Source-verifiedLevel 3

Microwave Pulse

A microwave pulse is the physical waveform that implements a quantum gate on a superconducting qubit — a nanosecond-scale burst of shaped electromagnetic radiation, defined by five parameters and ultimately stored as two arrays of I and Q samples.

What it means

On superconducting quantum processors, every gate in a circuit diagram is ultimately executed as one or more precisely shaped microwave pulses.At this layer a gate is the control envelope ε(t) in a time-dependent Hamiltonian H(t) = H₀ + ε(t)·H_c: H₀ describes the qubit at rest, ε(t) is the waveform we send, and the resulting time-ordered evolution U is the gate that actually happens.A pulse is specified by five parameters: duration (tens of nanoseconds), carrier frequency (resonant with the qubit transition, typically 4–8 GHz), amplitude (with duration, it sets the rotation angle on the Bloch sphere), phase (which selects the rotation axis: 0° gives an X rotation, 90° a Y rotation), and the envelope shape.Control electronics synthesize the waveform using IQ modulation, so at the lowest level a pulse is just two arrays of numbers — the in-phase (I) and quadrature (Q) samples — that a DAC or AWG converts into an analog voltage.Pulse-level programmers no longer write circuits but Schedules: timelines that assign pulses to drive, coupler and readout channels, like tracks in a music sequencer.

Everyday analogy

A circuit gate is a note on a musical score, and the pulse is the pianist's finger: the same note played with a different finger motion makes a different sound. A gate is one very short wave lasting about 40 nanoseconds — roughly 2.5 million of them fit into a single blink of an eye. Or think of a recipe: the line just says 'stir-fry', but the pulse is the actual wrist motion — too hard and the oil splashes (leakage to higher levels), too soft and it burns (decoherence wins).
The word 'pulse' comes from Latin pulsus, 'a beating, heartbeat', from pellere 'to strike' — the very same word a doctor uses when taking your pulse. A microwave pulse is exactly that: one short, precisely timed beat delivered to the qubit's heart. 'Envelope' comes from French envelopper, 'to wrap': the envelope is the smooth outline that wraps the fast microwave oscillation, just as a paper envelope wraps a letter.

Common misconceptions

  • A gate is NOT a fixed pulse — its exact shape depends on calibration data and changes from chip to chip and even day to day. The same Hadamard compiles to a different waveform on a different morning.
  • Gates look digital, but their body is analog — the clean CZ box in a circuit diagram hides a continuously tuned waveform where a 0.1% amplitude error or a 1 ns timing slip changes the result.

Key takeaways

  • A pulse is defined by five parameters: duration, carrier frequency (~4–8 GHz, resonant with the qubit), amplitude, phase, and envelope shape.
  • The phase selects the rotation axis (0° → X rotation, 90° → Y rotation); amplitude and duration together set the rotation angle.
  • At the control level a pulse is ultimately two arrays of numbers — I and Q samples — that a DAC/AWG converts into an analog waveform.
  • A fast entangling gate lasts about 40 ns; roughly 2.5 million of them fit into one blink of an eye.

Check your understanding

Which pulse parameter selects the axis of rotation on the Bloch sphere?

  1. A.Duration
  2. B.Amplitude
  3. C.Phase
  4. D.Carrier frequency
Show the answer

Answer: C. Phase

Why: The relative phase of the carrier sets the rotation axis: 0° drives an X rotation, 90° drives a Y rotation. Amplitude and duration together set the rotation angle, not the axis.

At the lowest control level, what is a pulse ultimately made of?

  1. A.A unitary matrix
  2. B.Two arrays of numbers: I and Q samples
  3. C.A single frequency value
  4. D.A line of QASM code
Show the answer

Answer: B. Two arrays of numbers: I and Q samples

Why: Control electronics use IQ modulation: the pulse is stored as an in-phase (I) sample array and a quadrature (Q) sample array, which a DAC/AWG converts into the analog microwave waveform.

Builds on

QubitBloch Sphere

Primary source: Krantz et al., Appl. Phys. Rev. 6, 021318 (2019), doi:10.1063/1.5089550

Pulse parameters, IQ modulation, control-Hamiltonian picture per Krantz SIV; pedagogy per Patra & Raina arXiv:2604.21565 (verified 2026-07-03).

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