What is Karma??

Karma - which means

• - to 'act' as a living entity.

•  - "sum of person's actions in one of his successive states of existence, viewed as deciding his fate for the next".  

•  - In Sanskrit karma means "volitional action that is undertaken deliberately or knowingly". This also dovetails self-determination and a strong will power to abstain from inactivity. 

• - Karma is the differentia that characterizes human beings and distinguishes him from other creatures of the world.  

•  - The theory of karma harps on the Newtonian principle that every action produces an equal and opposite reaction.

   

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How to make a transistor based on electron spin (Quantum mechanics)

In spin electronics, it is the spin of the electron and not its electrical charge that could be used for computing. Indeed, this could be done entirely without electrical currents, and would be more energy efficient as it is easier to switch a spin than carry an electric current. In such devices, the spin can assume two orientations, which can be used to represent the 1 and 0 of computer bits.

The fundamental unit of a computer is the transistor. So what about the equivalent for spin electronics, the spin transistor? Well, the concept of a transistor that only switches an electron’s spin instead of charge was proposed 20 years ago by Supriyo Datta and Biswajit Das, but was never realized. The problem has been to control the spin of an electron in a clear and efficient way by electrical voltages while it is in transit through a nanoscale device.

( The Spin Hall effect transistor. (c) Science 330, 1801 (2010) )

Work by Jörg Wunderlich from the Hitachi Laboratory in Cambridge, Tomas Jungwirth from the Institute of Physics in Prague and the University of Nottingham in the UK, and their colleagues now published in Science comes the closest yet to the Datta-Das spin transistor: they present a spin Hall effect transistor.

Unlike the Datta-Das transistor, which basically is the concept of the conventional transistor transferred to spin electronics, the spin Hall effect transistor is a little more elaborate. The researchers excite electrons with a predefined spin (yellow cylinder in the figure). As the electrons travel from there to the other end of the device they scatter and get diverted either to the left or the right, depending on the direction their spin is pointing at. If the electrons all have spins pointing in the same direction, as in the experiment, they all get deflected in the same direction. This creates a Hall voltage along a crossbar (R_H in the figure), even though no electric current flows in this device.

Another crossbar close to the point where the electrons with uniform spin are excited ensures no electrical currents can flow, because this crossbar is electrically grounded and therefore sets any voltages to zero. The spin orientation, however survives. Otherwise there would be no Hall voltage in the experiments.

So far, there is nothing too surprising, the spin Hall effect is known. What is new in this structure is that a transistor element is added: the gate. In a conventional transistor the gate voltage (V_G) controls the flow of electrons and turns the transistor on and off, which means it can switch the bits from 1 to 0. The researchers now apply a similar voltage to their structure. And even though no electric current flows, this voltage turns the transport of spin, and hence the Hall voltage, on and off.

Even though this structure is a good deal more complicated than a conventional transistor, it demonstrates the principle that a pure spin current can be controlled in a similar way to a conventional transistor. Of course, we are still far away from applications. For example, the spin signal is created optically and not electronically. But what it accomplishes beautifully is to show how close we have come to realize a spin transistor. Twenty years after the paper of Datta and Das it is about time!

References:

Datta, S., & Das, B. (1990). Electronic analog of the electro-optic modulator Applied Physics Letters, 56 (7) DOI: 10.1063/1.102730

Wunderlich, J., Park, B., Irvine, A., Zarbo, L., Rozkotova, E., Nemec, P., Novak, V., Sinova, J., & Jungwirth, T. (2010). Spin Hall Effect Transistor Science, 330 (6012), 1801-1804 DOI: 10.1126/science.1195816

Get those computers spinning (Quantum physics)

( Photo by Philippe Teuwen via wikimedia. )

 

This Article discuss about new ways of computing using the quantum mechanical property of spin. Taken together, these provide a brief glimpse into the different ways researchers have progressed in incorporating spin into electronic devices.

The fundamental element of a computer chip is the transistor. The transistor is where the bits are switched from 0 to 1 and vice versa. Transistors are made from semiconductors such as silicon and operate by moving electrical charges between two contacts. But electrical charges are not the only possibility to operate a computer. Another one is to use spin.

What is spin and why do we care?

