J. S. Lundeen

Experimental Application of Decoherence-Free Subspaces in an Optical Quantum-Computing Algorithm

For a practical quantum computer to operate, it is essential to properly manage decoherence. One important technique for doing this is the use of decoherence-free subspaces (DFSs), which have recently been demonstrated. Here we present the first use of DFSs to improve the performance of a quantum algorithm. An optical implementation of the Deutsch-Jozsa algorithm can be made insensitive to a particular class of phase noise by encoding information in the appropriate subspaces; we observe a reduction of the error rate from 35% to 7%, essentially its value in the absence of noise.

Experimental realization of the quantum box problem

Abstract The three-box problem is a gedankenexperiment designed to elucidate some interesting features of quantum measurement and locality. A particle is prepared in a particular superposition of three boxes, and later found in a different (but nonorthogonal) superposition. It was predicted that appropriate “weak” measurements of particle position in the interval between preparation and post-selection would find the particle in two different places, each with certainty. We verify these predictions in an optical experiment and address the issues of locality and of negative probability.

Conditional-phase switch at the single-photon level.

We present an experimental realization of a two-photon conditional phase switch, related to the c-phis; gate of quantum computation. This gate relies on quantum interference between photon pairs and generates entanglement between two optical modes through the process of spontaneous parametric down-conversion (SPDC). The interference effect serves to enhance the effective nonlinearity by many orders of magnitude, so it is significant at the quantum (single-photon) level. By adjusting the relative optical phase between the classical pump for SPDC and the pair of input modes, one can impress a large phase shift on one beam which depends on the presence or absence of a single photon in a control mode.

Nonlinear Optics with Less Than One Photon

We demonstrate suppression and enhancement of spontaneous parametric down-conversion via quantum interference with two weak fields from a local oscillator (LO). Effectively, pairs of LO photons up-convert with high efficiency for appropriate phase settings, exhibiting an effective nonlinearity enhanced by at least 10 orders of magnitude. This constitutes a two-photon switch and promises to be applicable to a wide variety of quantum nonlinear optical phenomena.

Practical measurement of joint weak values and their connection to the annihilation operator

Abstract Weak measurements are a new tool for characterizing post-selected quantum systems during their evolution. Weak measurement was originally formulated in terms of von Neumann interactions which are practically available for only the simplest single-particle observables. In the present work, we extend and greatly simplify a recent, experimentally feasible, reformulation of weak measurement for multiparticle observables [Phys. Rev. Lett. 92 (2004) 130402]. We also show that the resulting “joint weak values” take on a particularly elegant form when expressed in terms of annihilation operators.

Experimental observation of nonclassical effects on single-photon detection rates

It is often asserted that quantum effects can be observed in coincidence detection rates or other correlations, but never in the rate of single-photon detection. We observe nonclassical interference in a singles rate, thanks to the intrinsic nonlinearity of photon counters. This is due to a dependence of the effective detection efficiency on the quantum statistics of the light beam. Such measurements of detector response to photon pairs promise to shed light on the microscopic aspects of silicon photodetectors, and on general issues of quantum measurement and decoherence.

Electromagnetically induced opacity for photon pairs

It is shown that quantum interference with classical beams may be used to suppress or enhance the rate of spontaneous photon-pair production from a nonlinear crystal. Sum-frequency generation of the classical beams is simultaneously enhanced or suppressed via interference with a classical pump. In the extreme case, a crystal which is transparent to individual photons may block all photon pairs, converting them to 2w. This constitutes a coherent nonlinear response at the single-photon level, enhanced by a factor of approximately 10 10 Experimental data and a theoretical description are presented, and an attempt is made to delineate the classical and quantum aspects of these effects.

Comment on “Manipulating the frequency-entangled states by an acoustic-optical modulator”

A recent theoretical paper by Shi et al. [Phys. Rev. A 61, 064102 (2000)] proposes a scheme for entanglement swapping utilizing acousto-optic modulators without requiring a Bell-state measurement. In this Comment, we show that the proposal is flawed and no entanglement swapping can occur without measurement.

Comment on 'Linear optics implementation of weak values in Hardy's paradox'

A recent experimental proposal [S.E. Ahnert and M.C. Payne, Phys. Rev. A 70, 042102 (2004)] outlines a method to measure the weak value predictions of Aharonov in Hardys paradox. This proposal contains flaws in the state preparation method and the procedure for carrying out the requisite weak measurements. We identify previously published solutions to some of the flaws.

Experimental weak measurements in Hardy's Paradox

Hardys Paradox brings to light some of the striking difficulties in the interpretation of indirect quantum measurements. We weakly measure the location of the particles in Hardys Paradox. The results provide a self-consistent resolution.