 the International Olympiad Medalist, a function which is held by the Homi Baba Centre in collaboration with the Infosys Foundation Bangalore and the TIFR Endowment Fund. So as is the norm in this function, which we hold every year on this day, 22nd December, this is actually the birthday of Srinivas Ramanujan. I'm sure all of you know his name, the famous Indian mathematician. So this function is always held on this day, on 22nd December. So according to the tradition that we have been following, there are two lectures before the formal function, which will start at 12 noon. Before that we have two lectures today by two eminent speakers and it's my pleasure to welcome now the first speaker of today, Professor Rupa Manjari Ghosh, the Vice Chancellor of Shivnodar University, Uttar Pradesh. Professor Ghosh is a professor of physics with special interest in quantum optics and laser physics. Professor Ghosh obtained her MSc degree in physics from the University of Calcutta in 1980 and PhD in physics from the University of Rochester in 1987. She was at the Jauhar Lal Nehru University from 1988 till 2012, where she held many important academic and administrative positions, including Dean of School of Physical Sciences. She joined the Shivnodar University in 2012 as the founding director of the School of Natural Sciences. Later she held several other leading positions at Shivnodar and became its Vice Chancellor in 2016. She has also served as the chief advisor for the National Council of Educational Research and Training, NCRT, science textbooks for Class 9 and 10. Besides her contribution to science research and training from university to school level, she is also well known for her stand in efforts to bring in gender justice and environmental consciousness in the higher education system. Her research interests are in experimental and theoretical quantum optics, laser physics, non-linear optics, magneto-optics, and quantum information. She has made pioneering contributions in quantum optics and quantum information, particularly in the creation and use of sources of single photons and entangled photon pairs. Professor Ghosh was awarded the Sri Shakti Science Samman in 2008. And today she will speak on light and atoms. So welcome, Professor Ghosh. Thank you very much. It's a real pleasure and honor for me to be with you this morning. This is my first visit, believe it or not, to this center, though, you know, to TIFR and other places I visited many times. And I thought when Anvish commanded that I come and talk about experimental physics a bit, I thought I would get recharged by all the energy that I see in this morning in this room. So let me get started. This is not a very, very formally structured one, but I will talk about a bit of our research, but hopefully some points would get through. So feel free to interrupt me if you do not understand something or you want some clarification. So I started the very basic kind of a definition that NCRT textbook writers are often tempted to give. So often people ask me what is really physics and in NCRT textbooks you would find. This is what we wrote in one of the introductions that physics is a systematic attempt to understand natural phenomena in as much depth and detail as possible. Why the hell do you do this? You do this so that you can use this knowledge to predict, modify and control phenomena. Now that is an important part. When you understand, then only you can try to control. And that itchy feeling is with all physicists. There are two qualifying remarks I have put in here that in physics often you find people as segregating themselves as theoretical physicists and experimental physicists. The area I work in is possible to do both. And it is important to remember that physics is actually an experimental science and theories are constructed to explain experimental facts. The way if you look at the title that I have put in is actually the evolution of physics happens when theory and experiment go hand in hand. And beautiful offshoots in terms of applications often come out. It is also important because of the kind of implications that I will talk about today that physics tries to answer the question how things happen in nature. And it does not bother going into the details of why it happens that way. For example, you heard that I work in quantum optics or quantum mechanics. If you ask me why that works and nothing else works, I do not have an answer. Believe in God if you wish to. But the idea is that we separate it out through methodologies that we call scientific methodologies into the domain of what science is or physics theory is all about and what the domain of belief or philosophy etcetera are. So I think it is very important for us to keep that distinction in mind when we approach the scientific domain through methodologies devised by ourselves sort of to protect. And remember that we try to answer the question how things happen in nature and not get into why it happens that way and no other way. So I can go on and on but let me just get started with one example of what I just said that once you start understanding phenomena you would be able to control it. The domain that everybody in physics talks about is non-linearity, non-linearity, non-linearity you must have heard some bit of it. But essentially let me confine myself to optics. Now what is meant by a non-linear optical phenomena in a very general way first let us understand what we mean by a linear system. So think of a pendulum and I kick it depending on the strength of the kick. If I kick it harder it displaces more. So the response of the system is proportional to the strength of the kick. Now in optics you talk in terms of the response which is given as the polarization which is the induced dipole moment per unit volume. So I shine in light or shine in electric field E and the system response that is my stimulus system response by getting polarized. In a linear system the relationship between P and E is linear. So if I draw a curve I will get a straight line. So the constant that is sitting in in front is known as susceptibility and if I depending on the kind of unit I use I put in this epsilon naught or not that is not really important. Point is for the linear system the relationship between polarization and electric field is linear. So what is non-linearity? Non-linearity very easily you can imagine that when I am talking about this pendulum if I kick it really hard it is not going to just get linearly displaced it is going to do funny stuff and one way of representing all the funny stuff it does which is the reality really is to put in higher order terms. Keep on putting square E cube before so and so forth and the if I put in anisotropy that is not everything is directionally symmetric then I can write one component