Crosstalk and Calibration

Learn about crosstalk between qubits and how real devices are calibrated.

Crosstalk and Calibration

Even when you apply a gate to one qubit, neighboring qubits can be disturbed. This is crosstalk, and it is one of the most challenging noise sources on real quantum hardware because it is correlated: crosstalk errors affect multiple qubits simultaneously, which is harder to correct than independent errors.

What Is Crosstalk?

Crosstalk occurs when:

1.A microwave pulse intended for qubit A leaks into neighboring qubit B's frequency band
2.Two simultaneous two-qubit gates on shared qubits interfere destructively
3.Parasitic couplings between qubits create unintended ZZ interactions even when no gate is applied (always-on ZZ coupling)

On superconducting hardware, qubits are coupled via tunable couplers or fixed capacitive links. Even when a coupler is "off," residual coupling remains. This always-on ZZ coupling shifts qubit frequencies depending on neighboring qubit states.

Types of Crosstalk

Frequency crosstalk: Microwave pulses have sidebands that accidentally drive neighboring qubits at slightly different frequencies. Addressed by careful pulse shaping (DRAG pulses, cross-resonance calibration).

Gate crosstalk: Running two two-qubit gates simultaneously on adjacent qubit pairs causes their microwave drives to interfere, creating multi-qubit errors not present in isolated gates.

Spectator errors: When qubit A is being gated, nearby "spectator" qubits B and C experience parasitic evolution. For superconducting qubits, spectator errors can be 0.1-1% per neighboring gate: comparable to direct gate errors.

ZZ crosstalk (always-on): The residual coupling between qubits in the "off" state creates a ZZ Hamiltonian $H = χ Z_{A} Z_{B}$ . This shifts the frequencies of both qubits based on each other's state. Effect: a qubit's $∣0 ⟩$ and $∣1 ⟩$ resonance frequencies differ by $\sim 2 χ$ when the neighbor is $∣0 ⟩$ vs $∣1 ⟩$ . $χ$ is typically 10-100 kHz, causing ~0.01% error per microsecond of idle time.

Calibration

To minimize these errors, quantum hardware undergoes continuous calibration:

Single-qubit calibration:

•Drive frequency: Find the exact resonance frequency for each qubit
•Rabi calibration: Determine the exact pulse amplitude for a pi rotation
•DRAG calibration: Tune pulse shape to suppress leakage to non-qubit states
•Readout calibration: Measure P(0|prepared 0) and P(1|prepared 1) for each qubit

This is the opening of the lesson. The full walkthrough, the interactive circuit, and the graded challenge continue inside myqubit.

How this lesson works

A guided reading lesson with interactive knowledge checks. Concepts are explained step by step with circuit diagrams and runnable examples, and you confirm understanding before moving on.

Part of: Quantum Hardware & NISQ

Explore the physics of real quantum computers, understand noise, and learn near-term algorithms designed for today's noisy hardware.

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