Wednesday, December 31, 2008

How did my 2008 go ?

Everyday Life

The everyday life was usual as in the past. Some of the chores were to help my daughter with her homework, to organize and participate in a get together periodically like MO:MO parties, Dashain and Tihar celebrations, etc. Most of the time was spent in internet - improving personal website, visiting the facebook and writing blogs. Other day-to-day activities were checking e-mails and reading all kinds of news papers that are published all around the world !! There was not a single day that passed without clicking on the following websites:,,,,,,
There is no question that the following website was highly clicked on:
The visits to the following web pages was also made at high frequency:
Thus walking to the office after morning tea/coffee, spending the morning at university, coming back to home for lunch, going back to the university and spending the rest of the day until dinner becomes ready and come back to home and chat with friends and family - was a pendulum-like life of the year 2008. Oh, we also visited the Niagara Falls and the 1000 islands in the month of August.

Research and Academia

I was continuing my research work on the BEC-based atom interferometry, under a distant supervision of my adviser as he was in sabbatical for about a year and half in University of Colorado, Boulder. He came back to the University in January 2008. Therefore, I started learning more research skills under his direct supervision right from the beginning of the Spring semester. I participated in the 2008 March Meeting of the American Physical Society (APS) in New Orleans, Louisiana. I presented my research work on Single and double reflection atom Michelson interferometers in a weakly confining magnetic trap in that meeting. After my return from New Orleans, we had a campus wide poster presentation at WPI, which WPI celebrates annually as the Graduate Research Achievement Day (GRAD). I made a poster presentation there on Theoretical Analysis of single and double reflection atom interferometers in a weakly- confining magnetic trap. I was more than happy when the first research paper was published in Physical Review A in April, 2008 ! I was awarded the Graduate Research Government(GSG) Conference funding award by the GSG at WPI. I gave a presentation on Theoretical analysis of a free-oscillation atom interferometer in a weakly confining magnetic trap at Harvard University in May, 2008. I was awarded the Graduate Research Assistantship (GRA) for the summer of 2008. I also got an opportunity of teaching a summer course on Waves and Oscillations at WPI. I got an award of Graduate Teaching Assistantship (GTA) for the year 2008-2009 in August. I learnt research skills in theoretical atomic physics in this year more than ever in the past. Thus, I enjoyed 2008 with teaching, research and normal everyday work !

Monday, December 29, 2008

The coldest transistor ever !

This is a brief review of the paper published in Physical Review A.

An electronic transistor is a three-terminal, solid-state device, used to amplify a signal in electronic circuits. The three terminals are called - an emitter (E), a base(B) and a collector(C). Can we make a transistor out of a Bose-Einstein Condensate (BEC)? The answer is - YES ! A transistor can be designed by using a BEC in an asymmetric triple-well potential. Let's consider a triple-well potential with wells labeled - Left(L), Middle(M) and Right(R). The left well, L, has a lot of cold atoms in the BEC state, so it acts as an emitter (E). If there are no atoms or a very few atoms in the middle well, M, and no atoms in the right well, R, no atoms can tunnel through the middle well to reach the right well, which acts as collector(C). This is because of the mismatch of the chemical potential between the three wells. If the atomic population in M is increased to some value, there will be a large flux of atoms reaching R, tunelling through M. Because when the number of atoms are increased in M, the chemical potential rises due to the nonlinearity caused by atom-atom interactions, making tunnelling possible. Thus, M is analogous to the base of an electronic transistor. Thus by controlling the atomic population in M, the atomic population in R can be controlled/amplified. Hence, this system clearly shows a transistor-like behavior and is the coldest transitor ever as it functions at BEC temperature, which is some micro/nanoKelvins! A BEC transistor may prove to be useful in precision measurements.


There are hundreds of research groups all around the world, working with the ultra cold atoms. Some of them are working with just the cold atoms where as many others are working with the Bose-Einstein Condensates(BECs). Bose-Einstein Condensation (BEC) was predicted by Albert Einstein in the early 1920s, when he applied the BOSE STATISTICS to the massive particles-the atoms. Bose had developed his statistics to study the behavior of light particles, called the photons. Although the phenomenon of BEC was made that early, the world had to wait for about 70 years to realize a Bose-Einstein condensate experimentally. The reason was the lack of the technology to cool a gas to a temperature of some nanoKelvins at which the BEC could be observed. The BEC was realized experimentally in Rubidium-87 gas in JILA, Colorado in 1995. In the same year, it was also realized in Sodium-23 in MIT and in Lithium-7 in Rice University. Now there are several laboratories in the world which prepare a BEC and manipulate it in different ways. One of the potential applications of a BEC is in making sensors like interferometers/gyroscopes.

