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arXiv:1108.1543 · Physica Scripta

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

Assaf Shaham, H. S. Eisenberg

Published Feb 1, 2012• 8 citations
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Abstract

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.

DOI
10.1088/0031-8949/2012/t147/014029

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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.
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.
We describe experiments with photon pairs to evaluate, correct for, and avoid sources of error in optical quantum information processing. It is well known that a simple beamsplitter can non-deterministicially prepare or select entangled polarization states. We use quantum process tomography (QPT) to fully characterize this effect, including loss and decoherence. The QPT results identify errors and indicate how well they can be corrected. To evade decoherence in a noisy quantum channel, we identify decoherence-free subspaces using experimental channel characterization, without need for a priori knowledge of the decoherence mechanism or simplifying assumptions. Working with pairs of polarization-encoded photonic qubits, we use tomographic and adaptive techniques to identify 2- and 3-state decoherence-free subspaces for encoding decoherence-free qubits and qutrits within the noisy channel.