Spin is a quantity that is related to the rotation of fundamental particles around their own axis, similar to a spinning top. The concept of spin is deeply rooted in quantum mechanics, pioneered by people such as Wolgang Pauli and Niels Bohr. Of course, the analogy of such a fundamental property to a spinning top does not work fully. If you want to learn more about the intriguing world of spin, take a look at Dave Goldberg’s blog post.

But how does spin have any relevance in computing? Well, if the particle with spin also has an electrical charge, as the electron does, this also creates a magnetic field, similar to that of a tiny compass. This magnetic field can be used to store information just like an electric charge. Whether the compass points upwards or downwards then corresponds to the 0 and 1 of a bit.

What are the benefits of using spin? Open a computer and just look at the efforts that are needed to cool the computer processor so that it doesn’t overheat. In conventional electronics there is a lot of energy loss and heat generation. Spin can be switched with much less energy. Furthermore, spin is a property of all materials, not just semiconductors. And last but certainly not least, unlike the memory of a transistor, the information stored in spin is not necessarily lost if a computer is turned off. This could be useful for computers that don’t need to be booted after being turned off.

( Wolfgang Pauli and Niels Bohr look at a spinning top. Bohr and Pauli are pioneers of quantum mechanics, where the concept of spin is rooted. Photograph by Erik Gustafson, courtesy AIP Emilio Segre Visual Archives, Margrethe Bohr Collection )

Spins aligned

To use spin for computing, one first needs to have electrons whose spins all point in the same direction. There are a number of ways to achieve this. Magnetic fields are an obvious one, as they act on spins in the same way as the Earth’s magnetic field on compass needles.

The approach that Marius Costache and Sergio Valenzuela from the Universitat Autònoma de Barcelona in Spain have now chosen has the added benefit that it also transports the spin across a device — without actually transporting any electrons.

In their work, the first spin-related paper in this week’s Science, they fabricate a small superconducting ‘island’ that is connected from both sides with electrical wires. The island is so small that if an excess electron is brought on the island its electrical charge is sufficient to deter other electrons from travelling across the island. Also, the energy of that single electron is above the energy of the ‘sea’ of superconducting electrons.

The researchers now apply an external magnetic field in upward direction. This lowers the energy of electrons with spin aligned in the same direction and increases the energy barrier for those in downwards direction. This makes a perfect spin filter that is selective for electrons with the right spin orientations. But because of the superconducting properties of the island, no real electrons are passing through, only the spin orientation is transferred from one side to the other. The drawback is of course that this only works at the low temperatures at which superconductors operate.

Manipulating spin

The second paper in Science deals with another issue: how to manipulate the spin in a thin magnetic layer? Christian Pfleiderer from the Technical University of Munich and colleagues use a known approach to turn this spin around, which is an electrical current made of electrons that have spin in the direction the switching will take place. This ‘spin-torque effect’ is something like a brute-force approach as the sheer mass of electrons of spin in the other direction eventually changes the spin of the magnetic layer. It is as if you walk against a large crowd of people that eventually force you to walk in their direction. Because of the large electron current required, this torque effect has been only possible in nanostructures, where heating effects won’t be a problem. Pfleiderer and colleagues have now discovered that the spin-torque works very well in the magnet MnSi, where the spins form a complex assemblies, so-called skyrmions. These skyrmions reverse their direction much easier than conventional magnets, so that the electrical currents required are orders of magnitude smaller. The drawback here is that this still is a quite elaborate experimental far away from the device stage.

Storing spin

Magnets are of course ideal to store spin information for a long time. But there magnetism is set by the atoms and not by free electrons. To combine spin and electronics, however, it would be desirable to find ways to store spin with electrons. This is possible with the spins in the atomic nucleus, as these can be addressed by electrons and nuclear spins have comparatively long lifetimes. Christoph Boehme from the University of Utah and colleagues have now stored spin information in the atomic nucleus of phosphorus atoms embedded in a silicon chip. They demonstrate that the lifetimes of these spins are larger than 100 seconds. The benefit is that this is done completely electronically, and it is in silicon, which is the best material to combine spin electronics and regular electronics. On the other hand, this approach may not yet have reached the easy of implementation and reliability required for applications.

