PolarisationsQubit

PolarisationsQubit is a qubit implemented using the polarization state of a photon. It encodes quantum information in the two-level subspace of polarization, typically horizontal |H> and vertical |V>, with other bases such as diagonal |D> and anti-diagonal |A> or left/right circular |L> and |R> used for measurements and operations.

Implementation involves preparing a photon in a superposition α|H> + β|V> to define the qubit state. Initialization is achieved with single-photon sources and polarization control elements such as waveplates and polarizers. State preparation and basis changes are performed with optical components like polarizing beam splitters and half-wave plates to rotate between bases. Readout is accomplished by projecting the polarization onto a chosen basis using polarizers and detectors that register the outcome.

Quantum gates on a PolarisationsQubit are primarily realized with linear optical elements. Single-qubit gates use waveplates or electro-optic modulators to implement rotations on the Bloch sphere. Two-qubit gates are more challenging and typically rely on nonlinear optical interactions or measurement-induced entangling operations within linear-optics quantum computing schemes, often employing interference with ancilla photons and photon detectors.

Noise and decoherence primarily arise from changes in polarization due to birefringence in optical components and transmission media. Mitigation strategies include using polarization-maintaining fibers or waveguide structures, active polarization control, and reference-frame alignment to preserve states during transmission.

Applications of PolarisationsQubits are widespread in quantum communication and photonic experiments, where polarization provides convenient interfaces for entanglement generation and state tomography. For scalable quantum computation, alternative encodings or integrated photonic platforms are often pursued to reduce loss and improve stability.