Superconductivity most realistic technology for quantum computers.
Rabiya Tanveer,Dept. of physics,chaitanya Degree & P.G College,Warangal,A.P,India.
Affilation:1.Nano Science & technology consortium,Noida,U.P,India.
2.photonics21-european technology platform.
Abstract: This article describes the applications of phenomenon of superconductivity in nanochips ,which could the basis for most realistic approach for quantum computing. Nano scale superconducting electronic devices could potentially revolutionized the electronics in an unimaginable way.
Superconductivity and magnetism appear in condensed matter due to strong electron-electron correlation. Since superconductivity is suppressed by magnetic field, magnetism, with its spontaneously generated high internal magnetic field, is normally considered as an antagonistic state.
Superconductivity was discovered in 1911 .At that time superconductivity is the subject of an enormous amount of research in physics, It is a surprising, new, and very important phenomenon. Superconducting materials have an electrical resistance of zero, and so can carry large electrical currents without power dissipation or heat generation.The phenomenon of Superconductivity is considered as a macroscopic phenomenon that it exists at the molecular scale, which opens up a novel route for studying this phenomenon.
Superconductors are used in various applications ranging from supercomputers to brain imaging devices. The phenomenon of superconductivity has various applications for eg : in superconducting quantum interference devices and in physical realizations of qubits. Single-electron transistors are often constructed of superconducting materials which functions on Josephson effect to achieve novel effects. The resulting device is called a “superconducting single-electron transistor.
Superconductivity has a lot of promising work in low-dimensional electron gases, carbon nanotubes, and nanowires.Nanotechnology has undoubtedly emerged as one of today’s hottest fields. It helps engineers and physicist to understand nanoscale electronic devices eg;Nano scale FETs .Nanoscale FETs could potentially push the moore’s law much farther and helps to develop in powerful computers.
Nano scale superconducting electronic devices could potentially revolutionized the electronics in an unimaginable way. For example, transistors are important in digital circuits because they utilize the electronic properties of semiconductors and can thus be used as switches. Nanoscale FETs attempt to scale this down by contacting a nanostructure with metal electrodes and modulating the carrier density in the channel via a gate voltage. To retain th e switching action in nanoscale FETs, currents which flow in them must be dissipationless .So a new nanostructures are to be developed with superconducting materials.eg; carbon nanotubes, n-type InAs nanowires, Ge/Si core/shell nanowires heterostructures, and graphene.
This nanostructure acts as a Josephson junction, and thus a super current is found to flow through it. The geometry is similar to conventional FET geometries: the nanostructure bridges two conductive electrodes (a superconducting metal such as Al) which act as a source and a drain when a suitable voltage is applied across them.The electrodes can be deposited by using optical or electron-beam lithography and microfabrication/etching techniques coupled with metal evaporation techniques. The nanostructure then acts as a conduction channel that can be tuned via the electric field effect of a highly doped Si back gate separated using several hundred nm of SiO2 dielectric .
The transport of electric current in a conductor is associated with the displacement of electrons: Collisions between these electrons and the crystal ions cause resistance and release heat. In superconductors electrons exits in the form of pairs,known as cooperpairs ,below the superconducting transition temperature, they allow themselves to synchronize their motion with the ions, and all occupy the same quantum state. Electrons in their normal state can be as free particles and undergo collisions with each other, where as in superconducting state electon pair are coupled with each other and move in the same direction without colliding.each other.
The electron has a charge, but like a tiny magnet, it also has a magnetic moment called spin. In a singlet superconductor, the electron pairs are formed by electrons of opposite spin, which cancels the pair’s magnetic moment. But when the material is placed in a strong magnetic field, the spins are forced to orient themselves along the field, as the field acts on each spin individually. This breaks the pairs and destroys superconductivity.
The magnetic fields inside a magnetically ordered material tends to act in the same manner and thus that superconductivity and magnetism tend to avoid each other.But
when a single crystal of CeCoIn5 (a metal compound consisting of cerium, cobalt and indium) is cooled to a temperature of minus 273.1 degrees, close to absolute zero,it is observed that magnetism and superconductivity coexist and disappear at the same time when they heat the sample or increase the magnetic field.This discovery is extraordinary, since magnetic order exists exclusively when this sample is in the superconducting state. In this unique case, magnetism and superconductivity do not compete with each other. Instead, superconductivity generates magnetic order.Thus superconductivity is a condition required to establish this magnetic order. It is observed that magnetic field exits in superconductors in the form of tiny magnetic dots,which actually enhance the
superconductivity instead of destroying it.