Of course, these three papers only provide a very specific view into some areas explored for spin electronics. And while the concept of electronics being to a large extend driven by spin instead of electrical charges is very appealing, it is still a long way for most of these approaches to become technologically viable. Nevertheless, the variety of ideas and approaches currently pursued is impressive and a good indicator of the ingenuity of researchers and the interesting physics coming out of this field as it moves closer towards applications.

References:

Costache, M., & Valenzuela, S. (2010). Experimental Spin Ratchet Science, 330 (6011), 1645-1648 DOI: 10.1126/science.1196228

Jonietz, F., Muhlbauer, S., Pfleiderer, C., Neubauer, A., Munzer, W., Bauer, A., Adams, T., Georgii, R., Boni, P., Duine, R., Everschor, K., Garst, M., & Rosch, A. (2010). Spin Transfer Torques in MnSi at Ultralow Current Densities Science, 330 (6011), 1648-1651 DOI: 10.1126/science.1195709

McCamey, D., Van Tol, J., Morley, G., & Boehme, C. (2010). Electronic Spin Storage in an Electrically Readable Nuclear Spin Memory with a Lifetime 100 Seconds Science, 330 (6011), 1652-1656 DOI: 10.1126/science.1197931

Light does matter (Quantum physics)

Light is special. In our everyday experience it behaves like a wave, which gets reflected, refracted and shows interference with other light of the same wavelength. At the same time, light also consists of particles, so-called photons. This duality is quite fundamental: the Hanbury Brown and Twiss experiment for example only works because of the particle-like properties of light.

This amazing and perhaps confusing duality, where light in one experiment appears to be a wave and in others it behaves like particles, is now laid bare in a paper published in Nature. There, Jan Klaers, Martin Weitz and colleagues from the University of Bonn in Germany take one of the classical properties of light waves and turn it upside down — by demonstrating a related effect that only works when considering the particle qualities of light!

( The experimental setup. A laser beam injects photons into a cavity filled with light, and a camera observes the photons coming out of the cavity. Reprinted by permission from Macmillan Publishers Ltd. Nature 468, 545-548 (2010).)

The classical effect they use is that light waves can all oscillate synchronously. This is exactly what happens in a laser, and is typical behaviour for a class of particles to which photons belong to, the bosons. Bosons love to be all in the same state.

A similar synchronous behaviour can also occur for other bosons, including certain atoms, which then all assume the same quantum state. This state is called a Bose-Einstein condensate, after Satyendra Nath BoseAlbert Einstein, who described it first in 1924. It is a Bose-Einstein condensate of light that Weitz and colleagues have now demonstrated. (after whom bosons are named) and

( Bose-Einstein condensation of light, as observed by a camera looking at the photons from the cavity. The narrowing of the spatial distribution as the light intensity is increased (bottom image) is a tell-tale sign. Reprinted by permission from Macmillan Publishers Ltd. Nature 468, 545-548 (2010). )

 Well, so if photons are bosons anyway, what is the big deal, what is the difference? I have to confess this was my first thought when only reading the title of the paper. A Bose-Einstein condensate of light? Simply turn on any laser, and there you have something similar. But there is a subtle difference: in the Bose-Einstein condensate of atoms, the number of atoms is conserved — atoms may drift in and out of the experimental system, but they obviously are not artificially created on the spot. This is different to a laser, where new light is created all the time.

But how to get light behave like that, like discrete atomic particles? The trick is to confine the light between two mirrors that are very close together (Fig. 1). Light only fits between the mirrors if its wavelength is short enough. But a maximum wavelength of light also means that the light has a certain minimum energy (the energy of light is inverse to its wavelength).

The minimum energy of light between the mirrors is high enough to rule out the creation of new photons, because the thermal energy from the heat is too small, and other energy sources aren’t available either. No new photons are created, and the system behaves more like a bunch of particles. Even so, at this stage we still haven’t achieved a Bose-Einstein condensate, all we have is light bouncing back and forth between two mirrors.

As in the case of atoms, the photons need to be cooled down. The purpose is to bring them closer together in energy, and this can done by a cooled dye solution that is placed between the mirrors. The interaction of the photons with the dye molecules brings them in tune with the temperature of the solution, which again is very much like what would happen for regular particles.

