An electron 'feels' a magnetic field without even entering the region where it exists—like a compass pointing north through a wall.
In practice: Used in ultrasensitive magnetic field sensors and for studying topological phases in materials.
In 1959, Yakir Aharonov and David Bohm theoretically predicted that in quantum mechanics, electromagnetic potentials play a more fundamental role than fields. They proposed a thought experiment: electron interference changes even when they move in a region with zero electric and magnetic fields, if a non-zero vector potential A is present. This challenged the classical view that only fields are physically significant. Experimental confirmation in 1960 proved them right.
How it works
The effect underlies the operation of SQUIDs (superconducting quantum interference devices)—instruments that measure magnetic fields a billion times weaker than Earth's. It also appears in carbon nanotubes and topological insulators.
💡 David Bohm himself, one of the authors, believed it supported his philosophical theory of 'hidden variables' about the implicate order of the universe, but most physicists interpret the effect in the standard way.
Imagine a tube with a current-carrying wire wound around it; the magnetic field exists only inside the tube. Electrons flying outside should not feel the field—since the field is zero where they fly. But quantum mechanics shows otherwise: the electron waves going around the tube from different sides shift in phase, and the phase difference depends on the magnetic flux inside the tube. This shift is seen as a change in the interference pattern (alternating bright and dark fringes).
How it works
The effect underlies the operation of SQUIDs (superconducting quantum interference devices)—instruments that measure magnetic fields a billion times weaker than Earth's. It also appears in carbon nanotubes and topological insulators.
💡 Aharonov and Bohm proposed two versions: magnetic (related to A) and electric (related to φ). The electric effect is experimentally more challenging but also confirmed—an electron senses a voltage change without passing through the field.
The Aharonov-Bohm effect is a quantum phenomenon where the interference pattern of charged particles shifts in the presence of an electromagnetic potential (vector A or scalar φ), even if the particles pass through regions with zero E and B fields. Formally, the wave function Ψ acquires an additional phase factor: exp(ie/ħ ∫ A·dl), giving a phase difference Δφ = eΦ/ħ, where Φ is the magnetic flux enclosed by the path. This effect demonstrates the nonlocality of quantum mechanics: a field at one point influences particles that never go there.
Discovery
Proposed by Yakir Aharonov and David Bohm in 1959 as a consequence of the quantum mechanics formalism. Experimentally confirmed in 1960 by Roberts Chambers, who used a tiny magnet and an electron interferometer. Later, with the development of mesoscopic physics, the effect was rediscovered in semiconductors and carbon nanotubes. In 1985, Michael Berry linked the geometric phase to the Aharonov-Bohm effect.
How it works
It is applied in magnetic flux measurements (SQUIDs), studies of ballistic transport in nanostructures, and as a tool for creating topological qubits. Limitations: requires high coherence of electron waves (low temperatures and small sizes), otherwise interference is washed out.
Caveats
Does an Aharonov-Bohm effect exist for the gravitational potential?; How is the effect related to time-reversal symmetry breaking in topological materials?; Can it be used for quantum information transfer without fields?
\Delta \varphi = \frac{e}{\hbar} \Phi
Δφ — phase difference between two electron paths; e — elementary charge, e ≈ 1.602×10⁻¹⁹ C; ħ — reduced Planck constant, ħ ≈ 1.0546×10⁻³⁴ J·s; Φ — magnetic flux enclosed between the two alternative trajectories (in Webers, Wb). For an isolated solenoid, Φ = B·S, where B is the induction inside and S is the cross-sectional area.
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