Photonic Quantum Computing

Photonic Quantum Computing

Photonic quantum computing uses individual photons, particles of light, as qubits. It is the most radically different approach from superconducting and trapped-ion systems, operating at room temperature and offering a natural path to quantum networking.

The Photon as a Qubit

A photon's quantum information can be encoded in several degrees of freedom:

Polarization encoding: Horizontal polarization = $|0⟩$, vertical polarization = $|1⟩$. Gates are implemented with waveplates and beam splitters. This is the most intuitive encoding.

Dual-rail encoding: The qubit is in which of two optical modes (paths or time bins) contains the photon. $|0⟩$ = photon in mode A, $|1⟩$ = photon in mode B. This maps naturally to integrated photonic circuits.

Squeezed light (continuous variable): Instead of single photons, use quantum states of many photons (squeezed coherent states). Encodes quantum information in continuous amplitude and phase quadratures.

Linear Optical Gates

Single-qubit gates (phase shifters, Hadamard via beam splitters) are easy to implement with linear optics. Two-qubit gates are the fundamental challenge.

The cost: each two-qubit gate requires $O(n^2)$ ancilla photons and succeeds only probabilistically. This overhead makes fault-tolerant optical quantum computing resource-intensive.

Integrated Photonic Chips

Modern photonic quantum computers use silicon photonics or indium phosphide to fabricate miniaturized optical circuits on chips:

PsiQuantum and Xanadu are two leading companies building photonic quantum computers using silicon photonics at scale.