Precise creation, characterization, and manipulation of single optical qubits

Abstract

We present the theoretical basis for and experimental verification of arbitrary single-qubit state generation, using the polarization of photons generated via spontaneous parametric downconversion. Our precision measurement and state reconstruction system has the capability to distinguish over 3 million states, all of which can be reproducibly generated using our state creation apparatus. In order to complete the triumvirate of single qubit control, there must be a way to not only manipulate single qubits after creation and before measurement, but a way to characterize the manipulations themselves. We present a general representation of arbitrary processes, and experimental techniques for generating a variety of single qubit manipulations, including unitary, decohering, and (partially) polarizing operations.

Experimental noise-resistant Bell-inequality violations for polarization-entangled photons

“…In realistic applications, where pure entangled states become mixed states due to different types of noise, violations of Bell's inequalities provide a method to characterize the robustness of the entanglement against noise. For this purpose, different methods for creating two-photon polarization mixed states have been proposed, analyzed, and tested [13,14,15].…”

A Reconfigurable Quantum Local Area Network Over Deployed Fiber

“…That is, setting the fast axis of the QWP for both receivers to 45 • with respect to horizontal, we tune the HWP angle on one of the two photons to maximize coincidences and define the D/D measurement for both receivers: all other basis state projections are obtained by rotating the HWP by fixed amounts relative to each D setting. This correction can be understood intuitively on the Poincaré sphere as a rotation that maintains an equal H/V superposition but changes the relative phase between them [68]. Specifically, defining QWP and HWP angle pair (θ Q , θ H ), we have the measurement settings H = (0…”

Creating multiphoton-polarization bound entangled states

“…Since arbitrary single photon polarization states can be created [43] and arbitrary twophoton pure states can be created via spontaneous parametric downconversion [42], the above UPB bound entangled state can be created by mixing the four constituent two-way separable three-photon pure states. The third photon polarization appears in the four BB84 states and is implemented by simple rotations from a fixed polarization state such as |H ; see Fig.…”

“…The exact forms of U and |Φ are not very illuminating and for the actual implementation the approximate forms are sufficient. All the above local unitaries can be implemented by waveplates [43]. We only need to randomly generate any of the above four states and the associated single photon states, and the statistical mixture of the outcome will be the desired bound entangled state.…”