---
title: "How microwaves read a superconducting qubit"
description: "Follow a weak microwave probe through a cold resonator, amplifiers and two voltage clouds to see how a superconducting qubit is read."
canonical_url: "https://sandboxphysics.com/stories/instruments/superconducting-qubit-readout/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/instruments/superconducting-qubit-readout/"
last_modified: "2026-11-12"
---

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Inside the instrument

# How microwaves read a superconducting qubit

A superconducting qubit shifts the preferred frequency of a nearby resonator. A weak microwave probe leaves with a different amplitude and phase for each qubit state. Room-temperature electronics turn that difference into two voltages. Repeated measurements form two clouds that help assign a state.

Sandbox Physics · 2026-11-12

## Let a neighbouring circuit carry the message

The ground and excited states are two energy states of the qubit circuit. Attaching a meter also introduces loss. A common solution places a resonator nearby: a superconducting circuit that stores microwaves and responds most strongly around one frequency.

The qubit and resonator interact even when their frequencies are far apart. Each qubit state slightly shifts the resonator response. This is dispersive coupling. A probe at a fixed frequency then emerges with a state-dependent strength and timing within its oscillation cycle.

Sources: [3] [5]

## Down the cold cable and back again

Coaxial cables connect a room-temperature source to the cold chip. They carry thermal noise as well as the intended pulse. Attenuators absorb incoming power and release heat into refrigerator stages. Each attenuator also emits noise at its own temperature, so its location and thermal contact matter.

The returning signal needs amplification. Isolators suppress amplifier noise travelling backwards towards the chip. A 2021 Bluefors system example connects these cold lines to pulse control, IQ mixers and digital acquisition in one wiring diagram.

A cold platform supplies an environment. The qubit lifetime must be measured independently; cable contact, device loss and environmental noise also affect the experiment.

Sources: [1] [2]

## Two voltages preserve the wave's timing

Phase tells us where a wave sits within one oscillation cycle. A receiver compares the returning wave with a known reference called the local oscillator. Two mixers use references a quarter-cycle apart, producing I and Q voltages. Their position on a two-dimensional plot describes amplitude and phase.

An analogue-to-digital converter records integer counts. Independent reference signals reveal unequal gains and zero offsets in the two receiver channels. Blank measurements reveal the noise of the complete chain.

Sources: [1] [5]

## A single measurement lands inside a cloud

Repeated ground-state and excited-state preparation produces two clouds of noisy voltage points. A receiver can integrate along their direction of separation and set a dividing line. Wider separation and narrower clouds make assignment easier.

Waiting has a cost. The cavity field needs time to build after the probe arrives. During a long measurement, an excited qubit may relax and its signal move towards the ground-state response. Walter and colleagues studied this competition in their 2017 work on rapid single-shot readout, combining pulse timing, cavity response and relaxation.

Sources: [3] [4]

## Probe power has a working range

A stronger probe often separates the clouds more clearly. At high cavity photon number, the low-power frequency-shift law can fail. Higher transmon energy levels also affect that shift. A critical photon number provides a rough warning; an operating range still requires independent device characterization.

Sources: [5]

## Freeze the rule before testing new signals

One set of known preparation commands can establish a dividing line. Freeze it, check another set, then assign unfamiliar signals from their recorded voltages alone. This tests whether the same rule works on fresh observations.

A preparation command states what the experiment intended to prepare. Preparation can fail, and the qubit can relax during readout. Agreement with those commands describes their combined performance. Reporting quantum-state fidelity requires further separation of these errors.

Sources: [4] [1]

## [Follow the complete microwave readout chain](https://sandboxphysics.com/labs/transmon-artificial-atom/?instrument=dispersive-readout)

Change probe power and integration time, then compare the voltage clouds and held-out command score.

The model propagates a two-port conditional cavity field, records ADC voltages and applies a frozen blind classifier with an independently specified lifetime.

## Sources and further reading

1. [Lake et al. (2021) — The Bluefors Dilution Refrigerator as an Integrated Quantum Measurement System](https://bluefors.com/wp-content/uploads/2021/06/Application-Note-Bluefors-Dilution-Refrigerator-as-an-Integrated-Quantum-Measurement-System.pdf)

   Read printed pages 1–5 and visually inspected Figure 2 on printed page 4: two-port cavity transmission, distributed attenuation, HEMT isolation, IQ mixing, ADC and timing. Web PDF access failed; the publisher PDF was retrieved directly. Its device parameters are context, not the teaching device.

2. [Krinner et al. (2018), arXiv:1806.07862v1 — Engineering cryogenic setups for 100-qubit scale superconducting circuit systems](https://arxiv.org/pdf/1806.07862v1)

   Read printed pages 2–7, especially Eq.1–3: anchored coax, heat conduction, two-sided thermal-voltage PSD and normally ordered thermal-photon attenuation cascade. A stage temperature is not a qubit-state calibration.

3. [Gambetta et al. (2007), arXiv:0709.4264v2 — Quantum trajectory approach to circuit QED](https://arxiv.org/pdf/0709.4264v2)

   Read printed pages 2–4: unequal cavity ports, Eq.3.5 conditional coherent amplitudes and pointer-state separation. The teaching model propagates a conditional classical field with independently specified relaxation, not a stochastic quantum master equation.

4. [Walter et al. (2017), arXiv:1701.06933 — Realizing Rapid, High-Fidelity, Single-Shot Dispersive Readout of Superconducting Qubits](https://arxiv.org/pdf/1701.06933)

   Read pages 1–4: device and pulse sequence, sampled single-shot quadratures, integration, relaxation and assignment distributions. The identifier is 1701.06933; the published performance and Purcell-filter design are not reproduced.

5. [Blais et al. (2020), arXiv:2005.12667v1 — Circuit Quantum Electrodynamics](https://arxiv.org/pdf/2005.12667v1)

   Read printed pages 15–16, Eq.40–44, and pages 25–32, Figure14 and Eq.114–115: multilevel dispersive pull, state-dependent rough critical-photon screen, microwave IQ detection, quantum/added noise and integrated distinguishability. This is a scoped read of the 82-page review.

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