Geometric Phase of a Harmonic Oscillator in Circuit QED Master’s Thesis in Physics Presented by Marek Pechal Supervised by Simon Berger and Dr. Stefan Filipp Handed in to Prof. Dr. Andreas Wallraff, Laboratory for Solid State Physics, ETH Zurich Zurich, March 21, 2011 To read our E-mail, how mean of the spies and their quantum machine; Be comforted though, they do not yet know how to factorize twelve or fifteen. Volker Strassen, 1998 Abstract Quantum information processing is a concept which uses the laws of quantum mechanics to process and transport data in a fundamentally more efficient way than classical in- formation technology. One of the schemes proposed to achieve reliable coherent control of quantum devices and to overcome decoherence effects, the biggest obstacle on the way towards practical implementations of quantum information processing, is based on geometric effects. The so-called geometric phase is a property of quantal evolution which only depends on the trajectory followed by the system in the space of quantum states and not on the dynamical properties of the evolution – its time scale and the eigenenergies of the underlying Hamiltonian. In this thesis, the geometric phase of an adiabatically manipulated harmonic oscillator realized as a transmission line resonator in a supercon- ducting circuit is measured. We observe the expected scaling of the geometric phase with the square of the drive signal amplitude, its dependence on the direction and the shape of the path traced by the quantum state and its insensitivity to the length of the adiabatic sequence. We investigate dephasing effects caused by deviations from the ideal adiabatic regime. The results of our measurements are compared with the theoretical predictions and good quantitative agreements is found. In light of our results, a har- monic oscillator, a well-understood simple system, appears to be a convenient tool for studying the geometric phase and its potential use for quantum information processing. Contents Contents v List of Figures vii 1 Introduction 1 2 Superconducting qubits and circuit QED 3 2.1 Superconductingqubits . 3 2.1.1 Josephsonjunctions .. .. .. .. .. .. .. .. 3 2.1.2 Quantization of a circuit – Cooper pair box . ... 4 2.1.3 Thetransmonqubit .......................... 7 2.2 CircuitQED .................................. 12 2.2.1 The Jaynes-Cummings Hamiltonian . 14 2.2.2 Thedispersiveregime . 16 2.3 Dispersive readout and Qubit tomography . ..... 21 3 Geometric phase 27 3.1 Adiabatic limit in classical mechanics . ....... 27 3.2 Adiabatic transport in quantum mechanics and geometric phase . .. 29 3.3 Geometric phase in simple quantum systems . ..... 35 3.3.1 Two-levelsystem ............................ 35 3.3.2 Harmonicoscillator. 36 3.3.3 The tripod system – non-abelian geometric phase . ..... 40 3.4 Ambiguity of the non-adiabatic geometric and dynamical phase . 41 3.5 Generalizations of the geometric phase . ...... 45 4 Geometric phase of a resonator in circuit QED 47 4.1 Ramseymeasurementofthephase . 50 4.2 Exact solution of the harmonic oscillator evolution . .......... 52 4.2.1 Example – circular trajectory drive . 54 4.3 Dephasing due to residual qubit-resonator entanglement.......... 57 4.4 Two-qubitgeometricphasegate. 59 5 Experiment 62 5.1 Experimentalsetup............................... 62 5.1.1 Thesample ............................... 62 5.1.2 Thedilutionrefrigerator . 63 5.1.3 Measuringinstruments. 63 5.2 Methods..................................... 65 5.2.1 Resonatorspectroscopy . 65 5.2.2 Qubitspectroscopy. 65 5.2.3 Qubitmanipulationpulses. 67 5.2.4 Ramseyfringes ............................. 68 v 5.2.5 Qubit relaxation time measurement . 69 5.2.6 Rabioscillations ............................ 70 5.2.7 Calibration of the resonator drive strength . ...... 71 5.3 Geometricphasemeasurements . 74 5.3.1 Practical considerations . 74 5.3.2 Measurement of the total phase . 74 5.3.3 The total phase as a function of the pulse length . 75 5.3.4 Thephasesfordifferentpathshapes . 77 5.3.5 Direct measurement of the geometric phase . 79 5.3.6 Observed non-linearity of the geometric phase . ...... 83 5.3.7 Measurement of the pulse-induced dephasing . 83 6 Conclusion 85 6.1 Summaryoftheresults ............................ 85 6.2 Outlook ..................................... 86 A Geometric phase in the context of differential geometry 87 A.1 Paralleltransport............................... 87 A.2 The parallel transport law in a specific basis . ....... 90 B Parameters of the sample 94 Index 95 Bibliography 96 Acknowledgements 100 vi List of Figures 1 Equivalent circuits for a real and a split Josephson junction........ 4 2 Typesofsuperconductingqubits . 5 3 Thetransmonqubit .............................. 8 4 Energylevelsofatransmonqubit. 