Kevin J. Resch

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.

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.

Total reflection cannot occur with a negative delay time

It was recently predicted [ in ibid., vol. 33, p. 519, 1997] that the frustrated Gires-Tournois interferometer exhibits a negative delay time for reflection. Given its 100% reflectivity, this appears to contradict causality. We demonstrate that an additional, positive, contribution comes from consideration of the transverse dimension. We prove that this contribution is always large enough to enforce a positive total delay.

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.

Photon-exchange effects on photon-pair transmission

It has been proposed that photon-exchange effects associated with virtual atomic absorption could have widespread application in quantum information processing. Here we investigate simpler exchange effects associated with real absorption as modeled by an equivalent linear optical filter. Using nonclassical pairs of photons with variable time separation, we observe a maximum suppression of pair transmission by at least 5% with respect to the result for independent photons.

Quantum algorithms in the presence of decoherence: optical experiments

Summary form only given. We demonstrate the application of linear optical interferometers to performing quantum algorithms. In particular, we show how the Deutsch-Jozsa algorithm can be implemented in the presence of noise by using decoherence-free subspaces.

Quantum process tomography with entangled photons

Summary form only given. Using entangled photons from parametric down-conversion as optical qubits, we characterize single- and multiple-qubit operations by quantum process tomography. This allows complete characterization of the operations, including decoherence.

Conditional coherence via phase-sensitive postselection

A single beam of light created via spontaneous parametric down-conversion has a random phase. We show it is possible to induce a well-defined absolute phase in one beam by overlapping the other with a weak coherent state and conditioning on detection of a photon. This post-selection fixes the phase of the down-conversion even though stimulated emission is negligibly small.

Nonlinear optics with two photons (or less)

Nonlinear optical effects are generally limited to the high-intensity regime due to the small value of typical nonlinear coefficients. We demonstrate novel effects where a strong spectator field mediates a large nonlinear interaction between two photons. Such effects open the door to a new field of {\it quantum} nonlinear optics, potentially useful for two-photon all-optical switches (e.g., optical quantum logic gates) as well as other nonlinear effects such as the formation of quantum solitons. I will talk about enhanced upconversion, quantum state preparation, two-photon cross-phase modulation, and potential quantum-information applications to tasks such as Bell state determination.