A narrow constriction in a semiconductor that allows controlled passage of individual quasiparticles, acting as a turnstile.A quantum point contact is a narrow constriction in a semiconductor structure, typically formed in a two-dimensional electron gas, where the width is comparable to the electron's de Broglie wavelength. This confinement forces electrons to pass through in quantized modes, leading to a staircase-like increase in conductance as the width is widened. The device acts as a turnstile for individual quasiparticles, allowing precise control over the number of charge carriers that traverse it, which is fundamental to quantum transport studies.
The concept was experimentally realized in 1988 by Bart van Wees and colleagues at Delft University, who observed the quantized conductance steps in a GaAs/AlGaAs heterostructure. The theoretical framework was laid earlier by Rolf Landauer and Markus Büttiker.
The constriction is created by applying voltages to metallic gates on the surface of a semiconductor heterostructure, depleting the electron gas beneath and forming a narrow channel. As the channel width increases, the transverse momentum of electrons becomes quantized into discrete subbands. Each subband contributes a conductance quantum of 2e²/h (where e is electron charge and h is Planck's constant). By tuning the gate voltage, one can open or close these subbands, effectively controlling how many electrons pass per unit time. In the turnstile regime, an alternating voltage or microwave field pumps electrons one by one, creating a quantized current.
Quantum point contacts are used as sensitive charge detectors in quantum dot experiments, as single-electron sources for metrology (defining the ampere), and as building blocks for quantum information processing.
The conductance quantization in quantum point contacts is so precise that it is now used to define the standard for electrical current in the International System of Units.
