Assaf Halevy

Entanglement Swapping between Photons that have Never Coexisted

The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the nonlocality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spacelike separation, but also to particles with timelike separation. In order to demonstrate these principles, we generated and fully characterized an entangled pair of photons that have never coexisted. Using entanglement swapping between two temporally separated photon pairs, we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed two-photon state demonstrates that entanglement can be shared between timelike separated quantum systems.

Compact 2D nonlinear photonic crystal source of beamlike path entangled photons

We experimentally demonstrate a compact two-photon path entanglement source based on 2D nonlinear quasi phase matching technique. Photon pairs are directly generated into well defined and easy to collect non-collinear beamlike modes.

Projection of two biphoton qutrits onto a maximally entangled state

We propose and demonstrate the projection of two quantum three state systems (qutrits) onto a maximally entangled state. The qutrits are represented by the polarization of biphotons — pairs of indistinguishable photons.

Resource efficient source of multiphoton polarization entanglement.

Current photon entangling schemes require resources that grow with the photon number. We present a new approach that generates quantum entanglement between many photons, using only a single source of entangled photon pairs. The different spatial modes, one for each photon as required by other schemes, are replaced by different time slots of only two spatial modes. States of any number of photons are generated with the same setup, solving the scalability problem caused by the previous need for extra resources. Consequently, entangled photon states of larger numbers than before are practically realizable.

Entanglement dynamics in the presence of controlled unital noise

Quantum entanglement is notorious for being a very fragile resource. Significant efforts have been put into the study of entanglement degradation in the presence of a realistic noisy environment. Here, we present a theoretical and an experimental study of the decoherence properties of entangled pairs of qubits. The entanglement dynamics of maximally entangled qubit pairs is shown to be related in a simple way to the noise representation in the Bloch sphere picture. We derive the entanglement level in the case when both qubits of a Bell state are transmitted through any arbitrary unital Pauli channel, and compare it to the case when the channel is applied only to one of the qubits. The dynamics of both cases was verified experimentally using an all-optical setup. We further investigated the evolution of partially entangled initial states. Different dynamics was observed for initial mixed and pure states of the same entanglement level.

Direct observation of the degree of correlations using photon-number-resolving detectors

Optical parametric down-conversion is a common source for the generation of non-classical correlated photonic states. Using a parametric down-conversion source and photon-number resolving detectors, we measure the two-mode photon-number distribution of up to 10 photons. By changing the heralded collection efficiency, we control the level of correlations between the two modes. Clear evidence for photon-number correlations are presented despite detector imperfections such as low detection efficiency and other distorting effects. Two criteria, derived directly from the raw data, are shown to be good measures for the degree of correlation. Additionally, using a fitting technique, we find a connection between the measured photon-number distribution and the degree of correlation of the reconstructed original two-mode state. These observations are only possible as a result of the detection of high photon number events.

Entanglement Dynamics in the Presence of Unital Noisy Channels

The entanglement level of two initially entangled qubits, subjected to an uncorrelated unital noisy channel is simply manifested by the radii of its Bloch sphere mapping. We demonstrate this relation experimentally using an all-optical setup.

Compact 2D Nonlinear Photonic Crystal source of Beamlike Path Entangled Photons

We experimentally demonstrate a compact two-photon path entanglement source based on 2D nonlinear quasi phase matching technique. Photon pairs are directly generated into well defined and easy to collect non-collinear beamlike modes.

Direct measurements of the non-classicality degree in photon-number correlations

Using photon-number resolving detectors, we directly measure the two-mode photon-number distribution of parametric down-conversion for different degrees of correlation. We present two quantitative measures for the degree of non-classicality.

The biaxial nonlinear crystal BiB 3 O 6 as a polarization entangled photon source using non-collinear type-II parametric down-conversion

We describe the full characterization of BiB3O6 as a polarization entangled photon source using non-collinear type-II parametric down-conversion and experimentally demonstrate entanglement generation with up to 2.5 times higher rates compared to beta-BaB2O4.