Industry & Era
Quantum Volume
A single-number benchmark metric (introduced by IBM) measuring the largest random circuit a quantum computer can successfully execute.
Definition
Quantum Volume is a benchmark introduced by IBM intended to capture a quantum processor's overall practical capability in a single number, accounting for qubit count, connectivity, and gate error rates together, rather than focusing on any one specification in isolation.
In depth
Quantum volume is a single-number benchmark introduced by IBM in 2017 and formalized by Cross and coauthors in 2019. It asks how large a random square circuit, with n qubits and n layers of random two-qubit gates, a machine can run well. A circuit passes if the machine produces heavy outputs, the more likely half of the bit strings according to the ideal distribution, more than two thirds of the time, with statistical confidence. The quantum volume is 2 to the n for the largest n that passes, so each added qubit of capability doubles the number.
The test captures several things at once: the number of qubits, gate fidelity, connectivity, crosstalk, and compiler quality. Because it uses random circuits spread over the whole device, it penalizes uneven quality and limited connectivity, and it favors machines with high fidelity and all-to-all coupling. Quantinuum's trapped-ion systems have led the benchmark, reaching a quantum volume of more than 33 million, or 2 to the 25th, in 2025, after a series of roughly tenfold annual increases. Superconducting systems from IBM reported values in the hundreds to low thousands in earlier years.
Limitations are clear. The verification requires classical simulation of the circuits to compute heavy outputs, so the test cannot go beyond about 40 to 50 qubits, and as machines pass this size, quantum volume saturates as a measure. It tests random circuits, which are not the structured circuits of useful algorithms. For these reasons, newer benchmarks such as application-based suites and mirror-circuit benchmarks have been developed, though quantum volume remains a common headline figure.
Technical Definition
Quantum Volume is determined by running increasingly large random square circuits (equal qubits and circuit depth) and finding the largest size the device can execute with output statistics matching theoretical expectations above a defined threshold, expressed as 2 raised to that largest successful size.
Visual Explanation: An Analogy
Think of Quantum Volume like a decathlon score in athletics — rather than ranking athletes by a single event (like raw qubit count), it combines performance across multiple relevant dimensions into one overall comparative number.
Key facts
| Item | Detail |
|---|---|
| Definition | 2 to the n, where n is the largest square random circuit passing the heavy-output test |
| Pass criterion | Heavy outputs with probability above 2/3 with statistical confidence |
| Record | Quantinuum H2 reported 2^25, about 33.5 million, in 2025 |
| Introduced | IBM, 2017; formalized in 2019 |
| Limit | Verification needs classical simulation, so it saturates near 40 to 50 qubits |
Where the idea comes from
IBM proposed the metric in 2017 and published it with collaborators in 2019. Quantinuum, Honeywell, and IonQ adopted it, and it has been the most widely quoted hardware benchmark.
Real-World Use Cases
- Used to compare quantum processors across different hardware platforms on a more standardized basis than qubit count alone
- Referenced in cross-platform benchmark studies, like the comparison covered in our Research Papers section
How it differs from related ideas
- Quantum volume tests whole-circuit performance, while randomized benchmarking tests individual gate error.
- Algorithmic qubits use application circuits, while quantum volume uses random ones.
- Cross-entropy benchmarking measures fidelity on random circuits and is used for supremacy experiments.
Frequently asked questions
What is quantum volume?
A benchmark that measures the largest random square circuit a quantum computer can run successfully, reported as 2 to the power of the circuit size.
Who has the highest quantum volume?
Quantinuum's trapped-ion H2 reported 2^25, about 33.5 million, in 2025.
What are its limits?
It uses random circuits that do not resemble useful algorithms, and it needs classical simulation to check results.
Related Terms
Keep exploring
- Algorithm: Random Circuit Sampling (Quantum Supremacy Experiments)
- Hardware: Quantinuum H2
- Hardware: IBM Heron (r1, r2, and r3)