Quantum noise refers to random disturbances affecting quantum systems.Quantum noise refers to the random fluctuations that inevitably affect quantum systems, arising from both the discrete nature of quantum particles and the system's interaction with its environment. This noise is not just a nuisance — it is a fundamental feature of quantum mechanics, setting limits on measurement precision and causing decoherence. However, in some contexts, quantum noise can be harnessed: it can enhance the sensitivity of certain sensors or even drive beneficial effects like stochastic resonance.
Quantum noise has been studied since the early days of quantum mechanics, with key contributions from Werner Heisenberg's uncertainty principle. The concept of quantum noise in amplifiers was developed in the 1960s, and its role in quantum information became prominent in the 1990s.
Quantum noise manifests in several ways. Shot noise comes from the discrete arrival of photons or electrons, causing random fluctuations in measurements. Quantum projection noise arises when measuring a superposition state, which collapses randomly. Environmental noise, such as thermal fluctuations or electromagnetic interference, also perturbs quantum systems. In quantum metrology, noise sets the standard quantum limit, but techniques like squeezed light can reduce noise in one variable at the expense of another, beating this limit.
Quantum noise is a critical factor in quantum computing, where it limits qubit fidelity. In quantum sensing, understanding noise allows researchers to design protocols that are robust against it, and in quantum key distribution, noise levels are monitored to detect eavesdroppers.
In some quantum systems, adding a small amount of noise can actually improve the signal-to-noise ratio — a phenomenon called stochastic resonance, which has been observed in quantum sensors and even in biological systems.
| S_I | variable | power spectral density of current noise — power spectral density ampere squared per hertz |
| e | variable | elementary charge — elementary charge coulomb |
| I | variable | average electric current — electric current ampere |
| P_signal | variable | Power of the signal component — power watt |
| P_noise | variable | Power of the noise component — power watt |
| f | variable | frequency of the quantum field oscillation — frequency hertz |
| m | variable | mass of the particle associated with the field — mass kilogram |
| c | constant | universal constant relating space and time in relativity — speed of light = 299792458 metre per second |
| h | constant | fundamental constant relating energy to frequency in quantum mechanics — planck constant = 6.62607015e-34 joule second |
| t | variable | Measurement time — time second |
| N | variable | Number of particles or events — particle number |
| \propto | operator | Indicates proportional relationship — proportionality |