Mechanism of Superconducting
According to the Bardeen-Cooper-Schreiffer theory of superconductivity, electrons with opposite spins form pairs that can move through a material without resistance. A magnetic field can destroy superconductivity in two ways:.by breaking up the electron pair, or by trying to make both of the electron spins point in the same direction.
These effects also limit how much current can flow through the superconductor because of the disruptive effect of the magnetic field produced by the current itself.
Until now, only a few compounds remained superconducting under the influence of an applied magnetic field. Moreover, the number of materials in which an applied field could actually induce superconductivity – by the so-called magnetic field induced superconductivity effect – were very few.
Lange and co-workers placed a layer of cobalt-palladium ferromagnetic dots, each 800 nanometres in diameter and separated by 1.5 micrometres, on top of a superconducting thin film made of lead. Each dot produces a stray magnetic field that destroys the superconductivity in the thin film. The researchers then applied an external magnetic field, which enhanced the destructive effect of the dot’s magnetic field in the area directly beneath the dots and, to compensate, reduced it everywhere else in the film. The overall effect was an increase in the current carried by the superconductor.
This new ‘field compensation effect’ is not restricted to specific superconductors, the researchers say, so magnetic field induced superconductivity could be achieved in any superconducting thin film. The team believes that using handouts and nanopillars, which have larger stray fields, could allow superconducting materials to remain in higher magnetic fields. The nano-dot array could also be used to design logical devices for use in quantum computers.
The Superconducting materials in which current flows without resistance, have tantalizing applications. But even the highest-temperature superconductors require extreme cooling before the effect kicks in, so researchers want to know when and how superconductivity comes about in order to coax it into existence at room temperature.
Now a team has shown that, in a copper-based superconductor, tiny areas of weak superconductivity hold up at higher temperatures when surrounded by regions of strong superconductivity.
Researchers says that the Superconducting and normal currents can leak back and forth between adjacent layers of superconducting material and metal. In copper-based ceramic superconductors, made up of many different elements, superconductivity varies within nanometers depending on which atoms are nearby. These tiny regions can influence each other in much the same way that thin layers of metal and superconductor interact.
The researchers of Princeton University, Brookhaven National Laboratory, and the Central Research Institute of Electric Power Industry in Japan has used Scanning Tunneling Microscopy to investigate for the first time how this happens on the nanoscale. They observed that when the superconducting material is heated , they observed that superconductivity died out at different temperatures in regions just a few nanometers apart. The Superconductivity just not depends on the characteristics of the local region, but also on what was going on nearby. Regions of stronger superconductivity seemed to help regions of weaker superconductivity ,at higher temperatures.
Researchers might exploit this interplay by micromanaging a superconductor’s structure, so that regions of strong superconductivity have the maximum benefit to weak regions, potentially resulting in a new material that’s superconducting at a higher overall temperature than is possible with randomly arranged ceramic superconductors.
Magnetic impurities destroy superconductivity in conventional low-temperature superconductors, whereas high-Tc superconductors may depend on some kind of magnetic mechanism. Davis and his colleagues to investigate this phenomenon directly at the atomic scale in a superconductor for bismuth strontium calcium copper oxide to determine the influence of individual impurity atoms on electronic structure in their immediate neighborhoods.
Superconductivity is the flow of charged particles through a material without resistance, which happens when electrons form so-called Cooper pairs. Cooper pairs form below the superconducting transition temperature (Tc), which is only a few degrees above absolute zero in conventional superconductors, as cold as liquid helium or colder. Phonon quantized vibrations of the materials crystal lattice, helps to create regions of positive charge between the two electrons, “holes” which overcome the mutual repulsion of the electron’s negative charges.
With each Cooper pair there is another kind of pair, formed by each electron and its accompanying hole. These “quasiparticles” are fictitious representations of real particle systems, including the quantum states by which they are identified, but they make computation manageable in a way impossible for complete quantum solutions.
