Microwave Control of Qubits
Superconducting and many spin qubits are driven by shaped microwave pulses whose frequency, amplitude, and phase set the rotation on the Bloch sphere.
Resonant drive
To rotate a qubit you apply an oscillating field at its transition frequency. For superconducting and many spin qubits that frequency is in the microwave range, roughly 4 to 8 GHz. A pulse resonant with the qubit drives Rabi oscillations between |0> and |1>; the longer or stronger the pulse, the larger the rotation angle, up to a full population swap and beyond.
IQ modulation
Control electronics rarely synthesize GHz pulses directly. Instead a continuous local-oscillator tone is mixed with two lower-frequency baseband signals, the in-phase (I) and quadrature (Q) components, generated by digital-to-analog converters. Adjusting I and Q sets the pulse's amplitude and phase, and thus the axis and angle of the Bloch-sphere rotation. The rotating-frame picture makes the drive look like a static field about a chosen axis.
Pulse shaping
- Gaussian or cosine envelopes to limit spectral spread and off-resonant excitation
- DRAG corrections to suppress leakage into higher levels
- Virtual-Z gates, applied by shifting the phase of later pulses at zero time cost
Delivery
Room-temperature electronics generate the pulses, which travel down heavily attenuated coaxial lines into the refrigerator so that thermal noise from warmer stages does not reach the qubit. Each qubit typically needs its own drive line, making wiring a central scaling constraint. Crosstalk, where a pulse meant for one qubit slightly drives its neighbors, must be characterized and compensated.
Precise microwave control is what turns a two-level circuit into a programmable gate, and its fidelity ceiling is set jointly by pulse calibration, coherence, and the cleanliness of the delivery chain.