Photons as Flying Qubits

Photons as Flying Qubits

Quantum networks need a carrier for quantum information: something that can be generated, encoded, transmitted, and measured. Photons are the natural choice: they travel at light speed, interact minimally with their environment (low decoherence in flight), and can be created and detected with mature technology.

Why Photons?

Speed: Photons travel at the speed of light in the transmission medium. No other carrier approaches this.

Low decoherence: Unlike trapped ions or superconducting qubits, photons in fiber or free space interact weakly with their environment. A photon traveling through 100 km of fiber retains its quantum state (though many are lost due to absorption).

Telecom compatibility: Photons at 1310 nm and 1550 nm wavelengths travel in standard single-mode optical fiber: the same infrastructure used by the internet. Quantum networks can potentially piggyback on classical fiber infrastructure.

Single-photon sources and detectors: Spontaneous parametric down-conversion (SPDC) in nonlinear crystals produces entangled photon pairs. Single-photon avalanche diodes (SPADs) and superconducting nanowire single-photon detectors (SNSPDs) can detect individual photons with >90% efficiency.

Encoding Qubits in Photons

Multiple degrees of freedom can encode qubit information:

Polarization: Horizontal (|H⟩) and vertical (|V⟩) polarization encode |0⟩ and |1⟩. Superpositions of H and V encode arbitrary qubit states. Polarization is intuitive and easy to manipulate using wave plates and polarizing beam splitters.

Time-bin: Early |e⟩ and late |l⟩ arrival time encode the qubit. A photon split into two time slots by an interferometer can be in superposition of both. Time-bin encoding is robust against polarization mode dispersion in fiber (which would scramble polarization over long distances).

Orbital angular momentum (OAM): Photons with helical wavefronts carry angular momentum ±ℏℓ per photon. OAM modes with different ℓ values are orthogonal, enabling higher-dimensional qudits (not just qubits). Promising for free-space quantum networks.

Frequency: Different photon frequencies encode different qubit states. Frequency-bin encoding is compatible with dense wavelength-division multiplexing (DWDM): the technology used to send multiple classical channels on one fiber.