Quantum limit in physics refers to a limit on measurement accuracy at quantum scales.

It may refer to the minimum level of quantum noise

Quantum noise

Quantum noise is uncertainty of some physical quantity due to its quantum origin.In the case of number of particles , the quantum noise is also called shot noise. Most optical communications use amplitude modulation...

 which is obtainable without squeezed states. For instance, the sensitivity of optical measurements has a standard quantum limit. There is additionally a quantum limit for phase noise

Phase noise

Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities...

, reachable only by a laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

at high noise frequencies.

In spectroscopy, the shortest wavelength in an X-ray spectrum is called the quantum limit