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Superconducting Qubits: Dephasing and Quantum Chemistry UNIVERSITY of CALIFORNIA Santa Barbara Superconducting Qubits: Dephasing and Quantum Chemistry A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics by Peter James Joyce O'Malley Committee in charge: Professor John Martinis, Chair Professor David Weld Professor Chetan Nayak June 2016 The dissertation of Peter James Joyce O'Malley is approved: Professor David Weld Professor Chetan Nayak Professor John Martinis, Chair June 2016 Copyright c 2016 by Peter James Joyce O'Malley v vi Any work that aims to further human knowledge is inherently dedicated to future generations. There is one particular member of the next generation to which I dedicate this particular work. vii viii Acknowledgements It is a truth universally acknowledged that a dissertation is not the work of a single person. Without John Martinis, of course, this work would not exist in any form. I will be eter- nally indebted to him for ideas, guidance, resources, and|perhaps most importantly| assembling a truly great group of people to surround myself with. To these people I must extend my gratitude, insufficient though it may be; thank you for helping me as I ventured away from superconducting qubits and welcoming me back as I returned. While the nature of a university research group is to always be in flux, this group is lucky enough to have the possibility to continue to work together to build something great, and perhaps an order of magnitude luckier that we should wish to remain so. It has been an honor. Also indispensable on this journey have been all the members of the physics depart- ment who have provided the support I needed (and PCS, I apologize for repeatedly ending up, somehow, on your naughty list). My friends have allowed me that rare treasure for a graduate student: relaxation. I would never have gotten off the ground without B, C, & C O'Malley to push me to my best. And I do not know where I would be without Anna and Iris to come home to each day. ix x Curriculum Vitæ Peter James Joyce O'Malley Education 2016 Ph.D., Physics, University of California, Santa Barbara 2008 B.S., Physics and Astronomy, Haverford College, Haverford 2004 Bishop O'Dowd High School, Oakland First author publications \Qubit Metrology of Ultralow Phase Noise Using Randomized Benchmarking\, P. J. J. O'Malley, J. Kelly, R. Barends, et al., Physical Review Applied, 3(4), 044009 (2015). \Scalable Quantum Simulation of Molecular Energies\, P. J. J. O'Malley, R. Babbush, et al., submitted (2016). xi xii Abstract Superconducting Qubits: Dephasing and Quantum Chemistry by Peter James Joyce O'Malley One of the most exciting potential applications of a quantum computer is the ability to efficiently simulate quantum systems, a task that is out of the reach of even the largest classical supercomputers. Such simulations require a quantum algorithm capable of efficiently representing and manipulating a quantum system, as well as a device with sufficient coherence to execute it. In this work, we describe experiments advancing both of these goals. First, we discuss dephasing|currently a leading cause of decoherence in superconducting qubits|and present measurements accurately quantifying both low- and high-frequency phase noise sources. We then discuss two quantum algorithms for the simulation of chemical Hamiltonians, and experimentally contrast their performance. These results show that with continuing improvement in quantum devices we may soon be able to apply quantum computers to practical chemistry problems. xiii xiv Contents 1 Introduction1 1.1 Quantum computation: motivation and applications............1 1.1.1 Errors and error correction......................2 1.1.2 Potential applications of quantum computing...........4 1.2 Superconducting qubits............................5 1.2.1 History and overview.........................5 1.2.2 Advantages and disadvantages....................6 2 Dephasing9 2.1 A Bloch vector picture of dephasing.....................9 2.2 Frequency noise and the superconducting qubit.............. 10 2.2.1 Ramsey measurements........................ 11 2.2.2 Spin echo measurements....................... 13 2.2.3 Rabi measurements.......................... 14 2.3 Forms of frequency noise........................... 14 2.3.1 White noise.............................. 15 2.3.2 1/f noise................................ 16 2.3.3 Telegraph noise............................ 17 2.4 Sources of noise................................ 17 2.4.1 Flux noise............................... 18 2.4.2 Charge noise and quasiparticle tunneling.............. 19 2.4.3 Critical current noise......................... 20 2.4.4 Capacitance noise........................... 20 2.4.5 Resonator induced dephasing.................... 