Photonic Qubits are quantum bits that use particles of light (photons) to carry and process information, rather than electrons or ions.
In a classical computer, electricity flows through wires, and a "bit" is represented by high or low voltage (1 or 0).
Here is an explanation of how they work, utilizing examples of how information is encoded into light.
1. How Photonic Qubits Work (Encoding Examples)
Unlike atoms, which are physical matter, photons are packets of energy. To turn a photon into a qubit, we must choose a specific "degree of freedom" (a property of the light) to represent the
Here are the three most common examples:
Example A: Polarization Encoding (The Intuitive Approach)
We can encode data based on the direction the light wave is oscillating.
State |0$ \rangle$: The photon oscillates Horizontally.
State |1$ \rangle$: The photon oscillates Vertically.
Superposition: The photon oscillates Diagonally (a combination of both horizontal and vertical at the same time).
Analogy: Imagine shaking a rope. If you shake it side-to-side, that’s a 0. If you shake it up-and-down, that’s a 1. If you shake it diagonally, it is in a quantum superposition of both.
Example B: Path (Dual-Rail) Encoding (The Chip Approach)
This is the most common method for modern silicon photonic chips (like those from PsiQuantum). It uses two separate optical wires (waveguides).
State |0$ \rangle$: The photon travels through the Top Wire.
State |1$ \rangle$: The photon travels through the Bottom Wire.
Superposition: The photon goes through a "Beam Splitter" and travels through both wires simultaneously.
Example C: Time-Bin Encoding (The Communication Approach)
This is often used in quantum internet/communication because it is very stable over long distances in fiber optic cables.
State |0$ \rangle$: The photon arrives Early (e.g., in the first nanosecond).
8 State |1$ \rangle$: The photon arrives Late (e.g., in the second nanosecond).
Superposition: The photon is "smeared" out in time, existing in a state where it has arrived both early and late.
2. The Hardware: How do we "compute" with light?
Photonic quantum computers look very different from the copper-wire chips in your laptop. They essentially function as highly advanced pinball machines for light.
| Component | Function | Classical Equivalent |
| Laser Source | Fires single photons one by one. | Power Supply |
| Waveguide | Microscopic channels that guide light on a chip. | Copper Wire |
| Beam Splitter | Splits a photon into two paths (creates Superposition). | Logic Gate |
| Phase Shifter | Slows down light in one path to change its state. | Logic Gate |
| Photodetector | Measures the photon at the end (destroys the qubit). | Readout/Screen |
3. Advantages vs. Challenges
Why use Photons? (The Advantages)
Room Temperature Operation: Unlike Google or IBM's superconducting chips which need to be frozen to near absolute zero (-273°C), light works perfectly well at room temperature.
9 Connectability: Since most of the internet is already built on fiber optics, photonic quantum computers could easily connect to a "Quantum Internet."
10 Speed: The qubits travel at the speed of light.
11
The Hard Part (The Challenges)
Photons don't interact: Two flashlight beams passing through each other don't bounce off; they pass right through. To perform calculations, qubits must interact (entangle).
12 Engineers have to use clever tricks (like "measurement-based" computing) to force them to interact.Loss: If a photon gets absorbed by the glass or lost in a connector, the information is gone forever. This requires very complex error correction.
4. Real-World Implementations
There are two major companies leading this field with different approaches:
PsiQuantum (Discrete Variable): They use the Path Encoding method mentioned above.
13 They are building massive silicon chips with standard semiconductor manufacturing to route single photons through distinct paths.14 Xanadu (Continuous Variable): They use a different approach called "Squeezed States" (or qumodes).
15 Instead of a single photon being 0 or 1, they use complex states of light fields involving multiple photons. This is mathematically more like an analog computer but running quantum algorithms.
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