Scalability and coherence are two essential requirements for the experimental implementation of quantum information and quantum computing. Here, we report a breakthrough toward scalability: the simultaneous generation of a record 15 quadripartite entangled cluster states over 60 consecutive cavity modes (Q modes), in the optical frequency comb of a single optical parametric oscillator. The amount of observed entanglement was constant over the 60 Q modes, thereby proving the intrinsic scalability of this system. The number of observable Q modes was restricted by technical limitations, and we conservatively estimate the actual number of similar clusters to be at least 3 times larger. This result paves the way to the realization of large entangled states for scalable quantum information and quantum computing.

We present direct measurements of a new geometric phase acquired by optical beams carrying orbital angular momentum. This phase arises when the transverse mode of a beam is transformed following a closed path in the space of modes. The measurements were done via the interference of two copropagating optical beams that pass through the same interferometer parts but acquire different geometric phases. The method is insensitive to dynamical phases. The magnitude and sign of the measured phases are in excellent agreement with theoretical predictions.

We describe five quantum mechanics experiments that have been designed for an undergraduate setting. The experiments use correlated photons produced by parametric down conversion to generate interference patterns in interferometers. The photons are counted individually. The experimental results illustrate the consequences of multiple paths, indistinguishability, and entanglement. We analyze the results quantitatively using plane-wave probability amplitudes combined according to Feynman's rules, the state-vector formalism, and amplitude packets. The apparatus fits on a 2Јϫ4Ј optical breadboard.

We report on our research effort to generate large-scale multipartite optical-mode entanglement using as few physical resources as possible. We have previously shown that cluster-and GHZ-type N -partite continuous-variable entanglement can be obtained in an optical resonator that contains a suitably designed second-order nonlinear optical medium, pumped by at most O(N 2 ) fields. In this paper, we show that the frequency comb of such a resonator can be entangled into an arbitrary number of independent 2×2 and 2×3 continuous-variable cluster states by a single optical parametric oscillator pumped by just a few optical modes.

We generated −2.2 dB of broadband amplitude squeezing at 1064 nm in a periodically poled KTiOPO 4 (PP-KTP) waveguide by coupling of the fundamental and second-harmonic cw fields. This is the largest amount of squeezing obtained to date in a KTP waveguide, limited by propagation losses. This result paves the way for further improvements by use of lower-loss buried ion-exchanged waveguides. © 2009 Optical Society of America OCIS codes: 270.6570, 270.1670 The experimental implementation of continuousvariable (cv) quantum information [1], an ambitious and exciting endeavor requires the creation of strongly squeezed light. Squeezed light has been produced using a number of methods, but the most successful experiments to date (ranging from −9 to −10 dB of squeezing) have used optical parametric oscillators, which feature a second-order nonlinear material placed in a resonant optical cavity [2][3][4]. In such systems, intracavity losses are amplified by the resonator buildup and thus present a serious hindrance to increasing the squeezing level. It would therefore be beneficial to suppress the optical cavity by use of nonlinear optical waveguides in which the transverse field confinement yields an increase of the nonlinear efficiency that can make up for the buildup of a reasonably high finesse cavity [5,6]. If need be, some cavity modes can still be exquisitely well defined by seeding the nonlinear waveguide with an optical frequency comb [7,8]. Waveguides are ideally suited for applications, such as integrated circuits, owing to their small size [9] and could also help alleviate gain-induced diffraction, which has been seen with traveling waves in bulk crystals [10]. Moreover, removing the optical cavity yields an increase of the squeezing bandwidth by several orders of magnitude [11], which is of interest for fast quantum processing. Finally, comb-seeded waveguides are of great interest for a recently proposed method to generate massively scalable cv entanglement [12]. Over a decade ago, several experiments tried to exploit the increase in nonlinear efficiency that waveguides provide in an attempt to obtain large amounts of traveling-wave squeezing with pulsed inputs [13][14][15][16]. However, owing to propagation losses in the waveguide, these works achieved a maximum of −1.5 dB of squeezing. Recently, advances in waveguide fabrication techniques have allowed for better than −4 dB of pulsed traveling-wave squeezing in MgO-doped periodically poled LiNbO 3 (PPLN) waveguides [17]. Undoped PPLN waveguides were used to obtain squeezing and entanglement with a cw input [11]. Other media for nonlinear optical waveguides include quasi-phase-matched KTP [13], quasi-phase-matched LiTaO 3 (LT) [14], and periodically poled stoichiometric LT [18]. Although the first measurement of squeezed light from an optical waveguide was made in KTP [13], squeezing in KTP waveguides has not been explored in recent years. There are, however, a number of reasons to do so. High-squeezing experiments have been carried out in bulk KTP and PPKT...