20 2.5 Conclusion................................... 21 3 The Ramsey Tomography Oscilloscope 23 3.1 Introduction of the Ramsey Tomography Oscilloscope........... 24 3.2 Ramsey Tomography Oscilloscope results in transmon-type qubits............................. 26 3.2.1 Flux noise in Xmons......................... 26 3.2.2 Flux noise in gmons......................... 27 3.2.3 Flux noise in early SiXmons..................... 28 xv 3.2.4 Comparison with theory....................... 29 4 Qubit metrology of ultralow phase noise using randomized benchmark- ing 33 4.1 Introduction.................................. 34 4.2 RB Ramsey.................................. 37 4.3 Measuring telegraph noise.......................... 39 4.4 Measuring error from coherent qubit-qubit interactions.......... 42 4.5 Measuring different gate implementations.................. 44 4.6 Summary................................... 45 5 Chemistry on a quantum computer: a brief introduction 47 5.1 Representing electrons with qubits: The Bravyi-Kitaev Transform.... 48 5.2 The canonical quantum chemistry algorithm................ 51 5.3 The Variational Quantum Eigensolver.................... 55 6 Scalable Quantum Simulation of Molecular Energies 57 6.1 Variational quantum eigensolver....................... 60 6.2 Phase estimation algorithm......................... 64 6.3 Experimental Methods............................ 67 6.4 Conclusion................................... 68 A Perturbative treatment of the Xmon Hamiltonian 71 B Spectral density of phase noise 75 C Ramsey Tomography Oscilloscope data processing 79 D Appendices for Chapter 4 81 D.1 Theoretical relation of RB error to φ2 ................... 81 h i D.2 Types of phase noise............................. 84 D.3 T1, Ramsey, and spin echo fits........................ 86 D.4 Flux noise................................... 87 D.5 RB Ramsey across the qubit spectrum................... 90 D.6 Charge noise.................................. 91 D.7 Calculation of ΩZZ .............................. 91 D.8 Fits to gate errors in Figure 4.4....................... 93 D.9 Telegraph noise measured in other devices................. 94 E Appendices for Chapter 5 97 E.1 The electronic structure problem...................... 97 E.2 Experimental methods for VQE....................... 100 E.3 Experimental methods for PEA....................... 103 E.4 Unitary coupled cluster............................ 106 xvi Bibliography 111 xvii Chapter 1 Introduction 1.1 Quantum computation: motivation and applica- tions The continued increase in computing power over the past decades has been an unprece- dented boon to society in general, and scientific research in particular. From early analog simulations of nuclear physics in the Manhattan project to modern attempts to model the human brain [67] or the entire universe [129], nearly all areas of research make use of computing power in one way or another. Computers can solve problems that are easy to understand, yet difficult to execute|such as computing electromagnetic fields at all points in a space, or searching a massive database|as well as problems for which the solution does not have an intuitive explanation|such as neural networks capable of outperforming the best humans in the game of Go [113]. However, there are problems that will take even the most powerful supercomputer longer than the age of the universe to solve. Frustratingly, such a problem is presented by the scientific theory that underpins all of physics: quantum mechanics. The fact that particles become entangled|meaning their state cannot be described individually, but 1 that the quantum state must describe the system as a whole|and that the superposition of such states are equally valid themselves, means that increasing the size of a quantum system increases the resources required to simulate it exponentially. Concretely, if we can simulate a system of n electrons, then simulating a system of n + 1 electrons will be twice as hard. Even if computing power continues to double every few years, it will take many, many such doublings to significantly increase the size of a quantum system we can simulate. For this reason, we say that an algorithm that solves a problem (for example, simulating a physical system) is “efficient" or \scalable" if the resources it requires scale polynomially with the size of the problem. However, in 1982 Richard Feynman proposed [38] a solution to this: use the very quantum mechanical systems that are so hard to simulate as a simulator themselves. Since then, the idea of a universal quantum computer made of quantum bits (\qubits") has been developed, and many algorithms for such a computer have been shown to out- perform their classical counterparts. While it is unknown whether an arbitrary
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