Quantum-Polarization State Tomography Master Thesis submitted by Omer¨ Bayraktar Quantum Electronics and Quantum Optics Department of Applied Physics Royal Institute of Technology (KTH) and Physik-Department E11 Technische Universit¨atM¨unchen 2016 Graduation thesis on the subject Quantum Optics for the degree Master of Science in Condensed Matter Physics from the Department of Physics of the Technical University of Munich and Master of Science in Engineering from the School of Engineering Physics of the Royal Institute of Technology (KTH). TRITA-FYS 2016:09 ISSN 0280-316X ISRN KTH/FYS/--16:09--SE Abstract In quantum state tomography a set of informationally complete observables is mea- sured on a finite set of replicas of the same system, from which the quantum state of the very system is inferred. In qubit and continuous-variable quantum information, quantum state tomography is an indispensable benchmark. Improved security in quan- tum cryptography, computational speed-up, increased resolution in quantum metrol- ogy and other fundamental questions have increasingly drawn attention to qudits, i.e., high-dimensional states. However, qudit state tomography has been limited to physi- cal systems such as nuclear spins, the orbital angular momentum degree of freedom of photons and hybrid optical qudits. In this work the quantum state tomography of N- photon polarization states is demonstrated, rendering possible the characterization of multiple-qudit systems encoded in the quantum-polarization of N-photon states. This provides a benchmarking tool for experiments exploiting the quantum-polarization of multiple N-photon states. Sammanfattning Tomografi av kvanttillst˚andinneb¨ar att en grupp informationskompletta observabler m¨ats p˚aen ¨andlig m¨angd kopior av samma tillst˚and,och genom m¨atresultaten kan man sluta sig till vilket detta tillst˚and ¨ar. Inom f¨altet kvantinformation ¨ar kvanttill- st˚andstomografiett oumb¨arligt verktyg. F¨or att ¨oka s¨akerheten inom kvantkrypto, ¨oka ber¨akningshastigheten f¨or kvantalgoritmer, ¨oka uppl¨osningen i m¨atningar av kvanttill- st˚andsamt f¨or andra grundl¨aggande fr˚agorhar intresset ¨okat f¨or "kvantditar", dvs tillst˚andmed fler distinkta egentillst˚and ¨an tv˚a.Dock har tomografi av kvantdittill- st˚andbegr¨ansats till vissa fysiska system, t ex spinnegentillst˚andenhos atomk¨arnor, r¨orelsem¨angdsmomentet hos fotoner och optiska "hybridkvantditar". I detta arbete demonstreras tomografi av N-fotontillst˚and,vilket m¨ojligg¨or karakterisering av sam- mansatta kvantdittillst˚andsom kodats med hj¨alp av polarisationen. Denna metod kan anv¨andas som m˚attstock f¨or andra experiment d¨ar polarisationen hos N-fotontillst˚and studeras. Kurzfassung Die Quantenzustandstomographie dient der Bestimmung eines unbekannten Quanten- zustandes, indem wiederholt Messungen an identischen Kopien des selben Zustandes durchgefuhrt¨ werden. Sind die Messungen im tomographischem Sinne vollst¨andig, so gelingt die eindeutige Bestimmung des ursprunglich¨ unbekannten Quantenzustandes. In der Quanteninformationsverarbeitung mit Quantenbits und kontinuierlichen Va- III riablen hat sich die Quantenzustandstomographie zu einer Art Zertifizierungsmetho- de entwickelt. Desweiteren gewinnen h¨oherdimensionale Zust¨ande, sogenannte Qudits, weiter an Bedeutung, da sie die Sicherheit von Quantenkryptographie steigern, eine erh¨ohte Sensitivit¨at in quantenmetrologischen Anwendungen vorweisen und potentiel- le Kandidaten fur¨ die Beantwortung weiterer fundamentaler Fragen sin. Somit stellen Qudits die n¨achste Entwicklungsstufe in der Quanteninformationsverarbeitung dar. Je- doch ist die Quantenzustandstomographie von Qudits bisher auf physikalische Systeme wie den Kernspin, Bahndrehimpuls von Photonen und andere hybride, optische Qudits beschr¨ankt gewesen. Im Rahmen dieser Arbeit wird eine Methode fur¨ die Quantenzu- standstomographie von N-Photon Quantenpolarisationszust¨anden vorgestellt und ex- perimentell untersucht. Somit wird eine Verallgemeinerung der bisher g¨angigen Quan- tenzustandstomographie von optischen Qubits, die im Polarisationszustand von einzel- nen Photonen kodiert sind, und eine m¨ogliche Zertifizierungsmethode fur¨ Experimente mit optischen Qudits, die auf der Basis von N-Photon Quantenpolarisationszust¨anden implementiert werden, bereitgestellt. IV Contents Table of Contents V List of Figures VII 1 Introduction 1 2 Quantum State Tomography 3 2.1 Exact Quantum State Tomography ..................... 3 2.1.1 Single-Qubit State Tomography ................... 4 2.1.2 Multiple-Qubit State Tomography ................. 7 2.1.3 Qudit State Tomography ...................... 8 2.2 Quantum-Polarization State Tomography ................. 10 2.2.1 Polarization Measurements ..................... 