10 5 Chargedispersionofatransmonqubit . 11 6 Illustration of the rotating wave approximation . ....... 12 7 CircuitQEDarchitecture ........................... 13 8 Theresonatoranditscouplingtothequbit . 14 9 TheJaynes-Cummingsladder . .. .. .. .. .. .. .. 16 10 Resonator dispersive shift by a multilevel qubit . ........ 19 11 Circuit diagram of resonator response measurement . ........ 22 12 Resonator transmission in a continuous dispersive measurement . 22 13 Dynamical resonator transmission in a pulsed dispersive measurement . 24 14 Qubittomography ............................... 26 15 Theegg-and-spoonrace ............................ 28 16 An example of an adiabatic change in a quantum system . ..... 31 17 TheBlochsphere................................ 36 18 Solid angle traced by a state vector on the Bloch sphere . ....... 37 19 Shift of a harmonic potential by a linear term . ..... 38 20 Path of the coherent state quadrature in the complex plane ........ 39 21 Structureofthetripodsystem . 40 22 Two interpretations of the unconventional geometric phase ........ 43 23 Qubit-dependent path of the coherent state quadrature . ........ 48 24 Resonator drive with a circular trajectory . ...... 49 25 Ramseymeasurement ............................. 51 26 Exact trajectories of the resonator state driven by a circular pulse . 54 27 Geometric interpretation of the exact resonator state trajectories . 55 28 Phases induced by a drive with a circular trajectory . ....... 57 29 Qubit-dependent final displacement of the resonator state ......... 58 30 Interpretation of the coherent state overlap phase . ......... 59 31 Optical microscope image of the sample . 62 32 Circuitdiagramofthesample. 63 33 Schematic diagram of the experimental setup . ..... 64 34 Resonatortransmissionspectrum . 65 35 Qubitspectrum................................. 66 36 DRAGpulse .................................. 68 37 Ramsey and relaxation time measurement . 69 38 Rabifrequencymeasurement . 70 39 ACStarkshiftmeasurement. 72 40 Measured and theoretically calculated qubit spectra . ......... 73 41 Ramsey measurement of the total accumulated phase . ...... 75 vii 42 Ramsey oscillations for a strongly non-adiabatic pulse . .......... 76 43 Total accumulated phase as a function of the pulse length . ........ 76 44 Alternative shapes of the drive path . 78 45 Measuredphasesfordifferentpathshapes . 79 46 Spinechomeasurement ............................ 80 47 Ramsey oscillations in the spin echo measurement . ...... 81 48 Measured geometric phase difference versus the enclosed area ....... 82 49 Observed non-linearity of the geometric phase . ....... 83 50 Dephasinginducedbytheresonatorpulse . 84 51 Adiabatic evolution as a parallel transport in a fibre bundle........ 88 52 Paralleltransportonasphere . 91 viii 1 INTRODUCTION 1 Introduction The theoretical concept of quantum computation emerged almost three decades ago when Richard P. Feynman [1] noted that general quantum systems cannot be efficiently simulated on classical computers. He pointed out that a new kind of computer, itself quantum mechanical in nature, is much better suited for simulating other quantum systems. This idea served as a direct evidence that there are certain computational tasks, at least in the field of quantum simulations, for which a suitable quantum computer outperforms a classical one. Just a few years after Feynman, David Deutsch [2] came up with the concept of a univer- sal quantum computer and showed that it is indeed computationally more powerful than its classical counterpart – the famous Turing machine. His quantum computer architec- ture operating with two-state quantum systems as elementary carriers of information, commonly known as qubits, has proven to be very convenient thanks to its similarity to the classical model of computation. This similarity makes it particularly easy to see that a quantum computer can simulate a classical one. The crucial aspect of efficient quantum computation is that a qubit can exist not only in its two basis states but also in their arbitrary linear superposition. This means that a quantum computer can process multiple inputs at the same time, thus allowing certain tasks to be solved significantly faster than on a classical computer. Since Deutsch’s pioneering theoretical work, a range of quantum algorithms have been found that offer more efficient solutions than classical ones for many concrete computa- tional problems unrelated to simulating quantum mechanics. Some of the more famous examples are the Deutsch-Jozsa algorithm [3], Grover’s search algorithm [4] and Shor’s factoring algorithm [5]. The latter is widely known thanks to its serious implications for cryptography. A natural set of requirements that a physical system has to meet to realize