Quantum point contact

A Quantum Point Contact (QPC) is a narrow constriction between two wide electrically-conducting regions, of a width comparable to the electronic wave length (nano- to micrometer). Quantum point contacts were first reported in 1988 by a Dutch group (Van Wees et al.) and, independently, by a British group (Wharam et al.).

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Fabrication

There are different ways of fabricating a QPC. It can be realised for instance in a break-junction by pulling apart a piece of conductor until it breaks. The breaking point forms the point contact. In a more controlled way, quantum point contacts are formed in 2-dimensional electron gases (2DEG), e.g. in GaAs/AlGaAs heterostructures. By applying a voltage to suitably-shaped gate electrodes, the electron gas can be locally depleted and many different types of conducting regions can be created in the plane of the 2DEG, among them quantum dots and quantum point contacts.

Another means of creating a point contact is by positioning an STM-tip close to the surface of a conductor.

Properties

Geometrically a quantum point contact is a constriction in the transverse direction which presents a resistance to the motion of electrons. Applying a voltage V across the point contact a current will flow, the size given by I = GV, where G is the conductance of the contact. This formula resembles Ohm's law for macroscopic resistors. However there is a fundamental difference here resulting from the small system size which requires a quantum mechanical point of view.

At low temperatures and voltages, electrons contributing to the current have a certain energy/momentum/wavelength called Fermi energy/momentum/wavelength. The transverse confinement in the quantum point contact results in a quantisation of the transverse motion much like in a waveguide. The electron wave can only pass through the constriction if it interferes constructively which for a given size of constriction only happens for a certain number of modes N. The current carried by such a quantum state is the product of the velocity times the electron density. These two quantities by themselves differ from one mode to the other, but their product is mode independent. As a consequence, each state contributes the same amount e² / h per spin direction to the total conductance

G = NG_Q.

This is a fundamental result; the conductance does not take on arbitrary values but is quantised in multiples of the conductance quantum G_Q = 2e² / h which is expressed through electron charge e and Planck constant h. The integer number N is determined by the width of the point contact and roughly equals the width divided by twice the electron wave length. As a function of the width (or gate voltage in the case of GaAs/AlGaAs heterostructure devices) of the point contact, the conductance shows a staircase behaviour as more and more modes (or channels) contribute to the electron transport. The step-height is given by G_Q.

An external magnetic field applied to the quantum point contact lifts the spin degeneracy and leads to half-integer steps in the conductance. In addition, the number N of modes that contribute becomes smaller. For large magnetic fields N is independent of the width of the constriction, given by the theory of the quantum Hall effect.

An interesting feature, not yet fully understood, is a plateau at 0.7G_Q, the so-called 0.7-structure.

Applications

Apart from studying fundamentals of charge transport in mesoscopic conductors, quantum point contacts can be used as extremely sensible charge detectors. Since the conductance through the contact strongly depends on the size of the constriction, any potential fluctuation (for instance, created by other electrons) in the vicinity will influence the current through the QPC. It is possible to detect single electrons with such a scheme. In view of quantum computation in solid-state systems, QPCs may be used as readout devices for the state of a qubit.

Further reading

• H. van Houten and C.W.J. Beenakker (1996). "Quantum point contacts". Physics Today 49 (7): 22–27.
• C.W.J.Beenakker and H. van Houten (1991). "Quantum Transport in Semiconductor Nanostructures". Solid State Physics 44.
• B.J. van Wees et al. (1988). "Quantized conductance of point contacts in a two-dimensional electron gas". Physical Review Letters 60: 848–850.
• D.A. Wharam et al. (1988). "One-dimensional transport and the quantization of the ballistic resistance". J. Phys. C 21: L209.
• J.M. Elzerman et al. (2003). "Few-electron quantum dot circuit with integrated charge read out". Physical Review B 67: 161308.
• K. J. Thomas et al. (1996). "Possible spin polarization in a one-dimensional electron gas". Physical Review Letters 77: 135.
• Nicolás Agraït, Alfredo Levy Yeyati, Jan M. van Ruitenbeek (2003). "Quantum properties of atomic-sized conductors". Physics Reports 377: 81.