As a last step, to get the Bose-Einstein condensate going we need a sufficient number of particles. The images taken of light passing through the cavity show this impressively (Fig. 2). Broadly speaking, at low light intensities there aren’t enough photons to synchronise with each other to form a condensate. As a result, the light is broadly distributed. At high enough intensities, however, the Bose-Einstein condensate is clearly evident through the narrow distribution of light in a single beam. This is to some degree comparable to the narrow beam coming out of a laser, and shows the relationship between both effects.

Indeed, in many ways the properties of light in a laser and in this Bose-Einstein condensate are similar. But the similarity arises not from the wave-like property itself, but from the dual nature of light that can act as a wave as well as a particle. In that respect the demonstration of Bose-Einstein condensation of light makes a full circle: light that behaves like matter that behaves like light. Simply beautiful.

Reference:

Klaers, J., Schmitt, J., Vewinger, F., & Weitz, M. (2010). Bose–Einstein condensation of photons in an optical microcavity Nature, 468 (7323), 545-548 DOI: 10.1038/nature09567

Master of the Universe !!

Aryabhatta - Iindian scientist (mathematician and astronomer)

Statue of Aryabhata on the grounds of IUCAA, Pune. As there is no known information regarding his appearance, any image of Aryabhata originates from an artist's conception.

Aryabhatta came to this world on the 476 A.D at Patliputra in Magadha which is known as the modern Patna in Bihar. Some people were saying that he was born in the South of India mostly Kerala. But it cannot be disproved that he was not born in Patlipura and then travelled to Magadha where he was educated and established a coaching centre. His first name is “Arya” which is a South Indian name and “Bhatt” or “Bhatta” a normal north Indian name which could be seen among the trader people in India.

No matter where he could be originated from, people cannot dispute that he resided in Patliputra because he wrote one of his popular “Aryabhatta-siddhanta” but “Aryabhatiya” was much more popular than the former. This is the only work that Aryabhatta do for his survival. His writing consists of mathematical theory and astronomical theory which was viewed to be perfect in modern mathematics. The calculation of 3.1416 is nearly the same with the true value of Pi which is 3.14159. 

Aryabhatta’s strongest contribution was zero.  

Another aspect of mathematics that he worked upon is arithemetic, algebra, quadratic equations, trigonometry and sine table.

The English interpretation of the above Shloka would be:

Add 4 to 100, multiply by 8, then add 62000, then divide by 20000. The result is "approximately" circumference of a circle of diameter 20000.

The answer of the above calculation is 62832/20000 = 3.1416.

By using the word "Asanna" (last word in the Sanskrit verse), Aryabhatta clearly states that the value so found is not exact, but an "approximate" one, something that was "approaching" the exact value. So he is referring to it being irrational.

This is quite a significant contribution, as it was not until the 17th century that Pi was proved to be irrational in Europe.

Aryabhatta was aware that the earth rotates on its axis. The earth rotates round the sun and the moon moves round the earth. He discovered the 9 planets position and related them to their rotation round the sun. Aryabhatta said the light received from planets and the moon is gotten from sun. He also made mention on the eclipse of the sun, moon, day and night, earth contours and the 365 days of the year as the exact length of the year. Aryabhatta also revealed that the earth circumference is 24835 miles when compared to the modern day calculation which is 24900 miles.

Aryabhatta have unusually great intelligence and well skilled in the sense that all his theories has became wonders to some mathematicians of the present age. The Greeks and the Arabs developed some of his works to suit their present demands. Aryabhatta was the first inventor of the earth sphericity and also discovered that earth rotates round the sun. He was the one that created the formula (a + b)² = a² + b² + 2ab. He also created a solution formula of solving the following equations:

1 + 2 + 3 + 4 + 5 + ……………… + n = n (n + 1)/2
1² + 2² + 3² + 4² + 5² + ……………….. + n² = n (n + 1) (2n + 1)/6
1³ + 2³ + 3³ + 4³ + 5³ + ………………….. n³ = (n (n + 1)/2)²
1⁴ + 2⁴ + 3⁴ + 4⁴ + 5⁴ + ………………….. + n⁴ = (n (n + 1) (2n + 1) (3n² + 3n – 1))/30