Physics
Photons as Qubits
Quick fact
A photon's polarization can simultaneously represent a 0 and a 1—a quantum superposition—making it a natural qubit.
Why this is interesting
You use light every day, but did you know that a single particle of light can be harnessed to perform quantum computations and send unhackable messages?
Read the full explanation
Understanding Photons as Qubits
Imagine a coin spinning in the air: it's not fully heads or tails until it lands. A quantum bit, or qubit, is like that spinning coin—it can exist in a blend of 0 and 1 at the same time. Photons, the particles of light, can be made to act as qubits. For example, we can use a property called polarization: horizontal polarization might mean '0', vertical means '1', and a diagonal or circular polarization means a superposition of both. By controlling how a photon is oriented, we encode quantum information. This is done using crystals, waveplates, and other optical devices. The key is that a single photon carries one qubit, and because photons barely interact with their surroundings, the quantum state stays intact longer than in other systems, making them perfect for quantum communication over long distances.
A deeper explanation
The mechanism behind using photons as qubits relies on encoding quantum information in one or more degrees of freedom of the photon: polarization (direction of the electric field), time-bin (early vs. late time of arrival), orbital angular momentum (twisted light), or phase (relative shift between paths). Polarization is the simplest: a photon passing through a polarizing beamsplitter can be put into a superposition of horizontal and vertical states using a half-wave plate. This superposition is a direct manifestation of the quantum principle that a qubit can be in a linear combination of basis states. Why are photons so attractive? They travel at the speed of light and have extremely weak interactions with each other and the environment, which preserves quantum coherence (the 'fragile' superposition) over many kilometers in optical fibers. This makes them the leading platform for quantum key distribution (secure communication) and quantum repeaters (extending quantum networks). In quantum computing, photons interact via nonlinear optical effects or measurement-induced gates, though building a universal photonic quantum computer remains challenging. The concept matters because it shows how fundamental quantum properties—superposition and measurement—are directly accessed and manipulated in a particle as ordinary as light.