We present a source of entangled photons that violates a Bell inequality free of the "fair-sampling" assumption, by over 7 standard deviations. This violation is the first reported experiment with photons to close the detection loophole, and we demonstrate enough "efficiency" overhead to eventually perform a fully loophole-free test of local realism. The entanglement quality is verified by maximally violating additional Bell tests, testing the upper limit of quantum correlations. Finally, we use the source to generate "device-independent" private quantum random numbers at rates over 4 orders of magnitude beyond previous experiments.This document has been published at http://prl.aps.org/abstract/PRL/v111/i13/e130406 in Phys. Rev. Lett.PACS numbers: 03.65. Ud, 03.67.Ac, 42.50.Xa, 03.67.Bg In 1935, Einstein, Podolsky, and Rosen suggested that certain quantum mechanical states must violate one or both of the fundamental classical assumptions of locality (sufficiently distant events cannot change the outcome of a nearby measurement) and realism (the outcome probabilities of potential measurements depend only on the state of the system). These nonclassical two-particle states exhibit multiple-basis correlations (or anti-correlations), and are referred to as "entangled". Because locality and realism are so fundamental to classical intuition, a central debate in 20th century physics [1] revolved around the following question: could an alternative to quantum mechanics-a local realistic theory-explain entanglements seemingly nonclassical correlations? In 1964, John Bell devised a way to in principle answer this question experimentally, by analyzing the limit of allowed correlations between measurements made on an ensemble of any classical system [2]. If performed under sufficiently ideal conditions, a violation of Bells inequality would conclusively rule out all possible local realistic theories. Although entanglement has been experimentally demonstrated and the Bell inequality violated in a myriad of non-ideal experiments [3][4][5][6][7][8][9][10][11][12], each of these experiments fails to overcome at least one of two critical obstacles.The first obstacle-the "locality loophole"-addresses the possibility that a local realistic theory might rely on some type of signal sent from one entangled particle to its partner (e.g., a signal containing information about the specific measurement carried out on the first particle), or from the measurement apparatus to the source (known as the freedom of choice loophole). These loopholes have thus far only been closed using entangled photons [8, 13]; photons traveling in different directions can be measured at places and times which are relativistically strictly simultaneous (i.e., in a space-like separated configuration). The second obstacle-the "detection loophole"-addresses the fact that even maximally entangled particles, when measured with low-quantum-efficiency detectors, will produce experimental results that can be explained by a local realistic theory. To avoid this, almos...

Using spontaneous parametric down-conversion, we produce polarization-entangled states of two photons and characterize them using two-photon tomography to measure the density matrix. A controllable decoherence is imposed on the states by passing the photons through thick, adjustable birefringent elements. When the system is subject to collective decoherence, one particular entangled state is seen to be decoherence-free, as predicted by theory. Such decoherence-free systems may have an important role for the future of quantum computation and information processing.

Complete and precise characterization of a quantum dynamical process can be achieved via the method of quantum process tomography. Using a source of correlated photons, we have implemented several methods, each investigating a wide range of processes, e.g., unitary, decohering, and polarizing. One of these methods, ancilla-assisted process tomography (AAPT), makes use of an additional "ancilla system," and we have theoretically determined the conditions when AAPT is possible. Surprisingly, entanglement is not required. We present data obtained using both separable and entangled input states. The use of entanglement yields superior results, however.

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To deploy and operate a quantum network which utilizes existing telecommunications infrastructure, it is necessary to be able to route entangled photons at high speeds, with minimal loss and signal-band noise, and-most importantly-without disturbing the photons' quantum state. Here we present a switch which fulfills these requirements and characterize its performance at the single photon level; it exhibits a 200-ps switching window, a 120:1 contrast ratio, 1.5 dB loss, and induces no measurable degradation in the switched photons' entangled-state fidelity (< 0.002). Furthermore, because this type of switch couples the temporal and spatial degrees of freedom, it provides an important new tool with which to encode multiple-qubit states in a single photon. As a proof-of-principle demonstration of this capability, we demultiplex a single quantum channel from a dual-channel, time-division-multiplexed entangled photon stream, effectively performing a controlled-bit-flip on a two-qubit subspace of a five-qubit, two-photon state. PACS numbers:Switching technologies enable networked rather than point-to-point communications. Next-generation photonic quantum networks will require switches that operate with low loss, low signal-band noise, and without disturbing the transmitted photons' spatial, temporal, or polarization degrees of freedom [1]. Additionally, the switch's operational wavelength must be compatible with a low-loss, non-dispersive transmission medium, such as standard optical fiber's 1.3-µm zero-dispersion band [2,3]. Unfortunately, no previously demonstrated technology [4]-[15] is capable of simultaneously satisfying each of the above requirements: waveguide electro-optic modulators (EOMs) [16] and resonators [17,18] can operate at very high speeds (10 GHz) but completely destroy any quantum information stored in the polarization degree of freedom; micro-electromechanical switches [6,19] do not degrade the photon's quantum state, but operate at very low speeds (<= 250 kHz); polarizationindependent EOMs [16] can operate at moderate speeds (∼100 MHz) but with relatively high loss; and finally, traditional 1550-nm devices based on nonlinear-optical fiber loops [7,20] generate unacceptably high levels of Raman-induced noise photons (> 1 in-band noise photon per 100-ps switching window [21]).Although the requirements for ultrafast entangledphoton switching are collectively daunting, they describe a device that is capable of selectively coupling the spatial and temporal degrees of photonic quantum information. In other words, a device that can encode multiple-qubit quantum states onto a single photon, enabling quantum communication protocols that exploit high-dimensional spatio-temporal encodings. In this Letter we describe the construction and characterization of an all-optical switch which meets each of the aforementioned requirements, and whose aggregate performance (in terms of loss, speed, and in-band noise) exceeds that of all available alternatives [4]-[20] by orders of magnitude [22].Moreover, this switc...

While the most direct method to increase the brightness of a type-I entanglement source is to increase the collected solid angle of the down-conversion, this leads to effective decoherence caused by an angle-dependent phase shift. Using specially designed compensation crystals, we have reversed this effect and created the brightest source of entangled photons to date, over two million measured pairs per second, recorded while measuring the largest reported violation of Bell's inequality (1239 sigma).