Photonic Qubit

Photons are natural carriers of quantum information. They travel at the speed of light, interact weakly with their environment (giving long coherence times), and connect naturally to the optical fiber infrastructure already deployed worldwide. These properties make photonic qubits attractive for quantum communication and, with significant engineering, for quantum computing.

Encoding schemes

There are several ways to encode a qubit in a photon:

Polarization encoding: the two basis states are horizontal and vertical polarization: $|H\rangle = |0\rangle$ and $|V\rangle = |1\rangle$. Waveplates act as single-qubit gates (a half-wave plate at 22.5 degrees implements the Hadamard). This is the most intuitive encoding and the most common in quantum optics laboratories.

Path encoding: a single photon travels through one of two optical paths, with $|0\rangle$ and $|1\rangle$ corresponding to the two arms of a beamsplitter interferometer. Beamsplitters implement beam-mixing operations; phase shifters implement $Z$ rotations.

Time-bin encoding: the qubit is encoded in whether the photon arrives in an early time bin ($|0\rangle$) or a late time bin ($|1\rangle$). This encoding is robust in optical fiber, where birefringence can scramble polarization over long distances.

Orbital angular momentum (OAM): photons can carry quantized angular momentum in their spatial mode structure, enabling high-dimensional (qudit) encoding, though generation and measurement of high-OAM modes is technically demanding.

Advantages

Room-temperature operation: unlike superconducting qubits (which require temperatures near 15 millikelvin) or some spin qubits (which require dilution refrigerators), photonic systems operate at room temperature. The photon source and detector may need cooling, but the optical network does not.

Low decoherence in transit: photons in vacuum or optical fiber do not couple strongly to thermal noise. Coherence lengths in fiber can span kilometers, which is why photons are the natural choice for quantum communication.

Native networking: photons are already the carriers of information in classical optical networks. Photonic qubits can be transmitted directly over fiber or free space, enabling quantum key distribution and quantum repeaters without transduction.

High clock rates: photonic systems can operate at GHz rates with pulsed lasers, with single-photon detectors achieving nanosecond timing resolution.

The fundamental challenge: two-qubit gates

Photons do not naturally interact with each other. In linear optics (beamsplitters, phase shifters, mirrors), two-photon operations are probabilistic.

The KLM theorem (Knill, Laflamme, Milburn, 2001) showed that universal linear optical quantum computing is possible using only linear optics, single-photon sources, and photon-number-resolving detectors, but only with ancilla photons and adaptive measurements (feedforward). The probability of a two-qubit gate succeeding can be boosted arbitrarily close to 1 with enough ancilla photons, but the resource overhead is significant.

Approaches to scalable photonic quantum computing

Linear optical quantum computing (LOQC): the KLM approach. Uses ancilla photons, postselection, and feedforward to implement near-deterministic gates. Requires fast optical switching to route photons conditionally on measurement outcomes.

Measurement-based quantum computing (MBQC) on photons: generate a large entangled cluster state of photons, then perform computation by adaptive single-qubit measurements. The challenge moves from implementing two-qubit gates to reliably generating large cluster states, which can be done by fusing smaller entangled states.

Integrated photonics: silicon photonics and lithium niobate platforms allow optical circuits to be fabricated on chip with thousands of components, dramatically reducing size and improving stability compared to bulk optics.

Continuous-variable photonics: rather than encoding in discrete photon number, encode in the quadratures of the electromagnetic field (like position and momentum for a harmonic oscillator). Squeezed states are the resource; Gaussian boson sampling is a leading near-term application.

Single-photon sources and detectors

Practical photonic quantum computing depends critically on high-quality single-photon sources and detectors. Ideal sources produce exactly one photon on demand, with high purity (no multi-photon events) and high indistinguishability (all photons are identical, enabling Hong-Ou-Mandel interference). Semiconductor quantum dots in microcavities currently achieve near-deterministic emission with indistinguishabilities above 99%.

On the detection side, superconducting nanowire single-photon detectors (SNSPDs) achieve efficiencies above 95% with timing jitter below 50 picoseconds and dark count rates below 1 Hz, essential for the fast feedforward operations that LOQC requires.

Companies and platforms

Company	Approach	Notable milestone
PsiQuantum	Fusion-based LOQC on silicon photonics	Large-scale foundry partnership for silicon photonics fabrication
Xanadu	Continuous-variable, Gaussian boson sampling	Borealis demonstration of quantum advantage (2022)
Quandela	Semiconductor quantum dot photon sources	High-efficiency near-deterministic single-photon sources
Nu Quantum	Photonic networking and qubit interconnects	Focus on quantum repeater components

See also

• Continuous Variable Quantum Computing
• Boson Sampling
• Quantum Network
• Qubit
• Decoherence