# Glossary¶

This is a glossary of terms used within the IBM Quantum site. If you are looking for the list of operations, go to the Operations glossary.

**amplitude**Amplitudes are complex numbers, and each possible outcome has a corresponding amplitude. Amplitudes are analogous to conventional probabilities, as the magnitude of the amplitude is correlated to the chance of measuring that outcome. Unlike conventional probabilities, amplitudes have phase and can interfere with each other.

**auxiliary qubit**Certain quantum operations require, or can be made more efficient using, extra qubits that do not store the inputs or outputs of the operation. Since these extra qubits do not contain useful information before or after the operation, their role is auxiliary. If the state of the auxiliary qubits is known before the operation, they are known as ‘clean’ qubits (and are usually set to ). If the state is unknown, they are referred to as ‘dirty’ qubits.

**backend**The term

*backend*can refer to either a quantum system or a high-performance classical simulator of a quantum system. Visit our Quantum services page to see all current IBM Quantum systems and simulators.**Bloch sphere**The Bloch sphere (named after Felix Bloch) is a visual representation of the state of a qubit.

*Note that the Bloch sphere is different from the q-sphere; for more details, see the*q-sphere visualization topic*in the IBM Quantum Composer docs.*Multiple states can also be simultaneously displayed (see below). The components of the Bloch representation of the qubit state are found from the expectation values of the , , and gates. A qubit described by a statevector has unit length and is found on the surface of the Bloch sphere. Qubits characterized by a density matrix will in general have length less than one, as determined by the purity of the state, and lie within the Bloch sphere.**classical register**See register.

**entanglement**Entanglement is a property of quantum systems comprised of more than one subsystem (i.e., qubits), where the quantum state of any one subsystem cannot be uniquely described independently of the remaining subsystems. Mathematically an entangled state is one that can not be written as as product of subsystem states. Subsystems of entangled states are mixed states requiring a density matrix representation. For a bipartite quantum system, the entanglement is equal to the entropy of the subsystems.

Fair-share queuing executes jobs on a quantum system in a dynamic order so that no provider can monopolize the system. The

*shares*in fair-share queuing represent the fraction of system time that is allocated to a given provider. Providers with the most device time have the highest priority in the fair-share algorithm. A provider’s*dynamic priority*depends on how much of the provider’s allotted system time has been consumed over a given floating window of time. When you send a job, it will be executed by the provider with the highest dynamic priority (or lowest fraction of allotted time used) at that moment. For more information, see the Fair-share queuing section.**global phase**A phase applied to a statevector as a whole, . States related by a global phase are equivalent in quantum mechanics; global phases can be ignored. This is a consequence of the fact that only energy differences, as opposed to absolute values, matter in determining the dynamics of physical systems. See An aside on global phase.

**job**A job ties together all of the relevant information about a computation on IBM Quantum: a quantum circuit, choice of backend, the choice of how many shots to execute on the backend, and the results upon executing the quantum circuit on the backend.

**OpenQASM**A quantum assembly language dialect (see QASM). For more information, see the OpenQASM code topic in the IBM Quantum Composer docs.

**phase**See relative phase.

**provider**Access to the various services offered by IBM Quantum is controlled by the

**providers**to which you are assigned. A provider is defined by a hierarchical organization of**hub**,**group**, and**project**. A hub is the top level of a given hierarchy (organization) and contains within it one or more groups. These groups are in turn populated with projects. The combination of hub/group/project is called a provider. Users can belong to more than one provider at any given time.**QASM**QASM is an abbreviation for quantum assembly language. It is a set of text-based instructions to describe and visualize quantum circuits. IBM Quantum uses a dialect called OpenQASM; see more in the OpenQASM code topic in the IBM Quantum Composer docs.

**quantum circuit**A quantum circuit is a computational routine consisting of coherent quantum operations on quantum data, such as qubits, and concurrent real-time classical computation. It is an ordered sequence of quantum gates, measurements, and resets, which may be conditioned on and use data from the real-time classical computation. A set of quantum gates is said to be universal if any unitary transformation of the quantum data can be efficiently approximated arbitrarily well as a sequence of gates in the set. Any quantum program can be represented by a sequence of quantum circuits and non-concurrent classical computation.

**quantum computer**A quantum computer is a device capable of executing coherent controlled quantum dynamics.

**quantum gate**A quantum gate is a reversible (unitary) operation applied to one or more qubits.

**quantum register**See register.

**qubit**A qubit (pronounced “cue-bit” and short for quantum bit) is the basic unit of quantum information. A qubit consists of two-levels that can be expressed using the “computational basis” states , and . Unlike a classical bit, the state of a qubit can be a linear combination (superposition) of both computational states. Read more about the qubit in the Field guide in the IBM Quantum Composer docs.

**register**A

*quantum register*is a collection of qubits on which gates and other operations act. A*classical register*consists of bits that can be written to and read within the coherence time of the quantum circuit.**relative phase**A phase difference between components of a superposition state. By convention, the first term in a superposition is made to be real, and the remaining states have phase values relative to this, e.g., .

**seed**A seed is the value introduced into the algorithm that generates pseudorandom numbers. The simulator creates randomness by generating results based on the seed.

**shot**Because the measurement of a qubit in a superposition state is random — the outcome is sometimes 0 and sometimes 1 — you must repeat the measurement multiple times to determine the likelihood that a qubit is in a particular state. When performing the experiment, you will be asked how many shots, or executions, to run in order to determine the qubit state probabilities.

**simulator**For a quantum computer comprised of a small number of qubits , we can simulate its behavior on a classical computer. In general such a computation requires storing complex numbers, where is the number of qubits. For circuits composed solely of Clifford gates, or circuits generating quantum states that are weakly entangled, special simulation techniques allow for simulating a greater number of qubits. See the Simulators overview topic to learn about the IBM Quantum simulators.

**statevector**Any single realization of a quantum system can be described through a complex vector known as its statevector. In a gate-based quantum computer the state of qubits has elements; the dimension of the statevector grows exponentially with .

**superposition**A superposition in quantum mechanics is a weighted sum, or linear combination, of two or more quantum states. A quantum computer with qubits can exist in a superposition of all of its computational basis states , through . Exploiting this ability is fundamental to most quantum algorithms.

**transpilation**Transpilation is the process where a quantum circuit is transformed into a new quantum circuit that performs the same task, but is restructured to be compatible with the physical layout of a particular quantum system and, where possible, optimize its performance.

**uncertainty principle**In quantum physics, we cannot simultaneously know two non-commuting variables (like the position and momentum of a particle). This implies that a quantum system in a perfectly definite state can be certain under one measurement and completely random under another. Moreover, if a quantum system starts out in an arbitrary unknown state, no measurement can reveal complete information about that state; the more information the measurement reveals, the more the state is disturbed. This is a underlying principle of quantum cryptography.

**universal fault-tolerant quantum computer**A universal fault-tolerant quantum computer is the grand challenge of quantum computing. It is a device that can properly perform universal quantum operations using unreliable components. See also universal quantum computer.

**universal quantum computer**A universal quantum computer is a machine that can simulate an arbitrary quantum state from an arbitrary initial quantum state. See also universal fault-tolerant quantum computer.