I struggle to find any information on Nielsen and Chuang or similar texts on the exact definition of the so-called IQ plane (I think this is a notion closely related to solid state quantum computers such as transmon).

Can you provide a simple explanation of what the IQ plane or recommend some references that clearly explain it and its relation to the estimation of quantum states when one performs quantum state tomography?


2 Answers 2


IQ is an abbreviation for In-phase and Quadrature phase components of the signal. Whenever a qubit state ie. Z observable is read using a system such as resonator, the phase and amplitude of the resonator at a given frequency of signal changes depending on the qubits state. This change hence creates a change in the IQ plane. Using this measurement technique, quantum tomographies are performed.

  • 1
    $\begingroup$ can you provide with a reference? $\endgroup$
    – Marion
    Commented Apr 30, 2021 at 10:47
  • $\begingroup$ Perhaps in sup E.10 and E.11 of Kelly, J. S. Fault-Tolerant Superconducting Qubits, University of California, Santa Barbara, 2015. web.physics.ucsb.edu/~martinisgroup/theses/Kelly2015.pdf. $\endgroup$
    – taper
    Commented Feb 24, 2023 at 11:26

I know it a bit late for this, but in case I can help future people searching for this, this paper explains it quite well at the start:

Efficient Z-gates for quantum computing

And maybe this other helps, complement it:

Improving Quantum Gate Fidelities by Using a Qubit to Measure Microwave Pulse Distortions


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