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Quantum Volume

Quantum Volume (QV) is a hardware-agnostic benchmark metric that measures the largest random circuit of equal width and depth that a quantum computer can successfully execute, capturing both qubit count and quality.

What it means

Quantum Volume, introduced by IBM in 2019, provides a single-number metric for comparing quantum computers.QV = 2^n where n is the largest number of qubits for which a random square circuit (depth n, width n) can be reliably executed with heavy output probability > 2/3.QV captures multiple hardware characteristics simultaneously: number of qubits, gate fidelity, connectivity, measurement errors, and crosstalk.A QV of 2^6 = 64 means the device can handle randomized circuits on 6 qubits with 6 layers of gates.Higher QV indicates a more capable quantum processor, but QV has limitations: it doesn't capture all aspects of quantum performance, and different applications may stress different hardware characteristics.Complementary metrics include CLOPS (circuit layer operations per second) and application-specific benchmarks.

Everyday analogy

Quantum Volume is like a single horsepower rating for a car -- it combines engine power, weight, and aerodynamics into one number, but doesn't tell you everything about how the car performs on every type of road.
Think of it as a standardized test score for quantum computers -- it gives a useful comparison metric, but doesn't capture every aspect of capability.

Common misconceptions

  • A higher qubit count does NOT automatically mean higher Quantum Volume -- gate quality and connectivity matter equally.
  • Quantum Volume is NOT the only benchmark that matters -- different applications may require different performance characteristics not captured by QV.

Key takeaways

  • QV = 2^n where n is the largest successful square circuit width.
  • Combines qubit count, gate fidelity, connectivity, and measurement quality into one metric.
  • Useful for comparing quantum hardware but not sufficient for predicting application performance.

Check your understanding

A quantum computer has Quantum Volume 128. What does this mean?

  1. A.It has 128 qubits
  2. B.It can run circuits on 128 qubits
  3. C.It can reliably execute random 7-qubit, 7-layer circuits (2^7=128)
  4. D.It can factor 128-bit numbers
Show the answer

Answer: C. It can reliably execute random 7-qubit, 7-layer circuits (2^7=128)

Why: QV = 128 = 2^7, meaning the device can successfully run randomized square circuits of width and depth 7 with heavy output probability above 2/3.

Builds on

QubitDecoherenceCNOT GateQuantum Measurement

Primary source: Cross, Bishop, Sheldon, Nation & Gambetta, Validating quantum computers using randomized model circuits, Phys. Rev. A 100, 032328 (2019), doi:10.1103/PhysRevA.100.032328

Metric definition established; no current-record claims in text.

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