Photonic Qubit

Quantum information encoded in photons, ideal for quantum communication and potentially scalable quantum computing.

Photonic qubits use single photons or optical modes to encode quantum information. They’re the workhorses of quantum communication and a promising path to scalable quantum computing.

Encoding Methods

Polarization Encoding

$$|0⟩=|H⟩\text{ (horizontal)},|1⟩=|V⟩\text{ (vertical)}$$

Simple, robust, limited to single-rail.

Path Encoding (Dual-Rail)

$$|0⟩=|10⟩\text{ (photon in mode A)},|1⟩=|01⟩\text{ (photon in mode B)}$$

Photon presence in one of two paths.

Time-Bin Encoding

$$|0⟩=|\text{early}⟩,|1⟩=|\text{late}⟩$$

When the photon arrives encodes the qubit.

Continuous Variable

Encode in quadratures of the electromagnetic field (like position and momentum of light).

Single-Qubit Operations

Simple with linear optics:

• Waveplates: Rotate polarization (any single-qubit gate)
• Beam splitters: Mix paths
• Phase shifters: Add phases

Single-qubit gates are essentially perfect!

Two-Qubit Operations

This is the hard part. Photons don’t naturally interact.

KLM Protocol

Knill-Laflamme-Milburn (2001): Use measurement and feedforward:

• Non-deterministic gates that work ~1/16 of the time
• Requires many ancilla photons and fast switching

Fusion-Based Quantum Computing

• Create small entangled states (resource states)
• Fuse them together via measurement
• Xanadu and PsiQuantum approach

Measurement-Based QC

Measurement-Based QC (MBQC): Create cluster states, compute via measurement.

Advantages

Challenges

Hardware Components

Major Players

• PsiQuantum: Million-qubit photonic computer goal
• Xanadu: Gaussian boson sampling, Borealis
• Quandela: European, integrated photonics
• ORCA Computing: Quantum memory approach

See also: Qubit, Quantum Teleportation