ENTANGLED PHOTON PAIRS: EFFICIENT GENERATION AND DETECTION, AND BIT COMMITMENT SIDDARTH KODURU JOSHI B. Sc. (Physics, Mathematics, Computer Science), Bangalore University M.Sc. (Physics), Bangalore University A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY CENTRE FOR QUANTUM TECHNOLOGIES NATIONAL UNIVERSITY OF SINGAPORE 2014 ii To, The road not taken... This thesis is a testament to the path I chose. I would nevertheless, take this contemplative moment to reflect and honor the alternatives: The experiments I did not perform. My grand parents whose hands I could have held. My parents. My girlfriend. My..... But, Science is lovely, dark and deep And I have riddles to solve before I sleep. iii Acknowledgements Being a third generation pure bred academic, this experiment in long term sleep deprivation culminating in a doctoral degree, is almost a right of passage. It is what I always saw myself doing. In reality, it has been all I thought it would and so much more too. But unlike the tribes one hears about on National Geographic, my right of passage was not done in isolation in a remote jungle. I was assisted and guided, helped and consoled, cheered on and lectured to and so much more. And without the help I received, well its best not to contemplate such abysmal scenarios. Christian Kurtsiefer, my guide, has, much to my enlightenment, been at the receiving end of my doubts, and requests for help. Despite my impeccable ineptitude in timing my interruptions, he has always taken the time to set both me and this experiment on the right tracks. For that I am grateful. \Technical difficulties” or more commonly known as \we don't know why it went boom" were the bane of this experiment. I really value his support, guidance, help, and patience. I have also lost track of the number of dinners he has treated this hungry student to, in appreciation of which I can only quote \So long and thanks for all the fish” { the dolphins. Alessandro Cer´ehas been of great help, not only in the experiment but in proof reading this thesis too. Of late, he has been the \go-to" man for discussing ideas and dispelling bewilderment. I owe him much. My girlfriend Kamalam Vanninathan, has stood by me throughout and been the pillar I can lean on. For that and more I thank her. My parents were instrumental in my success and needless to say they have my eternal gratitude. Antia Lamas Linares, was the one who first showed me the ropes. Brenda Chng, in the perpetually being reorganized lab, continued to show we where everything was till date. Bharath Srivathsan and Gurpreet Gulati, friends, iv office mates and brains for me to pick. Are some names I would like to single out. My other friends and coworkers who assisted in so many small ways, I owe you all a debt of deep gratitude. It was fun working with M. Kalenikin, C.C. Ming and Q.X. Leong and I value their assistance. Collaborating with Nelly Ng and Stephanie Wehner was both fun and interesting. To all those in the center from whom I have begged, borrowed or stolen equipment and parts over the years, you have my fond thanks. v Contents Summary ix List of Publications xi List of Tables xii List of Figures xiii List of Acronyms xxvi Definitions of some terms xxviii 1 Introduction 2 1.1 Thesis outline . 3 2 Theory 5 2.1 Spontaneous Parametric Down Conversion (SPDC) . 5 2.1.1 Quasi-Phase Matching . 7 2.2 The Bell test . 10 2.3 Loopholes in a Bell test . 13 2.3.1 Locality/communication loophole . 13 2.3.2 Detection loophole or fair sampling assumption . 15 2.3.3 Freedom of choice loophole . 17 2.3.4 Other loopholes . 17 2.3.5 Practical Considerations . 18 3 Highly efficient source of polarization entangled photon pairs 20 3.1 Detecting photon pairs . 21 3.1.1 Corrections to the efficiency . 22 vi CONTENTS 3.2 Generating entanglement . 23 3.2.1 Polarization correlation visibility . 27 3.2.2 Tunable degree of entanglement . 29 3.2.3 Locking the phase . 29 3.2.4 Stability over time . 30 3.3 Collection optimization . 31 3.3.1 Focusing pump and collection modes . 32 3.3.2 Optimizing the focusing of the pump and collection modes . 34 3.4 Efficiency . 38 3.5 Wavelength tuning . 39 3.6 Bandwidth . 42 4 Detectors 48 4.1 Introduction . 48 4.2 Avalanche Photo-Diodes (APDs) . 49 4.3 Measuring the APD detection efficiency . 52 4.4 Transition Edge Sensors . 54 4.4.1 Electro-thermal feedback . 57 4.4.2 The SQUID amplifier . 63 4.4.3 Adiabatic Demagnetization Refrigerator . 68 4.4.4 Detecting a photon . 70 4.5 Measurements with the high efficiency source and TESs . 74 4.5.1 Peak height distribution . 74 4.5.2 Background counts . 77 4.5.3 Heralding efficiency measurement . 78 4.5.4 Timing jitter . 79 5 Bit Commitment 81 5.1 Introduction . 81 5.2 Protocol and its security . 