Using the process of spontaneous parametric downconversion in a novel two-crystal geometry, we have generated a source of polarization-entangled photon pairs which is more than ten times brighter, per unit of pump power, than previous sources, with another factor of 30 to 75 expected to be readily achievable. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source capabilities, we obtained a 242-σ violation of Bell's inequalities in less than three minutes, and observed near-perfect photon correlations when the collection efficiency was reduced. In addition, both the degree of entanglement and the state purity should be readily tunable. [5], and quantum computation [6]. At present, the most accessible and controllable source of entanglement arises from the process of spontaneous parametric down-conversion in a nonlinear optical crystal. Here we describe a proposal for, and experimental realization of, an ultrabright source of polarization-entangled photon pairs, using two such nonlinear crystals. Because nearly every pair of photons produced is polarization-entangled, the total flux of emitted polarization-entangled pairs should be hundreds of times greater than is achievable with the best previous source, for comparable pump powers. The new technique has the added advantage that the degree of entanglement and the purity of the state may be readily tunable, heretofore impossible.It is now well known that the photons produced via the down-conversion process share nonclassical correlations [7]. In particular, when a pump photon splits into two daughter photons, conservation of energy and momentum lead to entanglements in these two continuous degrees of freedom [8]. Yet conceptually, the simplest examples of entangled states of two photons are the polarizationentangled "Bell states":where H and V denote horizontal and vertical polarization, respectively, and for convenience we omit the normalization factor (1/ √ 2). For instance, HV − V H is the direct analog of the spin-singlet considered by Bell [2]. To date there have been only two methods for producing such polarization-entangled photon pairs, and each has fairly substantial limitations. The first was an atomic cascade -a two-photon decay process from one state of zero angular momentum to another. The resulting photons do display nonclassical correlations (they were used in the first tests of Bell's inequalities [9,10]), but the correlations decrease if the photons are not emitted backto-back, as is allowed by recoil of the parent atom.This problem was circumvented with parametric downconversion, since the emission directions of the photons are well-correlated. In several earlier experiments downconversion photon pairs of definite polarization were incident on a beamsplitter, and nonclassical correlations observed for those post-selected events in which photons traveled to different output ports [11]. However, the photons were actually created i...

An analysis is made of the background level and counter efticiencies actually necessary to perform a loophole-free Einstein-Podolsky-Rosen experiment. Both requirements are correlated. Photon counters do not absolutely have to have more than 82.8Fo efficiency if the signal-over-noise ratio is very high.PACS number(s): 03.65.Bz

Using a spontaneous-down-conversion photon source, we produce true nonmaximally entangled states, i.e., without the need for postselection. The degree and phase of entanglement are readily tunable, and are characterized both by a standard analysis using coincidence minima, and by quantum state tomography of the two-photon state. Using the latter, we experimentally reconstruct the reduced density matrix for the polarization. Finally, we use these states to measure the Hardy fraction, obtaining a result that is 122s from any local-realistic result.PACS numbers: 03.65. Bz, 42.50.Dv Entanglement is arguably the defining characteristic of quantum mechanics, and can occur between any quantum systems, be they separate particles [1] or separate degrees of freedom of a single particle [2]. The latter can be used to realize interference-based all-optical implementations of quantum algorithms [3], while multiparticle entangled states are central in discussions of locality [4,5], and in quantum information, where they enable quantum computation [6], cryptography [7], dense coding [8], and teleportation [9]. More generally, entanglement is the underlying mechanism for measurements on, and decoherence of, quantum systems, and thus is central to understanding the quantum/classical interface.Historically, controlled production of multiparticle entangled states has proven to be nontrivial. To date, the "cleanest" and most accessible source of such entanglement arises from the process of spontaneous optical parametric down-conversion in a nonlinear crystal (for a review, see [10]). This entanglement is of a specific and limited kind: the states are maximally entangled, e.g., ͑jHV ͘ 6´jVH͒͘͞ p 1 1 j´j 2 , where H and V , respectively, represent the horizontal and vertical polarizations of two separated photons, and´1. There is no possibility of varying the intrinsic degree of entanglement, [ 11], to produce nonmaximally entangled states without compromising the purity of the state, i.e., introducing mixture [12]. Nonmaximally entangled states have been shown to reduce the required detector efficiencies for loophole-free tests of Bell inequalities [13], as well as allowing logical arguments that demonstrate the nonlocality of quantum mechanics without inequalities [14][15][16]. More generally, such states lie in a previously inaccessible range of Hilbert space, and may therefore be an important resource in quantum information applications.States with a fixed degree of entanglement,´Ӎ 4͞3, have been deterministically generated in ion traps [17], and there have been several optical experiments where nonmaximally entangled states were controllably generated via postselection, i.e., selective measurement of a product state, after the state had been produced [18,19]. The latter experiments are of considerable pedagogical interest in that they demonstrate the logic behind inequality-free locality tests. However, the underlying state is factorizable, and so is not truly entangled. In this Letter, we describe the controllable production...