Quantum Networks at NIST: Glossary

Cluster state is an entangled state of multiple qubits. They are different from multipartite entangled states such as GHZ or W states because it is more difficult to eliminate entanglement in a cluster state. Cluster states may be an alternative to quantum memory in quantum repeaters.

Entanglement is a phenomenon where the quantum state of two or more quantum systems cannot be described separately, even when they are separated by a large distance. In an entangled system, a measurement on one part can affect the system as a whole. Thus, a measurement outcome of one part of the system can predict a measurement outcome of the other.

Fidelity is a measure of similarity between quantum states. Typically, fidelity is used to compare a measured quantum state to a desired quantum state.

Fock state is a quantum state that describes a definite number of non-interacting identical particles. For instance, in quantum optics, a single-photon state is a Fock state of one photon.

Nitrogen-vacancy center (NV-center) is a type of crystal defect in diamond. It is comprised of a nitrogen atom substituting for a carbon atom in a lattice next to a lattice vacancy. NV-centers have a discrete quantum level structure and can be used to store and process qubits. Transitions between states can generate single photons.

Nonclassical (quantum) light is light that can only be described by quantum mechanics, i.e. when classical formalism does not suffice. Examples of nonclassical light include squeezed vacuum and single photons.

Photon number statistics is the probability distribution to find a certain number of photons in a field. In many cases, photon number statistics measurement is sufficient to demonstrate non-classicality of a light source.

Quantum communications is the exchange of certain information between two or more systems that cannot be operationally accomplished with classical information exchange.

Quantum dot is a small (typically few nm in diameter) semiconductor particle that exhibits optical and electronic properties that are differ from bulk semiconductors. Quantum dots commonly have discrete electronic states, similar to atoms. Those states can be used to process quantum information, and transitions between states may result in generating a single photon.

Qubit (qbit, quantum bit) is the binary unit of quantum information. A quantum bit can store a coherent superposition of both binary values at the same time.

Quantum key distribution (QKD) refers to a family of protocols that generate secret keys for two or more parties in a secure way. Theoretically, security of the key distribution is guaranteed by the laws of quantum mechanics.

Quantum network is a set of quantum nodes connected via quantum communication channels and auxiliary classical channels for stabilization, timing, and/or routing.

Quantum memory is a memory that can store one or more quantum states.

Quantum repeater is a device that enables quantum state transmission over long distances. Quantum states decay exponentially with transmission distance due to loss. A quantum repeater mitigates that effect.

Quantum Transducer is a device that converts a physical quantum state from one physical  system to another. For instance, to connect two ion-based quantum nodes over a large distance, an ion excitation is converted to a photon. Its carrier frequency may need to be changed to be compatible with a telecom band, typically via nonlinear frequency conversion. After propagation, the photon needs to be converted back to an ion excitation.

A Rydberg atom is an atom whose electron(s) is (are) excited to a high quantum state. The higher the state, the further the electron is from the nucleus, which enhances interactions of the Rydberg atom with an external electromagnetic field as compared to an atom in ground state, a useful property. For example, Rydberg interactions among atoms in the ensemble can be used to make single photons.

Single photon is a single excitation of a mode of electromagnetic field. Single photons are quantum states and thus cannot be prepared by classical light sources.

Single-photon detector is a detector that can distinguish between a vacuum state (no input photons) and a faint state of light (one photon or more) at its input. Some, but not all, single-photon detectors can measure photon number statistics; that is, they can distinguish between one, two, three, or more photons.

Single photon source is a source of single photons. Single-photon sources are typically characterized by measuring photon number statistics and showing that probability of two or more photons in the field is below that of the Poissonian (laser) source of the same power. Single atoms, single ions, quantum dots, and color centers in solids are examples of single-particle-based single photon sources. Other physical processes, such as spontaneous parametric down-conversion and spontaneous four-wave mixing can produce correlated photons; these are examples of heralded sources in which measuring one photon of the pair heralds the presence of the other.

Superposition (quantum). Any quantum state may be represented as a sum of several other quantum states. For example, a qubit can be in one of its basis states, |0>, |1>, or in a state that is a linear combination of both "0" and "1".