84 5.3 Experiment . 90 5.4 Experimental parameters . 93 5.5 Symmetrizing losses . 97 5.6 Results and Conclusion . 99 vii CONTENTS 6 Conclusion and outlook 101 6.1 Future outlook . 103 A Fast polarization modulator 104 A.1 Introduction . 104 A.2 The Pockels effect . 105 A.3 Experiment and results . 108 A.3.1 Setup . 110 A.3.2 Acoustic ringing during fast polarization modulation . 112 A.4 Conclusion . 118 B Measurement of Gaussian beams 119 B.1 Gaussian beams . 119 B.2 Waist measurement . 120 C Alignment of the high efficiency polarization entangled source 122 D Calibration of APD detectors 130 References 134 viii Summary In this work we use sources of polarization entangled photon pairs for applications in quantum communication and to study fundamental quantum mechanics. This thesis consists of two experiments: bit commitment and the generation and detection of polarization entangled photon pairs with a high heralding efficiency. Bit commitment is a two-party protocol that can be used as a cryptographic prim- itive for tasks like secure identification. Secure bit commitment was thought to be impossible [1]. Nevertheless, we experimentally implemented a secure protocol for bit commitment by measurements on polarization-entangled photon pairs, thereby demon- strating the feasibility of two-party protocols in the noisy-storage model [2]. Device independent protocols are of great interest to modern quantum communi- cation. These protocols require a high heralding efficiency ( 66.7 %) [3]. The current implementations of these protocols using single photons are limited by optical losses and the limited detection efficiency of standard single photon detectors. The Transition Edge Sensors (TES), developed at NIST, have a detection efficiency > 98 % [4]. We present a highly efficient polarization entangled source of photon pairs obtained from spontaneous parametric downconversion in a PPKTP crystal. Using TESs we observe a 75.2 % heralding efficiency (pairs to singles ratio) of these photon pairs which is well above the threshold (66.7 %) for implementing device independent protocols and for a loophole-free Bell test. Key aspects to arrive at this high efficiency were careful mode matching techniques, and elimination of optical losses. Many device independent protocols make use of a wide range of entangled states, our source is capable of producing both maximally entangled states (with a polarization correlation visibility of 99.4 %) and non-maximally entangled states (with a fidelity of 99.3 %). Further, to demonstrate non-local effects, Alice and Bob need to implement their ix 0. SUMMARY choice of polarization measurement bases within the \time of flight" of the photon pairs. We have constructed a fast (3 µs) transverse electro-optical polarization modulator for this purpose. x List of Publications Some of the results of this thesis have been reported in the following peer reviewed publications 1. Nelly Huei Ying Ng, Siddarth K Joshi, Chia Chen Ming, Christian Kurtsiefer, and Stephanie Wehner.. Experimental implementation of bit commitment in the noisy-storage model.. Nature communications, 3:1326, 2012. Some of the other results in this thesis have been presented in conferences and are reported in the following proceedings 1. Siddarth K Joshi, Felix Anger, Antia Lamas-Linares and Christian Kurtsiefer.. Narrowband PPKTP Source for Polarization Entangled Photons. In The European Conference on Lasers and Electro-Optics, CD P24, Optical Society of America, 2011. 2. Siddarth K Joshi, Chia Chen Ming, Qixiang Leong, Antia Lamas- Linares, Sae Woo Nam, Alessandro Cere` and Christian Kurtsiefer.. Towards a loophole free Bell test. In CLEO: QELS Fundamental Science, QMIC-2, Optical Society of America, 2013. xi List of Tables 2.1 Some optical properties of BBO. Data was taken from [5, 6]. The direction of the ordinary and extraordinary rays are represented by o and e respectively. 8 2.2 Some optical and thermal properties of KTP. Data was taken from [5, 6]. 10 4.1 Table comparing some of the available single photon detectors. The data in this table was compiled from various sources [4, 7, 8, 9, 10, 11] and our measurements. It is indicative of the typical performance of these classes of detectors. There are several other types of detectors which are also being studied by various groups [9, 12, 13]. 50 5.1 Parameters required for security proof of bit commitment. All the above quantities are conditioned on the event that Alice registered a valid click. 94 6.1 Table comparing the various high efficiency sources of photon pairs. We can see that our source is very similar to the others. Our efficiency after correcting for the detection efficiency of the APDs is the highest.