Quantum Interference Effect

QUANTUM INTERFERENCE EFFECT

Quantum superposition

It is a fundamental principle of quantum mechanics. It states that much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state. (Fig. 5.15)

Magnetic fields can produce and control interference effects between the electrons in solids. In order to observe interference effects between different electron waves, their phase has to be maintained.

The phase coherence length Lϕ is the distance travelled by an electron without changing its phase. The phase of an electron wave is generally destroyed when electrons interact inelastically with defects in the lattice.

In general, ballistic electrons with a mean free path le much larger than sample dimensions L, (i.e. le >> L,) travel through the lattice without scattering. Therefore they show interference effects.

Applications of Quantum Interference Effect

Quantum interference effect is being applied in a growing number of applications, such as the

• Superconducting Quantum Interference Device (SQUID).

• quantum cryptography

• quantum computing and quantum interference transistor.

Quantum, Interference Transistor (QUIT):

Electrons are made to propagate through two arms of the quantum wire ring as shown in the fig. 5.16.

Suppose an electron wave enters the ring from left to right. The wave entering through "A" gets split up into two partial waves. A constructive interference can be expected to occur at “B” similar to the optical anlogue as they travel through the same distance.

The constructive interference at the output of the device reduces the resistance of the ring. Various methods of introducing a phase difference of л between the two waves have been suggested. This leads to destructive interference which in turn will increase the resistance by reducing the current.

An external voltage can control the nature of interference and the current. This device is expected to act as a high-speed transistor.