Saturday, December 20, 2008

What is Interference?

If we observe the flame of a burning candle from the other side of three card boards having holes aligned on a straight line with the the flame, we will be able to see the flame. What happens when we slightly displace one of the three card boards so that all the three holes and the flame are no more in a straight line ? The answer is - we can not see the flame any more. This is a very simple table top experiment to show that light travels in a straight line in a given medium. Sometimes, this is also called the rectilinear propagation of light. But what happens when the medium changes or some special condition arises on the path of light? Reflection, refraction, diffraction, INTERFERENCE or polarization can be observed in such situation.

A ray of light falling on a smooth surface bounces to the previous medium at the glancing angle, which is termed as the reflection of light. If light passes from one medium to the next, its speed changes and takes a new direction at the boundary unless the ray hits normally to the boundary surface, which is called refraction of light. A ray of light passing through a very narrow hole, of the size of the wavelength of light, will spread and a pattern of bright and dark regions are obtained at a fairly large distance (relative to the size of the hole) behind the hole, which is termed as the diffraction of light. If a ray of light hits two narrow holes, very close to each other, the pattern obtained is really interesting-the width of the bright and dark regions are the same and the phenomenon is called the INTERFERENCE of light. When a ray of light passes through some medium like calcite crystal (even through air, water, etc), the light field (particularly electric field) is confined in a plane, and the phenomenon is called polarization.
Let's go beyond this. Are these phenomena specific to light? What's about other waves? The answer is - all light-like waves - called the transverse waves have these properties. There is another category of waves which shows sound-like behavior - called longitudinal waves also show all the above properties except polarization. Whatever I wrote above was known before the advent of QUANTUM MECHANICS.

What's about the matter-waves? Quantum mechanics treats the moving matter as a wave called a matter wave. Matter waves also show all the above properties as the light waves do !

Consider a beam of monochromatic light (light beam with a single frequency) passing through a closely spaced double slits. What do we observe behind the slits? We observe a nice pattern, called interference fringe, on the screen placed at a fairly large distance behind the slits. What happens here is the interference of light. The two slits act as the secondary sources (Huygens' Principle) called the coherent sources and the waves from there reach the screen in phase or out of phase. If the waves from the slits reach a point on the screen in phase, they will reinforce each other, producing a bright fringe/band. This is called a constructive interference. If the waves from the slits reach a point on the screen out of phase, they cancel each other and a dark fringe/band is produced in there. This is called a destructive interference. But let's think about somewhat weird situation that there is a single photon reaching the slits. Do we still see the interference pattern? If so, how do we explain this? The single photon can go through one or the other slit? What does it then interfere with?

Note: This post is on progress !

Tuesday, December 9, 2008

Inversion of ammonia molecule

Ammonia molecule has a pyramidal structure with the basal plane as an equilateral triangle defined by the three HYDROGEN atoms and a NITROGEN atom at the apex on the line passing normally through the center of the triangle. This structure can be represented by a symmetric double well potential. The nitrogen atom can come closer to the basal plane. As it comes closer to the basal plane, it experiences a repulsive force due to the hydrogen atoms. Thus there is a potential barrier. The nitrogen atom then passes to the other side by tunneling effect. This can not be explained classically and is a purely a quantum mechanical problem. The frequency at which the nitrogen atoms oscillates back and forth about the basal plane is called the inversion frequency. Here are some references-Ref-1 , Ref-2.

Tuesday, December 2, 2008

What is Schrodinger's Cat?

It is well-accepted fact that a quantum particle can exist in a superposition/combination state of two or more possible states. To show this, Erwin Schrodinger proposed a thought experiment in 1935, in which a cat locked in a box along with a radioactive atom and a vial of deadly poison could be somehow both alive and dead at the same time. Schrodinger's argument was that a quantum particle such as an atom can be in more than one different quantum states at the same time but a classical object such as a cat couldn't be in two different states.Thus, a Schrodinger's Cat State is generally understood as a quantum superposition state of two or more possible quantum states of a particle.



Here is a nice youtube video:

Note: This was first posted on the NPS Google group Web page: