Spingineering : quantum control in the presence of relaxation

For certain problems quantum resources can exponentially increase computing power, but these quantum resources are very fragile in practice. When a quantum system interacts with an external environment, it undergoes decoherence the loss of quantum correlation and relaxation the loss of energy and eventually all of the quantum information is lost. Here we show a general principle of using unitary operators to suppress relaxation processes. Unitary operations do not cool a quantum system and seem an unlikely candidate for preventing irreversible thermodynamic heating processes, but surprisingly most decoherence processes can be corrected or ameliorated using open loop control with unitary controllers. We examine the different mechanisms of decoherence and relaxation on simple spin systems and discuss when the modes can be corrected. We show experimentally the feasibility of our correction schemes using nuclear magnetic resonance. We also demonstrate control of the nuclear spins over long time scales. Finally, we discuss the applications of unitary correction to higher dimensional systems and the potential applications to quantum information processing. Thesis Supervisor: Neil Gershenfeld Title: Associate Professor Spingineering: Quantum Control in the Presence of Relaxation by Benjamin Harris Recht Submitted to the Media Arts and Sciences department in partial fulfillment of the requirements for the degree of Master of Science in Media Arts and Sciences at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2002 Thesis Advisor . Neil Gershenfeld Associate Professor Department of Media Arts and Sciences Massachusetts Institute of Technology Thesis Reader . Isaac Chuang Associate Professor Department of Media Arts and Sciences Massachusetts Institute of Technology Thesis Reader . Seth Lloyd Professor Department of Mechanical Engineering Massachusetts Institute of Technology Acknowledgments This thesis, being no different than any other, would have not been possible without some truly remarkable people. I would like to thank my advisor Neil Gershenfeld for his support and guidance through my first two years and my readers: Isaac Chuang for introducing me to and guiding me through the quantum world, and Seth Lloyd for always saying just the right thing at the right time. I owe a large thank you to Yael Maguire for inspiring this work and his eagerness to discuss my progress. Thanks to Matthias Steffen and Yael for making sure that my theorist hands never broke the ever fussy 500 MHz NMR spectrometer. I would also like to thank Aram Harrow and Ali Rahimi for invaluable discussions and suggestions. Thanks to Jason Taylor for bearing with me in the confines of our office and to the rest of the PHM crew: Matt, Rehmi, Ben, Rich, Ravi, Bernd, John-Paul, Dan, Caroline, Esa, Femi. Thanks to Linda Peterson and Pat Sokaloff for taking extra time to care about all of us lost and helpless graduate students. I also owe a great deal of thanks to Susan MurphyBottari, Mike Houlilhan, and Liz Hennessey for making the PHM machine run as smoothly as it does. I send much love and respect to the unlockedgroove crew and all of the people who frequent to the Appliance of Science for helping me maintain my sanity. Thank you Mom and Dad for 24 years of constant support and Marissa for never forgetting to tell me when I’m wrong. Thank you Lauren for being exactly who you are. Thanks to the MIT Media Laboratory and the Things That Think Consortium. This thesis would not have been possible without the constant support of Nicotine, Caffeine, Alcohol, and Cholesterol. Thank you for better living through chemistry.