Assaf Shaham

Measurements of the dependence of the photon-number distribution on the number of modes in parametric down-conversion

Optical parametric down-conversion (PDC) is a central tool in quantum optics experiments. The number of collected down-converted modes greatly affects the quality of the produced photon state. We use Silicon Photomultiplier (SiPM) number-resolving detectors in order to observe the photon-number distribution of a PDC source, and show its dependence on the number of collected modes. Additionally, we show how the stimulated emission of photons and the partition of photons into several modes determine the overall photon number. We present a novel analytical model for the optical crosstalk effect in SiPM detectors, and use it to analyze the results.

Realizing controllable depolarization in photonic quantum-information channels

Controlling the depolarization of light is a long-standing open problem. In recent years, many demonstrations have used the polarization of single photons to encode quantum information. The depolarization of these photons is equivalent to the decoherence of the quantum information they encode. We present schemes for building various depolarizing channels with controlled properties using birefringent crystals. Three such schemes are demonstrated, and their effects on single photons are shown by quantum process tomography to be in good agreement with a theoretical model.

Realizing a variable isotropic depolarizer.

We demonstrate an isotropic depolarizing channel with a controllable degree of depolarization. The depolarizer is composed of four birefringent crystals and half-wave plates. Quantum process tomography results of the depolarization effect on single photons agree well with the theoretical prediction. This depolarizer can be used to test quantum communication protocols with photons.

Quantum process tomography of single-photon quantum channels with controllable decoherence

We characterized unital quantum channels of single-photon polarization qubits. The channels are composed of two birefringent crystals and wave plates, where their decoherence properties are controlled. An experimental comparison of two different depolarizing configurations was made using a quantum process tomography procedure. The results are with a high fidelity to theoretical predictions.

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.

Effect of decoherence on the contextual and nonlocal properties of a biphoton

Quantum contextuality is a nonintuitive property of quantum mechanics, that distinguishes it from any classical theory. A complementary quantum property is quantum nonlocality, which is an essential resource for many quantum information tasks. Here we experimentally study the contextual and nonlocal properties of polarization biphotons. First, we investigate the ability of the biphotons to exhibit contextuality by testing the violation of the KCBS inequality. In order to do so, we used the original protocol suggested in the KCBS paper, and adjusted it to the real scenario, where some of the biphotons are distinguishable. Second, we transmitted the biphotons through different unital channels with controlled amount of noise. We measured the decohered output states, and demonstrated that the ability to exhibit quantum contextuality using the KCBS inequality is more fragile to noise than the ability to exhibit nonlocality.

Experimental Study of the Decoherence of Biphoton Qutrits

We have generated various indistinguishable biphoton states, representing quantum trits. Their coherence was controllably changed and fully characterized by two-photon state tomography. Entanglement dynamics of the biphotons has also been studied.

Controlled Decoherence of Quantum Systems

The transition between quantum to classical systems is studied by applying decoherence to polarized photonic states. We show how to control the depolarization quantum process, and its effect on various quantum states.

Violation of the KCBS Inequality Using Polarized Biphoton Qutrits

We studied the violation of the KCBS inequality using polarized biphoton qutrits. This inequality refutes any classical description for measurement outcomes of spin-1 systems. The inequality was also tested in the presence of controlled decoherence.

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.