The magnetism persist in superconductor, because Cooper pairs one electron’s spin points ‘up’ and the other’s points ‘down,’ which gives rise to oppositely oriented magnetic moments.”
If an external magnetic field is applied to non superconducting systems, electrons of similar energy are separated by their spins. This splitting doesn’t affect superconductors, because “magnetic fields cannot penetrate the surface region of superconductors.” Where as in a conventional superconducting material the magnetic impurities will destroy the magnetic superconductivity because the Cooper pairs are split apart in the vicinity of each magnetic atom, which in conventional superconductors destroys their superconductivity.”
This is not so for high-Tc superconductors, whose transition temperatures are warmer than liquid nitrogen. “Nickel atoms are magnetic, but nickel impurities have a weak effect on superconductivity in high-Tc superconductors eg: for bismuth strontium calcium copper oxide. Oddly, zinc impurities disrupt it, and zinc atoms are non-magnetic.”
Part of the explanation lies in the electronic states characteristic of high-Tc superconductors. All the highest-Tc superconductors found so far are copper oxide ceramics having the crystal structure of the mineral perovskite, with planes of copper and oxygen atoms (where superconductivity is thought to occur) interlayered with planes of other atoms.
The electronic states of Cooper pairs in high-Tc superconductors are markedly different from those in conventional ones: the two electrons revolve around each other much faster and farther apart, as do their associated quasiparticles. These wider orbits are analogous to the higher-energy d orbitals of electrons around an atom, and high-Tc superconductors are often called d-wave superconductors.
Moreover, the researchers discovered two peaks in energy near each nickel atom, corresponding to the opposing up and down spins of the quasiparticle pairs.
“This shows that a nickel atom retains its overall magnetic moment in the superconducting state and doesn’t disturb that state. Also it maintains the magnetic properties of the cuprate perovskite system.”
If this system is doped with zinc impurities, destroys the superconductivity of this systems — each zinc atom destroying superconductivity within a radius of 1.5 nanometers, possibly because zinc atoms form nonmagnetic voids — this is good evidence that high-Tc superconductivity depends on uninterrupted magnetic pathways to aid the flow of charge. Now it is possible to investigate microscopic organization of high-Tc superconductors on an atom by atom basis.
This investigation leads to measure the quantum spin states of individual atoms which opens the larger vistas of possibility, including a potential mechanism for getting information into and out of the would-be superfast quantum computers of the future.
The use of superconducting films in the Meissner state reduces the level of noise in micro-and nanochips. Superconducting quantum computing is a promising implementation of quantum information that involves nanofabricated superconducting electrodes coupled through Josephson junctions. As in a superconducting electrode, the phase and the charge are conjugate variables, there exists three families of superconducting qubits, depending if the charge, the phase or neither of the two are good quantum numbers. This refers respectively to charge qubits, flux qubits, and hybrid qubits.
Superconducting qubit
Superconducting technologies have the unique potential for realizing compact solid-state devices with controllable macroscopic quantum properties and long coherence time. They represent the most realistic approach for a technology of quantum computers. In superconductors, all electrons are condensed in the same macroscopic quantum state, separated by a gap from the many quasi-particle states. Superconductors can be weakly coupled with Josephson tunnel junctions. The current through a Josephson junction depends upon the phase differences between the superconductors which act as non-commuting conjugate quantum variables to the charges of isolated islands. That makes it possible to construct qubits using superconductors (SQUBIT).
So far, superconducting electronics has not been able to compete with Si- and GaAs-technology in the field of computers, not even for special supercomputers. However, in the emerging field of Quantum Computing the situation is completely different. Now “quantum coherence” is the key issue and superconductivity has great advantages due to its built-in principle of “macroscopic quantum coherence”. An important feature of superconducting junctions is a possibility to reach long decoherence times. The main reason for that is a weak sensitivity of properly designed SQBITs to external electric fields produced by charge fluctuations.
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Yu. Makhlin, G. Schön, and A. Shnirman. Quantum-state engineering with Josephson-junction devices. Rev. Mod. Phys. 73, 357-400 (2001).
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