11 2.2.2 Single-Qubit State Tomography ................... 12 2.2.3 Two-Qubit State tomography .................... 14 2.2.4 N-Photon Quantum-Polarization State Tomography ....... 15 2.2.5 Maximum Likelihood Estimation .................. 18 3 Quantum State Preparation 21 3.1 Spontaneous Parametric Down-Conversion ................. 21 3.1.1 Construction of the Hamiltonian .................. 22 3.1.2 Derivation of the Two-Photon State ................ 24 3.2 Two-Photon Interference ........................... 26 3.2.1 Two-Photon Wave Packet ...................... 26 3.2.2 Linear Optical Transformation of the Two-Photon State ..... 28 3.3 Two-Photon Correlation ........................... 33 4 Experimental Setup 37 4.1 Two-Photon Generation ........................... 37 4.2 Compensation of Temporal Delay ...................... 39 4.3 Characterization of Photon Pairs ...................... 40 4.3.1 Two-photon Spectra as Function of Crystal Temperature .... 40 4.3.2 Two-Photon Interference Experiment ............... 40 4.4 Two-Photon Correlation ........................... 41 V CONTENTS 4.5 Quantum-Polarization State Tomography ................. 42 4.5.1 Photonic Qubit State Tomography ................. 42 4.5.2 N-photon Quantum-Polarization Tomography ........... 43 4.6 Practical Notes for Alignment ........................ 45 4.7 Photon Counting ............................... 46 4.7.1 Single-Photon Detectors ....................... 46 4.7.2 Pulse Counting ............................ 47 4.7.3 Time-to-Digital Conversion ..................... 47 5 Results 51 5.1 Temperature-Dependent Two-Photon Spectrum .............. 51 5.2 Two-Photon Interference Experiment .................... 53 5.3 Demonstration of Spatial Quantum Beating ................ 56 5.4 Violation of CHSH Inequality ........................ 58 5.5 Photonic Two-qubit State Tomography .................. 61 5.5.1 Reconstruction of Two-Photon Quantum-Polarization State ... 65 5.6 N-Photon Quantum-Polarization State Tomography ........... 67 6 Summary and Outlook 71 A Reconstructed Two-Photon Quantum Polarization States 73 A.1 N = 2 N00N-State .............................. 73 A.2 Equipartition State .............................. 73 Bibliography 77 VI List of Figures 2.1 State representation on the Bloch sphere .................. 6 2.2 Polarization measurements .......................... 10 2.3 Single-photon ellipsometer .......................... 13 2.4 Two-qubit state tomography ........................ 14 2.5 N-photon quantum-polarization state tomography ............ 16 3.1 Photography of spontaneous parametric down-conversion ........ 25 3.2 Sketch of setup for measuring joint detection rates ............ 26 3.3 Two-photon wave packet in spontaneous parametric down-conversion .. 29 3.4 Sketch of setup for measuring two-photon interference .......... 29 3.5 Wave packets in two-photon interference .................. 31 3.6 Quantum beating and two-photon interference .............. 32 3.7 Sketch of setup for violation of CHSH inequality ............. 34 4.1 Experimental setup: Spontaneous parametric down-conversion ..... 38 4.2 Experimental setup: Compensator ..................... 39 4.3 Experimental setup: Two-photon spectrum analyzer ........... 40 4.4 Experimental setup: Two-photon interference ............... 41 4.5 Experimental setup: Correlation measurements .............. 42 4.6 Experimental setup: Overview ....................... 43 4.7 Interference fringes during alignment .................... 46 4.8 Signal processing with time-to-digital converter .............. 48 5.1 Temperature tuning of two-photon spectrum ............... 52 5.2 Two-photon interference with indistinguishable photons ......... 53 5.3 Enhancement in biphoton rate ....................... 54 5.4 Two-photon interference contrast and spatial distinguishability ..... 55 5.5 Two-photon interference with frequency entangled photons ....... 56 5.6 Two-photon interference contrast and spectral distinguishability .... 57 5.7 Correlation measurements with single-mode fibers ............ 58 5.8 Correlation with multi-mode fibers ..................... 59 5.9 Variations of coupling efficiency in two-qubit state tomography ..... 62 5.10 Two-qubit state tomography: Reconstructed density matrices ...... 63 5.11 Two-qubit state tomography: Experimental data and reconstruction .. 64 VII LIST OF FIGURES 5.12 Two-photon quantum-polarization state from two-qubit state tomography 66 5.13 Two-photon quantum-polarization state from fixed setup ......... 67 A.1 Further two-photon quantum-polarization states from fixed setup .... 74 VIII Chapter 1 Introduction The situation of experimental quantum mechanics during its first 60 years can be summarized by the following sentences, written by E. Schr¨odingerin 1952 [1]: