Null values and quantum state discrìmination.

We present a measurement protocol for discriminating between two different quantum states of a qubit with high fidelity. The protocol, called null value, is comprised of a projective measurement performed on the system with a small probability (also known as partial collapse), followed by a tuned postselection. We report on an optical experimental implementation of the scheme. We show that our protocol leads to an amplified signal-to-noise ratio (as compared with a straightforward strong measurement) when discerning between the two quantum states.

Violation of continuous-variable Einstein-Podolsky-Rosen steering with discrete measurements.

In this Letter, we derive an entropic Einstein-Podolsky-Rosen (EPR) steering inequality for continuous-variable systems using only experimentally measured discrete probability distributions and details of the measurement apparatus. We use this inequality to witness EPR steering between the positions and momenta of photon pairs generated in spontaneous parametric down-conversion. We examine the asymmetry between parties in this inequality, and show that this asymmetry can be used to reduce the technical requirements of experimental setups intended to demonstrate the EPR paradox. Furthermore, we develop a more stringent steering inequality that is symmetric between parties, and use it to show that the down-converted photon pairs also exhibit symmetric EPR steering.

Preservation of energy-time entanglement in a slow light medium.

We demonstrate the preservation of entanglement of an energy-time entangled biphoton through a slow light medium. Using the D(1) and D(2) fine structure resonances of Rubidium, we delay one photon of the 1.5 THz biphoton by approximately 1.3 correlation lengths and measure the fourth order correlation fringes. After the group delay the fringe visibility is reduced from 97.0+/-4.4% to 80.0+/-4.8%, but is still sufficient to violate a Bell inequality. We show that temporal broadening is the primary mechanism for reducing the fringe visibility and that smaller bandwidths lead to greatly reduced broadening.

All-optical delay of images using slow light.

Two-dimensional images carried by optical pulses (2 ns) are delayed by up to 10 ns in a 10 cm cesium vapor cell. By interfering the delayed images with a local oscillator, the transverse phase and amplitude profiles of the images are shown to be preserved. It is further shown that delayed images can be well preserved even at very low light levels, where each pulse contains on average less than one photon.

Large-alphabet quantum key distribution using energy-time entangled bipartite States.

We present a protocol for large-alphabet quantum key distribution (QKD) using energy-time entangled biphotons. Binned, high-resolution timing measurements are used to generate a large-alphabet key with over 10 bits of information per photon pair, albeit with large noise. QKD with 5% bit error rate is demonstrated with 4 bits of information per photon pair, where the security of the quantum channel is determined by the visibility of Franson interference fringes. The protocol is easily generalizable to even larger alphabets, and utilizes energy-time entanglement which is robust to transmission over large distances in fiber.