The effect where a quantum system's measurement or environment causes it to react or decohere.Quantum backaction is the phenomenon where the act of measuring a quantum system inevitably disturbs it, altering its subsequent evolution. This is a fundamental consequence of the Heisenberg uncertainty principle: to learn about a system, you must interact with it, and that interaction introduces uncontrollable noise. This is not a limitation of our instruments but a intrinsic feature of quantum mechanics, setting a fundamental limit on how precisely we can observe the quantum world.
The concept was formalized in the 1970s and 1980s by physicists like Vladimir Braginsky and Kip Thorne, who studied the limits of measuring gravitational waves with laser interferometers.
When a measurement device interacts with a quantum system, the two become entangled. The measurement outcome is correlated with a change in the system's state. For example, measuring the position of a particle with light imparts a random momentum kick, a process called radiation pressure noise. This backaction can be minimized but never eliminated, and it is a key consideration in designing ultra-sensitive measurements like those in gravitational wave detectors.
Quantum backaction is a central challenge in gravitational wave detectors like LIGO, where it sets a limit on sensitivity, and in quantum computing, where it affects qubit readout.
In some cleverly designed experiments, the backaction from one measurement can be used to predict and cancel the noise in a subsequent measurement, a technique called quantum nondemolition.
| Q_{\mathcal{D}} | variable | Kinematical backreaction term — kinematical backreaction per second squared |
| \Theta | variable | Expansion scalar — expansion scalar per second |
| \sigma^2 | variable | Shear scalar squared — shear scalar squared per second squared |
| \langle \cdot \rangl | operator | Spatial average over domain — spatial average |
| ρ | variable | Density matrix — density matrix |
| p_i | variable | Probability of state |i⟩ — probability |
| |i⟩ | variable | Eigenstate of the density matrix — quantum state |
| ⟨i| | operator | Dual vector (bra) of |i⟩ — bra operator |