12-06-2012, 02:27 PM
APPLICATIONS OF JOSEPHSON JUNCTION
HIGH FREQUENCY DETECTOR.doc (Size: 91.5 KB / Downloads: 39)
HIGH FREQUENCY DETECTOR
If a dc bias voltage and a microwave signal of frequency ω are applied to the junction simultaneously, a form of mode locking will occur when the frequency ω is equal to the oscillation frequency ωj. As a result steps of constant size at voltages Vn=nhw/2e will be formed. The voltage step is given by Vs = hw/2e .These constant voltage steps are called sapphire steps and these can be observed in the VI characteristics of the device as follows.
VOLTAGE STANDARD
One of the best known applications of JJs is that of a voltage standard. It is based on the AC Josephson effect. The formation of sapphire steps has already been noted. When a DC voltage is applied to a Josephson junction, an oscillation of frequency occurs at the junction.
f Josephson= 2e∆/h
Since this relationship of voltage to frequency involves only fundamental constants and since frequency can be measured with extreme accuracy, the Josephson junction has become the standard voltage measurement. It is independent of the junction material or other properties of the junction. Josephson junction standards can yield voltages with accuracies of one part in 1010. Since the frequency can be measured with very high precision, one can transfer the precision to the voltage. The frequency standard is based on the cesium atomic clock. Since voltage can be directly represented in terms of frequency, the cesium standard can also be employed for the voltage. JJ voltage standard is three or four orders better than the western cell standard.
JOSEPHSON DEVICES
Devices based upon the characteristics of a Josephson junction are valuable in high speed circuits. Josephson junctions can be designed to switch in times of a few picoseconds. Their low power dissipation makes them useful in high-density computer circuits where resistive heating limits the applicability of conventional switches.
JOSEPHSON JUNCTION QUANTUM COMPUTER
A superconducting qubit (or quantum bit), which consists of a micrometer, sized loop with three or four Josephson Junctions, has two persistent currents of opposite direction as its two states. Since Kirchoff’s current law is not valid in superconducting states such coexistence of states can easily be achieved.
The states of the qubits, manipulated with magnetic fields and measured with a SQUID, can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops by increasing its critical current making it possible to base processor array architecture on this qubits (quantum bits used in a classical way). Such a classical computer might also serve as pre and post processor.
DIELECTRIC BASE TRANSISTOR
The original concept of Dielectric Base Transistor is based on resonant tunneling from a superconducting emitter to a superconducting collector. The charge carriers tunnel resonantly through a dielectric barrier that contains defect states of well-defined energy. This barrier is a trilayer, consisting of defect containing, high permittivity insulator Lh having a high dielectric constant sandwiched between two dielectric layers L1, having a lower dielectric constant. Because Lh transmits an electric field well, a voltage can shift the energetic position of the defect levels applied to the base electrode connected to the edge of Lh.