Photonic qubits carry quantum information in quantum networks. In some networks, photonic qubits are used to distribute entanglement between nodes. These entangled qubits don’t contain secret information on their own; the value lies in the entanglement they create. Once shared, that entanglement can be used for Quantum Secure Communications, distributed quantum computing, or distributed quantum sensing. In other types of quantum networks, these photonic qubits may be carrying secret key material or other information, such as in certain variations of QKD protocols. Regardless of the specific application, the photons are always encoded with quantum information, using patterns of light that define the qubit's state.
How are photonic qubits encoded with information?
Photonic qubits are encoded by assigning the 0 and 1 states to different patterns of light. There are two main ways a photonic qubit is encoded:
- How many light patterns (modes) you use to represent it. This is called rail-based encoding.
- Which physical property of the photon (polarization, time, frequency, etc) creates those patterns. When this type of encoding is used, it is referred to as the degree of freedom.
Photonic Qubit Encoding: Rail-based Encoding
Single-rail encoding is an absence / presence encoding. In this type of encoding, the zero state is the absence of a photon, and the one state is the presence of a photon, but the photon may also be in a superposition of absence and presence.
In dual-rail encoding, there are two modes the photon could occupy. The state that the photon is in will depend on the mode (or rail) it occupies. In our example, if the photonic qubit is on the left rail, then it is in the zero state, and if the photonic qubit is on the right rail, it will be in the one state. The photon could also exist in a superposition of these rails. The “rails” refer to two separate and distinguishable modes, which may be different paths, polarizations, or spatial modes, and often implemented within the same fiber.
Photonic Qubit Encoding: Degrees of Freedom
In addition to rail-based encoding, photonic qubits can be encoded based on physical properties of the photon.
The most common degrees of freedom that photonic qubits are encoded in are:
- Time-bin encoding. This type of encoding uses a time difference to define the state that the photon is in: the photon arrives at an early time or it arrives at a later time.
- Frequency-bin encoding. Different frequencies are used to determine which state the photon is in. The two modes are two frequencies, f₁ or f₂.
- Polarization encoding. Polarization describes the direction in which the electric field of the photon oscillates. Commonly, horizontal (H) and vertical (V) polarizations are used as the two basis states, but diagonal/antidiagonal polarizations and left/right circular polarizations can also define a qubit.
Each type of qubit encoding has unique benefits and challenges. Some encoding methods are easier to generate, manipulate, or detect on specific hardware. Some encodings are easier to implement with existing technologies. Other encodings are more resilient to noise or loss. The physical medium, such as fiber optics vs. free space, also influences encoding choice because of how the medium impacts a qubit’s performance. Carefully selecting qubit encodings to fit your network’s specific use cases and environmental influences can help to ensure a successful quantum network deployment. Quantum network simulation can help organizations identify how well different types of photonic qubit encoding will perform in a given situation.
For more about qubits and how they are used in quantum networks, please see the white paper Qubits: Understanding Quantum Information.
