Entangled Photon Pairs: Efficient Generation and Detection, and Bit Commitment

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 eciency. Bit commitment is a two-party protocol that can be used as a cryptographic primitive for tasks like secure identication. 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 demonstrating the feasibility of two-party protocols in the noisy-storage model [2]. Device independent protocols are of great interest to modern quantum communication. These protocols require a high heralding eciency (66.7 %) [3]. The current implementations of these protocols using single photons are limited by optical losses and the limited detection eciency of standard single photon detectors. The Transition Edge Sensors (TES), developed at NIST, have a detection eciency >98% [4]. We present a highly ecient polarization entangled source of photon pairs obtained from spontaneous parametric downconversion in a PPKTP crystal. Using TESs we observe a 75.2% heralding eciency (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 eciency 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 delity of 99.3 %). Further, to demonstrate non-local eects, Alice and Bob need to implement their ix 0. SUMMARY choice of polarization measurement bases within the \time of ight" of the photon pairs. We have constructed a fast (3 s) transverse electro-optical polarization modulator for this purpose.