Quantum interference is a  phenomenon in area of quantum mechanics.^[1] where quantum states — typically wavefunctions describing particles like electrons or photons — combine in ways that produce measurable probabilities greater or smaller than classical physics would predict. It sits at the heart of why the quantum world looks nothing like everyday experience: a single particle can arrive at a detector along multiple paths simultaneously, and those paths either reinforce or cancel each other out.

The basic idea

In classical physics, waves interfere all the time — overlapping ripples on water create peaks and flat spots. Quantum interference borrows this language but applies it to probability itself. A particle travelling between two points does not follow one definite path; the mathematics assigns an amplitude to every possible route, and amplitudes add together before being squared into a probability.[1] Where amplitudes point the same way, outcomes become more likely (constructive interference). Where they oppose, outcomes vanish entirely (destructive interference).

The strangeness is that this happens even when particles are sent through an apparatus one by one. Each individual electron lands as a single dot on a screen, yet after thousands of dots the familiar striped pattern emerges — evidence that each particle’s own wavefunction interfered with itself.

The double-slit experiment

The classic demonstration sends particles at a barrier with two narrow openings. With both slits open, an interference pattern appears even though only one particle is in flight at any moment. Close one slit, or install a detector to record which slit the particle used, and the pattern disappears, replaced by two simple bands.[1] The act of gaining “which-path” information destroys the phase relationship needed for interference — a process physicists call decoherence.

Interference is not about particles bumping into each other. It is about alternatives interfering with each other before nature decides what actually happened.

Why it matters technologically

Engineers have learned to steer interference deliberately. In a quantum computer, algorithms arrange for wrong answers to interfere destructively while correct ones build up constructively, which is how devices like superconducting qubit processors claim speedups on specific problems.[2] Interference also underpins atomic clocks, SQUID magnetometers used in brain imaging, and proposals for gravitational-wave detectors beyond current sensitivity limits.

Open questions

Despite a century of successful use, what exactly is “interfering” remains debated among interpretations of quantum mechanics. Whether the wavefunction is physically real or merely a bookkeeping device for knowledge has no settled experimental answer — every interpretation reproduces the same fringe patterns.

References

1. Feynman, R. P., Leighton, R. B., & Sands, M. The Feynman Lectures on Physics, Vol. III (quantum behaviour and the double-slit experiment).
2. Nielsen, M. A., & Chuang, I. L. Quantum Computation and Quantum Information (interference in quantum algorithms).