecting three-qubit quantum information, Phys. Lett. A 299, 31–37 (2002) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Y. Han, Possible realization of entanglement, logical gates, and quantum-information transfer with superconducting-quantum-interference-device qubits in cavity QED, Phys. Rev. A 67, 042311 (2003) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Y. Han, An energy relaxation tolerant approach to quantum entanglement, information transfer, and gates with superconducting-quantum-interference-device qubits in cavity QED, J. Phys.: Condens. Matter 16, 1907–1914 (2004) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Y. Han, Quantum information transfer and entanglement with SQUID qubits in cavity QED: A dark-state scheme with tolerance for nonuniform device parameter, Phys. Rev. Lett. 92, 117902 (2004) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Y. Han, Simplified realization of two-qubit quantum phase gate with four-level systems in cavity QED, Phys. Rev. A 70, 044303 (2004) [BibTeX] [link] [pdf]
C. P. Yang, S. I. Chu, and S. Y. Han, Efficient many-party controlled teleportation of multiqubit quantum information via entanglement, Phys. Rev. A 70, 022329 (2004) [BibTeX] [link] [pdf]
Z. Y. Zhou, S. I. Chu, and S. Y. Han, Suppression of energy-relaxation-induced decoherence in Λ-type three-level SQUID flux qubits: A dark-state approach, Phys. Rev. B 70, 094513 (2004) [BibTeX] [link] [pdf]
Z. Y. Zhou, S. I. Chu, and S. Y. Han, Unified approach for universal quantum gates in a coupled superconducting two-qubit system with fixed always-on coupling, Phys. Rev. B 73, 104521 (2006) [BibTeX] [link] [pdf]

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Dept. of Chemistry
The University of Kansas