Quantum noise refers to the fundamental limits of signal precision that arise from the laws of quantum physics. Unlike classical noise—which comes from external sources like heat, interference, or imperfect electronics—quantum noise is built into the fabric of nature itself.

In communication systems, quantum noise becomes important when working with very weak signals, such as in free-space optical (FSO) links, deep-space RF communications, or quantum key distribution (QKD). As systems approach their sensitivity limits, quantum noise often becomes the factor that sets the ultimate performance ceiling.

What Causes Quantum Noise?

Quantum noise comes from the fact that energy—like light or radio waves—exists in discrete packets, or quanta. For example, light is made of photons, and radio waves are made of individual electromagnetic excitations. When you try to measure or amplify these signals, there’s a natural limit to how accurately you can do so.

This is a direct result of the Heisenberg uncertainty principle, which says you can’t perfectly measure certain pairs of properties (like the strength and phase of a wave) at the same time. These unavoidable fluctuations are what we call quantum noise.

Even if you had a perfect detector in a perfectly quiet room at absolute zero temperature, quantum noise would still be present.

Types of Quantum Noise in Communication

1. Shot Noise

Shot noise shows up because photons (or electrons) arrive randomly. Even if you send a steady light beam to a detector, the arrival times of individual photons vary slightly, creating statistical fluctuations.

• In FSO communication, this becomes noticeable when the signal is very weak—such as during bad weather or long-distance links.

• In radio telescopes or satellite RF receivers, shot noise can dominate once thermal noise is minimized using cryogenic (ultra-cold) amplifiers.

Shot noise is often the main limit to receiver sensitivity when all other noise sources are reduced.

2. Vacuum Fluctuations

Even in a “perfect vacuum,” quantum physics predicts small, random fluctuations in the electromagnetic field. These vacuum fluctuations can enter detectors or amplifiers through unused ports and cause noise in the output signal.

In optical communications, especially in coherent detection systems, vacuum fluctuations can limit how precisely a signal’s phase or amplitude can be measured.

3. Amplifier Noise (Quantum-Limited Amplifiers)

Every amplifier adds some noise. Even ideal amplifiers—not limited by heat or circuit imperfections—must still add a minimum amount of noise due to quantum rules.

This added noise sets a limit for how weak a signal can be and still be amplified without distortion. Systems that need extreme sensitivity, like deep-space communication receivers, are often designed to operate near this quantum limit.

4. Loss and Decoherence

When sending quantum signals—like in quantum key distribution (QKD)—loss and environmental effects cause decoherence, a kind of quantum fading. This acts like noise, reducing signal fidelity and increasing error rates.

For satellite-based quantum links, atmospheric turbulence, scattering, and absorption all contribute to this kind of quantum noise.

Why Quantum Noise Matters in Communication Systems

Quantum noise sets a fundamental limit on how well a communication system can detect, amplify, or transmit weak signals. It’s especially relevant in:

• Free-space optical (FSO) systems in low-light or long-range conditions

• Satellite and deep-space RF communications, where signal strength is extremely low

• Quantum key distribution (QKD), where quantum noise both limits performance and helps ensure security

• Cryogenic microwave receivers, used in radio astronomy or interplanetary communications

Engineers must understand these limits when designing high-sensitivity receivers, low-noise amplifiers, or secure communication systems.

Managing and Using Quantum Noise

While quantum noise can’t be eliminated, there are ways to reduce its impact or even take advantage of it:

• Squeezed light: Specially prepared light that shifts quantum noise out of the part of the signal you care about, improving measurement accuracy.

• Low-noise electronics and cryogenic receivers: Reduce classical noise to the point where quantum noise is the main limitation.

• Quantum error correction: For future quantum networks, this helps correct errors caused by noise and loss.

• Quantum-aware detection methods: Techniques like homodyne detection or Dolinar receivers are optimized for quantum-limited conditions.

 Examples by Application:

References

1. 1. Books and Journal Articles

      1. C. W. Gardiner and P. Zoller, Quantum Noise, Springer, 2004.

      2. M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information, Cambridge University Press, 2010.

      3. C. M. Caves, “Quantum limits on noise in linear amplifiers,” Physical Review D, vol. 26, no. 8, 1982.

      4. H. A. Haus, Electromagnetic Noise and Quantum Optical Measurements, Springer, 2000.

      Online Resources

      5. Quera, “What is Quantum Noise?”

      6. RP Photonics Encyclopedia, “Quantum Noise.”