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arXiv:2109.08641 · Journal of the Optical Society of America B

Deterministic preparation of optical qubits with coherent feedback control

Amy Rouillard, Tanita Permaul, Sandeep K. Goyal, Thomas Konrad

Published May 13, 2024• 0 citations
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Abstract

We propose a class of preparation schemes for orbital angular momentum and polarization qubits carried by single photons or classical states of light based on coherent feedback control by an ancillary degree of freedom of light. The preparation methods use linear optics and include the transcription of an arbitrary polarization state onto a two-level OAM system (swap) for arbitrary OAM values ± ℓ within a light beam, i.e., without a spatial interferometer. The preparations can be carried out with unit efficiency independent from the potentially unknown initial state of the system. The swap scheme also allows us to implement arbitrary unitary gates on OAM qubits (± ℓ ) by reducing them to polarization gates. In addition, we show how to translate measurement-based qubit control channels into coherent feedback schemes for optical implementation.

DOI
10.1364/josab.523407

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We propose a class of preparation schemes for orbital angular momentum and polarization qubits carried by single photons or classical states of light based on coherent feedback control by an ancillary degree of freedom of light. The preparation methods use linear optics and include the transcription of an arbitrary polarization state onto a two-level OAM system (swap) for arbitrary OAM values ± ℓ within a light beam, i.e., without a spatial interferometer. The preparations can be carried out with unit efficiency independent from the potentially unknown initial state of the system. The swap scheme also allows us to implement arbitrary unitary gates on OAM qubits (± ℓ ) by reducing them to polarization gates. In addition, we show how to translate measurement-based qubit control channels into coherent feedback schemes for optical implementation.
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Abstract A simple argument is presented that explicitly shows how to construct an arbitrary quantum gate acting on orbital angular momentum (OAM) of single photons. The scheme can be applied to implement subspace multiplexing, where a single high-dimensional OAM qudit represents effectively a stack of multiple independent lower-dimensional qudits. A special subclass of unitaries composed of single-photon controlled gates is studied in detail and notable examples of the general approach are discussed. The generalization of the simple argument leads to the parallelization scheme, which results in the savings of resources. The presented schemes utilize only conventional optical elements and apply not only to single photons but also to classical light.
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The orbital angular momentum (OAM) of photons offers a suitable support to carry the quantum data of multiple users. We present two optical setups that send the information of nnn quantum communication parties through the same free-space optical link. Those qubits can be sent simultaneously and share path, wavelength, and polarization without interference, increasing the communication capacity of the system. The first solution, a qubit combiner, merges nnn channels into the same link, which transmits nnn independent photons. The second solution, the OAM multiplexer, uses controlled-not (CNOT\mathrm{CNOT}CNOT) gates to transfer the information of nnn optical channels to a single photon. Additional applications of the multiplexer circuits, such as quantum arithmetic, as well as connections to OAM sorting are discussed.