Glossary

Abbreviation of quantum bit, which is the most basic unit of information in a discrete quantum computing system. As compared to a classical bit, a qubit has a probability of being either a 0 or 1, represented by a superposition of the quantum states corresponding to 0 and 1.  

Physical Qubit: The two-state physical quantum systems (ranging from photons to ions) located within a quantum processing unit (QPU). For example, the ground state and an excited state of an ion forms a physical qubit with the ground state representing 0 and the excited state representing 1. 

Logical Qubit: A high-level abstraction of a qubit, used for fault-tolerant quantum computing, that is composed of multiple physical qubits for error correction. 

A quantum information approach where qubits are used, often represented by distinct states like 0 and 1.

A quantum information approach where qumodes are used, involving continuous properties like position and momentum rather than discrete states. 

A quantum unit of information similar to a qubit but with more than two possible states. 

A quantum unit of information that holds continuous-variable information used in continuous-variable quantum computing and quantum photonics. 

A model for computation in which a sequence of quantum gates and measurements are applied to a set of qubits (or qudits or qumodes). Typically, this defines a single execution of a shot on a QPU.  

A unitary operation on one or more qubits (or qudits or qumodes). In particular, they are always reversible, as opposed to measurements, and they grow exponentially in the number of qubits. For example, a quantum gate acting on 100 qubits can be represented by a 2100 x 2100 unitary matrix, which is far beyond what a classical computer can deal with. 

A machine containing a quantum processing unit (QPU) that can physically realize a set of qubits and execute quantum circuits, to do computations in an entirely different way than classical computers. 

For small numbers of qubits, quantum circuits can be simulated effectively, which is used for testing implementations on classical computers. However, since the time to simulate a quantum circuit grows exponentially in the number of qubits, simulators are primarily used for verifying the correctness of implementations and testing proofs of concept. 

Utilizing the principles of quantum mechanics, such as superposition and entanglement, allows for entirely new approaches to solving problems. In several examples, such as Shor’s algorithm or Grover’s algorithm, quantum algorithms have been shown to be able to solve problems more efficiently than their classical counterparts.  

The number of qubits in a quantum circuit (and hence the number of horizontal lines in the circuit diagram). Note that in some cases, classical bits used to control parts of the circuit or read out measurements may be counted toward the width. 

A measure of how many “layers” of quantum gates, executed in parallel, it takes to complete the computation defined by the circuit. Because quantum gates take time to implement, the depth of a circuit roughly corresponds to the amount of time it takes to execute the circuit. 

The number, possibly broken down by type, of the gates in a circuit. For example, two-qubit gates are typically more difficult to implement than one-qubit gates and so it is often helpful to have counts of each. 

The act of extracting one classical bit of data from a qubit (or qudit or qumode). This is an irreversible operation, and destroys entanglement and phase coherence between with the rest of the system. 

Superposition of states is a principle in quantum mechanics that describes a system existing in multiple states simultaneously. Mathematically, it is represented by a probability distribution of measuring each of the states, and it is what allows a qubit to represent a combination of 0 and 1 with different probabilities. 

The unwanted disturbances (e.g. thermal) that affect quantum systems, leading to errors in quantum computations, and can cause qubits to lose their quantum properties which is a process known as decoherence. 

The class of problems that quantum computers can solve efficiently with error probability less than a set bound. 

The ability of a quantum computer to approximate any arbitrary quantum computation using a finite set of quantum gates. 

A metric that measures the overall power of a quantum computer based on the largest square circuit it can successfully implement. 

An algorithm for approximating any quantum gate with a sequence of gates, from a simpler and universal gate set, to arbitrary precision.  

The process of converting quantum algorithms into a form that matches the architecture and gate set of a specific quantum computer. 

A measure (typically an average) of how accurately a quantum operation or state aligns with the desired outcome.  

The ability of a quantum system to perform accurate computations even in the presence of errors.  

The duration a qubit can retain its quantum state before losing coherence due to environmental factors. 

The time required to perform a single quantum gate operation on a quantum system. 

The description of the pairs (or more generally sets) of qubits in a quantum system on which gate operations can be performed; also called the topology of the system. 

A quantum phenomenon where two or more particles have interdependent states even when physically separated.