Making Photonic Qubits

A couple of earlier posts here covered superconducting and trapped ion qubits. Every approach hits the same wall: scaling. Photonic qubits encode information on individual photons, which largely ignore their surroundings and pick up less noise as a result. Their specific scaling bottleneck is producing single photons reliably.

Think of quantum dots like Dots candy. Each one is a small,
self-contained unit that delivers exactly one thing when you bite into it.

A photonic qubit stores quantum information on a single particle of light. Simple to say. Hard to build.

Choosing Your Encoding

We've discussed encoding in earlier posts. Pick a property of a photon to represent 0 and 1. Polarization is the most common choice: horizontal = 0, vertical = 1. Because in quantum mechanics, a single photon can be in both states simultaneously.

Other options include arrival time (time-bin), path through a chip (path encoding), or photon presence or absence. Each trades off differently depending on what you need downstream.

Creating Single Photons

The first real problem is producing single photons on demand.

• Spontaneous Parametric Down-Conversion (SPDC): Shine a laser into a special crystal. Occasionally one laser photon splits into two. Detect one, and you know the other exists. The success rate is roughly 1 to 10 out of every 100 laser pulses. Most pulses produce nothing.
• Quantum Dots: These are tiny semiconductor structures that emit exactly one photon per excitation. The best versions now exceed 99% reliability. The cost is that they only work near absolute zero, around -452°F. The photons travel fine at room temperature, but the sources need dilution refrigerators.

Manipulation and Interaction

Single-qubit operations on polarization qubits are straightforward. Wave plates rotate polarization by precise amounts. This is well understood and cheap.

Two-qubit operations are the hard part. Photons normally ignore each other completely. In 2001, three physicists showed you could use measurements and fast switching to simulate an interaction between photons. It works, but the basic two-qubit gate historically succeeded about 1 time in 16.

Scaling with Fusion

A newer approach skips reliable two-photon interaction entirely. You create small entangled photon groups, then merge them with simple optical components. When a merge fails, error correction absorbs the loss. PsiQuantum and others are pursuing this fusion-based architecture to reach millions of qubits.

Manufacturing

Photonic chips use the same fabrication processes and factories as conventional silicon chips. No custom facilities are required. One catch is that while the chips are silicon, reading results often requires superconducting detectors. Photons are stable at room temperature, but a full-scale photonic quantum computer will still likely sit inside a cryogenic system to keep the detectors and quantum dot sources cold enough to function.

Photonic qubits have a manufacturing path that superconducting and trapped ion systems do not: existing silicon fabs. The open question is whether photonic qubits can be mass-produced at the volumes a million-qubit machine requires.