Quantum computing sounds complicated—and the underlying physics certainly is. But the basic idea of photonic quantum computing can be understood without a physics PhD. In this guide, we'll walk through how light can be used to perform quantum computations.

What Is a Photon?

Let's start with the basics. A photon is a particle of light. When you turn on a lamp or look at your phone screen, you're seeing billions of photons.

Photons have some special properties that make them useful for quantum computing. They travel at the speed of light (obviously!). They can be in quantum superposition—meaning they can exist in multiple states simultaneously. And they don't interact much with their environment, which helps preserve their quantum properties.

🎯 An Everyday Analogy

Think of photons as messengers zipping through a fiber-optic maze. At each junction, they can take multiple paths simultaneously—like a choose-your-own-adventure book where you're reading all the possible storylines at once. At the end, when you "measure" which path the photon took, the story collapses into a single narrative. That quantum parallelism is what gives quantum computers their power.

How Do Photons Encode Information?

In a classical computer, information is stored in bits—0s and 1s. In a quantum computer, we use qubits, which can be 0, 1, or a superposition of both.

Photons can encode qubits in several ways:

Polarisation Encoding

Light waves can oscillate in different directions. We might define horizontal polarisation as "0" and vertical polarisation as "1." A photon can be in a superposition of both polarisations—that's a qubit!

Path Encoding

A photon traveling through an optical circuit might take path A or path B. We can define "path A" as 0 and "path B" as 1. Using beam splitters, we can put the photon in a superposition of taking both paths simultaneously.

Time-Bin Encoding

A photon might arrive at a detector at time T1 or time T2. We can encode information in which time slot the photon arrives—or a superposition of both.

The Building Blocks

A photonic quantum computer uses several key components to generate, manipulate, and measure photons:

🔦 Photon Sources

Generate individual photons on demand. These might use spontaneous parametric down-conversion or quantum dots.

🔀 Beam Splitters

Split a photon's path, creating superposition states. Essential for quantum interference.

⚡ Phase Shifters

Adjust the quantum phase of photons as they travel through the circuit.

📏 Interferometers

Combine photon paths to create interference patterns, implementing quantum gates.

📡 Detectors

Measure photons at the end of the computation, collapsing superpositions into definite outcomes.

💻 Classical Control

Electronics that coordinate the optical components and process measurement results.

Putting It Together: A Photonic Quantum Circuit

In a photonic quantum computer, computation follows a sequence. First, photon sources generate qubits encoded in light. These photons travel through optical circuits containing beam splitters and phase shifters. The beam splitters create superpositions and allow photons to interact (entangle). Phase shifters fine-tune the quantum states. At the end, detectors measure the photons, and classical computers process the results.

🔬 The Mach-Zehnder Interferometer

One of the most important building blocks in photonic quantum computing is the Mach-Zehnder interferometer (MZI). It consists of two beam splitters with phase shifters in between. By adjusting the phases, we can implement arbitrary single-qubit quantum gates. Arrays of MZIs form the "processors" of photonic quantum computers.

Why Use Photons?

With all the different approaches to quantum computing, why focus on photons? Several reasons:

Low noise: Photons don't interact strongly with their environment, making them naturally resistant to the "decoherence" that plagues other quantum systems. This means quantum information survives longer.

Room temperature: Unlike superconducting qubits that need cooling to near absolute zero, photonic systems work at room temperature. No expensive cryogenic equipment required.

Speed: Photons travel at the speed of light. Quantum operations happen extremely quickly.

Networking: Photons are already how we transmit information through the internet. Photonic quantum computers naturally interface with fibre optic networks, enabling future quantum communication and distributed quantum computing.

The Challenges

Photonic quantum computing isn't without challenges. Generating single photons reliably remains technically demanding. Photons are also "flying qubits"—they're always moving, which makes some operations more complex than in systems with stationary qubits. And implementing two-qubit gates (essential for universal quantum computing) requires clever techniques since photons don't naturally interact with each other.

Researchers and companies are making steady progress on all these fronts, with integrated photonic chips showing particular promise for creating compact, scalable systems.

What Can You Do With It?

Photonic quantum computers are particularly well-suited for problems involving optimisation and sampling—finding the best solution among many possibilities, or generating samples from complex probability distributions. They're also natural candidates for quantum machine learning and quantum simulation of molecular systems.

And because they don't need cryogenic cooling, photonic systems are accessible for education and research in settings where superconducting quantum computers simply wouldn't be practical.

"Light has been our primary technology for communication for decades. It makes sense that it will also play a major role in the future of computation."

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