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arXiv:1006.5795 · Physical Review A

Realizing controllable depolarization in photonic quantum-information channels

Assaf Shaham, H. S. Eisenberg

Published Feb 3, 2011• 45 citations
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Abstract

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.

DOI
10.1103/physreva.83.022303

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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.
We show that a configuration of four birefringent crystals and wave-plates can emulate almost any arbitrary unital channel for polarization qubits encoded in single photons, where the channel settings are controlled by the wave-plate angles. The scheme is applied to a single spatial mode and its operation is independent of the wavelength and the fine temporal properties of the input light. We implemented the scheme and demonstrated its operation by applying a dephasing environment to classical and quantum single-photon states with different temporal properties. The applied process was characterized by a quantum process tomography procedure, and a high fidelity to the theory was observed.
We employed an electrically driven polarization controller to implement anisotropic depolarizing quantum channels for the polarization state of single photons. The channels were characterized by means of ancilla-assisted quantum process tomography using polarization-entangled photons generated in the process of spontaneous parametric downconversion. The demonstrated depolarization method offers good repeatability, low cost, and compatibility with fiber-optic setups. It does not perturb the modal structure of single photons, and therefore can be used to verify experimentally protocols for managing decoherence effects based on multiphoton interference.
The noisy quantum channel significantly causes the qubits to lose coherence. The depolarizing quantum channel is one of the most pessimistic quantum channels, which possibly causes the qubit coherence to be entirely lost. The success of the quantum information process depends on the strategy used to prevent the quantum system from interacting with the noisy quantum channel. Therefore, understanding depolarizing quantum operations is essential for investigating the effects of noisy channels on various quantum information processes. In this work, we have successfully implemented the depolarizing quantum channel for polarization photonic qubits in the laboratory. The depolarizing quantum channel was capable of continuously varying the degree of depolarization for all input-state polarizations.