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Monday, June 1, 2026

What Is Gate Fidelity?

A quantum computer does not just need qubits. It needs to do something useful with them.

A gate is a physical process

That means changing their quantum statesThe full description of a quantum system's condition at a moment in time, such as whether a qubit is 0, 1, or a mix. in carefully controlled ways: rotating one qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once., entangling two, or stringing many such operations together into a calculation. These operations are called quantum gatesA single operation that changes the state of one or more qubits. Strung together, gates make up a quantum calculation, like the logic steps inside a normal chip..

A quantum gate is not a tiny switch. It is a physical process.

Depending on the kind of quantum computer, the gate might be made by a laser pulse, a microwave signal, an electrical field, or some other carefully tuned interaction. The goal is always the same: move the qubit from one quantum state to another with extreme precision.

But the qubit does not live in a perfectly isolated world. The control signal may be slightly off. Nearby qubits may interfere with one another. The qubit may drift outside the two states being used for computation. The operation may simply take too long relative to the qubit’s coherence timeHow long a qubit holds its delicate quantum state before noise scrambles it. Longer coherence means more time to compute..

Each of these effects can nudge the final state a little away from where it was supposed to land.

Two Bloch spheres compared. On the left, an ideal gate moves the qubit from its initial state cleanly onto the target state. On the right, a physical gate sends the qubit to a scatter of points clustered around the target. Two insets below show that 99.9% fidelity leaves a loose scatter while 99.99% fidelity is tightly clustered on the target.
Figure 1: An ideal gate lands the qubit exactly on the target state; a physical gate scatters around it. Gate fidelity measures how closely the physical operation matches the ideal one. The lower insets show the idea schematically: moving from 99.9% to 99.99% fidelity represents a tenfold reduction in gate infidelity.

What the number measures

Gate fidelityA score (for example 99.9%) for how accurately a quantum gate does what it is supposed to do. Higher means fewer errors. measures how closely a physical quantum gateA single operation that changes the state of one or more qubits. Strung together, gates make up a quantum calculation, like the logic steps inside a normal chip. reproduces the ideal mathematical operation it is meant to perform. Higher is better.

From 99.9% to 99.99% is a tenfold cut in gate infidelity. The small-looking step is the whole story.

A gate fidelity of 99.9% means the real operation is, on average, very close to the ideal one. The matching gate infidelityThe error left over in a gate (100% minus the fidelity). Moving from 99.9% to 99.99% fidelity cuts this tenfold, which is a big deal. is roughly 0.1%.

The gap between 99.9% and 99.99% looks small. It is not. Measured by the same method and under comparable conditions, it is a tenfold cut in gate infidelity.

There is no single gate-fidelity number for a whole quantum computer. Single-qubit gatesAn operation that acts on just one qubit. Generally easier to perform accurately than a two-qubit gate. are often easier to perform accurately. Two-qubit gatesA gate that acts on two qubits at once, such as the CNOT. Much harder to perform accurately than a single-qubit gate, and the real test of a machine. create interactions between qubitsThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once., can generate entanglementA quantum link where two qubits' states become tied together, so acting on or measuring one affects the other. It is a key resource for quantum computing. between them, and are usually harder to execute with high fidelity.

Which is why an impressive gate-fidelity claim should immediately raise one simple question:

Which gate?