---
title: "How to measure one quantum system and keep it available"
description: "How can a detector learn about one quantum system without simply consuming it?"
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language: "en"
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  zh-CN: "https://sandboxphysics.com/zh/stories/quantum-evidence/single-quantum-measurement-backaction/"
last_modified: "2026-10-24"
---

[Quantum physics: reading the evidence](https://sandboxphysics.com/stories/quantum-evidence/)

Nobel A · Evidence and control

# How to measure one quantum system and keep it available

Sandbox Physics · 2026-10-15

How can a detector learn about one quantum system without simply consuming it?

## Keep the system, then measure it again

A light detector usually learns that a photon arrived by absorbing it. The photon is gone. For a physicist trying to follow one quantum system over time, that is a frustrating kind of measurement: the first answer ends the experiment. The work recognized by the 2012 Nobel Prize made another approach possible. Trap an ion or store light in a cavity, learn something about it, and return to the same system for the next observation.

David Wineland and his colleagues used light to control trapped ions. Serge Haroche and his colleagues sent atoms through stored light to probe it. Light and matter swapped roles, but both methods turned an individual quantum system into something experimenters could prepare, question and control repeatedly.

Read the evidence: NIST (2012), David J. Wineland wins Nobel Prize in Physics Nobel Prize in Physics 2012 — presentation speech

## A trapped ion can announce its state with light

An ion is an atom with an electric charge. Electromagnetic fields can hold it in a small region for repeated experiments. An ion has several allowed internal energy states. Laser light can make one state bright: the ion absorbs a photon, emits one, and returns to a state where it can repeat the cycle. Another internal state sits outside that cycle and stays comparatively dark. Many scattered photons amplify the difference between the two states.

The ion remains trapped during this readout. A time trace can show long bright stretches interrupted by dark intervals, like a telegraph signal. To identify quantum jumps, experimenters compare the count levels and the time spent in each state with the known level structure and laser drives. They also measure background and collection efficiency; a short gap can simply mean that no emitted photon reached the detector.

This is already a useful measurement, but it changes the ion. A superposition has a definite phase relationship between its alternatives, which later pulses can turn into interference. Fluorescence that tells bright from dark removes that relationship. Retaining the particle and retaining its superposition are different experimental goals.

Read the evidence: Leibfried et al. (2003), Quantum dynamics of single trapped ions, Rev. Mod. Phys. 75, 281

## Atoms can count light without swallowing it

Haroche’s experiments reversed the arrangement. A microwave cavity held the light, and passing atoms carried information to the detector. In the 2007 experiment, the atomic transition was tuned away from the cavity resonance. The field changed the relative timing of two components of the atom’s state, called their phase, without being absorbed by the atom. Later pulses turned that phase shift into a detectable internal state.

One atom gave only partial information. A succession of atomic results gradually distinguished the possible photon numbers. The photons remained in the cavity between probes, making it possible to follow number changes as the stored light decayed. With imperfect probes, several results were needed to tell a likely jump from ordinary uncertainty.

Read the evidence: Gleyzes et al. (2007), Quantum jumps of light recording the birth and death of a photon in a cavity Guerlin et al. (2007), Progressive field-state collapse and quantum non-demolition photon counting

## What “non-demolition” preserves

For the ideal off-resonant probe, the chosen quantity is photon number: the interaction reveals information about it without exchanging energy with the field. This is quantum non-demolition measurement. The name refers to that particular observable under suitable dynamics.

The field state still changes as the results arrive. Before enough atoms have passed, several photon numbers may remain possible; afterwards the conditional state can be concentrated around one. Coherence between different number components can be lost in the process. Cavity leakage and thermal exchange can also change the number itself, even when the probe interaction preserves it.

Read the evidence: Gleyzes et al. (2007), Quantum jumps of light recording the birth and death of a photon in a cavity Guerlin et al. (2007), Progressive field-state collapse and quantum non-demolition photon counting

## A measurement becomes part of the control

The important advance was the ability to ask the next question of the same system. A bright–dark readout reveals an ion’s internal state, sideband spectroscopy probes its quantized motion, and a cavity probe follows stored photon number. Each measurement is chosen for a particular property. Knowing what survives the readout lets the next laser pulse, atom or feedback action do useful work.

Read the evidence: NIST (2012), David J. Wineland wins Nobel Prize in Physics Nobel Prize in Physics 2012 — presentation speech

## Continue with the models

These teaching Labs produce simulated data, which are separate from the historical observations discussed above.

- [Watch an atom and a cavity trade one excitation](https://sandboxphysics.com/labs/cavity-quantum-exchange/)
- [See quantum jumps in one atom’s fluorescence](https://sandboxphysics.com/labs/fluorescence-quantum-jumps/)
- [Probe a trapped ion’s motion through its sidebands](https://sandboxphysics.com/labs/trapped-ion-sideband-spectroscopy/)
- [Compare the ways a quantum state loses coherence](https://sandboxphysics.com/labs/decoherence-channels/)

### Cavity excitation exchange

Can an empty mode exchange an excitation?

- **Try**: Switch off both loss channels. Start from the vacuum and run a time scan, then repeat with four photons, keeping the coupling and detuning the same.
- **Read**: Compare how often the atom is found excited in the two scans. Without loss, the reference exchange frequency for a number state rises with photon number. Cavity occupation and leakage are model predictions, listed separately.
- **Scope**: This is resonant exchange; the article’s QND probe is dispersive, which is a different protocol. Atomic populations alone cannot reconstruct the joint state. The Poisson preparation averages over phase, so it is not a coherent field with a resolved phase.

$$
\Omega_n=\sqrt{\Delta^2+4g^2(n+1)}
$$

Model reference: [Bina · Jaynes–Cummings model, §§ 4–9](https://arxiv.org/abs/1111.1143)

## Sources and reading notes

The 2012 Nobel sources and the original cavity papers distinguish a surviving system, a preserved observable and preserved coherence. Linked Labs use declared teaching models and synthetic data. Their measurement accuracy, lifetime and reconstruction do not provide historical calibrations.

1. [Leibfried et al. (2003), Quantum dynamics of single trapped ions, Rev. Mod. Phys. 75, 281](https://www.nist.gov/document/rmp1pdf)
2. [NIST (2012), David J. Wineland wins Nobel Prize in Physics](https://www.nist.gov/news-events/news/2012/10/nists-david-j-wineland-wins-2012-nobel-prize-physics)
3. [Nobel Prize in Physics 2012 — presentation speech](https://www.nobelprize.org/prizes/physics/2012/ceremony-speech/)
4. [Gleyzes et al. (2007), Quantum jumps of light recording the birth and death of a photon in a cavity](https://www.nature.com/articles/nature05589)
5. [Guerlin et al. (2007), Progressive field-state collapse and quantum non-demolition photon counting](https://www.nature.com/articles/nature06057)

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