of the polarization as linear term, square term, cube term and so and so forth. So what you understand is that this could be a convergent series so I can say that I approximate the system by its linearity when these terms are really really small very tentatively speaking but that happens because these constants that are sitting in here are actually really small in a magnitude. A small number multiplied by fact electric field somehow can make it still substantial. When for example I use a laser the electric field residing in the laser could be quite large as a result a small number multiplied by the square of a large number roughly can give you a response which is almost as good as the linear term. So it would be really stupid to discard terms that are equally large in my response expansion. So one more thing you can actually see that the second order term which is should have been the first common non-linear term is 0 in the bulk of a medium which has center of inversion symmetry. If I reflect one electric field and make it e to minus e then you can see that the term if you have center of inversion symmetry would remain the same making the second order susceptibility the same as minus of itself. So essentially that says that that constant is actually 0 provided you have center of inversion symmetry. The most common non-linearity you see is actually this chi 3 the third order susceptibility. So this is really one way of looking at non-linearity this is not the only way remember I said that this series has to converge and in phenomena that are extremely exciting that we call resonance sometimes these extra terms can be not so extra be the dominating one I will not get into that. So with this general thing what I want to talk about is how in this domain actually seemingly with the use of a medium light can talk to light. So normally in Maxwell's equation if you have seen those they are linear I mean you know light crosses light without interacting but if I put a material in there I can make light interact with light and one common example of this would be like a spontaneous photon fission when one photon goes into the material splits into two or I put in two photons I combine them and so and so forth. So this could be done either mediated by another light field or so and so forth. So from this very general introduction let me move to the crux of the matter for today for my task. So essentially what I am talking about is that if I have an intense electromagnetic field it induces a non-linear response in the medium because as a reaction the medium modifies the optical field. So so far you have actually thought of bulk constants as constants I put in electric field and there is a chi term sitting in there chi which is a constant multiplied by e gets you the polarization. What I am talking about now chi is no longer a constant it is actually field dependent because as light propagates it modifies the medium that medium in turn is modifying the propagation of the light. Very simple physical picture this is what non-linear it is all about. So what I am going to talk about is one such phenomenon actually a little complicated one but very very exciting that we have spent ten years probing and this also talks about bulk quantum mechanics at room temperature normally things that you do not associate with quantum mechanics. If you have heard of it you know this is a little different from Newtonian mechanics I will not talk about the mechanics mechanics part of it but I will actually use I will not get into the details of this either but essentially tell you that there is a system which we have probed to prove the kind of things we are talking about. So what is it that we are talking about it is about manipulating atoms and the nice thing about people who work with lasers is that you know these changes the manipulations I am doing are not permanent. So as long as the control laser field is on these changes you see and as soon as the control laser field is off you are back to your normal self. So they are actually quite interesting for many many applications and two kinds of effects normally you talk about when you see atom photon interactions of that kind one is what we call dissipative or absorptive effects right. I shine in light with an intensity I naught it traverses a particular distance the intensity that comes out is actually diminished. So you are familiar with such dissipation effect is normally described by a complex susceptibility. So it is an imaginary part of the complex susceptibility and even in the absence of any field this would lead to a broadening of the atomic energy levels and so on and so forth this is sort of known for some time. A conjugate part of it which is causally connected is the reactive or dispersive effect this is denoted by the real part of the susceptibility as in a prism you see dispersion effect and which is really connected always to imaginary part of the susceptibility this should lead to the other one lead to broadening of the energy level this would lead to actually a shift of the atomic energy levels and very famous Nobel prize winning work fall under understanding this real part of the susceptibility. So what am I talking about I am talking about a particular phenomenon that is called electromagnetically induced transparency in short EIT as the name implies this really transmission of a resonant light through otherwise opaque medium. So the medium was opaque I am shining light nothing was coming out I use a control to make it transparent so the original light sort of comes through and that would happen as long as the control is on. So this is something that I am going to use today and to illustrate the control that I was talking about earlier that you understand. So very basic understanding of what absorption is all about a very crude model is that suppose I concentrate on two energy levels inside my atom the energy levels are separated by energy of h bar omega naught and I shine a laser with a frequency of omega that is shown in red and omega omega naught may not be the same. You see an absorption when delta is 0 the difference between omega 0 and omega naught is nothing then you have maximum absorption corresponding dispersion curves look like that. These are very standard dispersion curves for absorption through a two level atom. So essentially when I shine in the laser light I actually lift this electronic or whatever population upstairs and it all gets absorbed so nothing comes out of it. The light does not come out because light has actually lifted the population upstairs and done its job so there is nothing left to come out. Now if I add a third level and use a second laser there is these profiles get drastically modified and this leads to what we call a transparency window where I had maximum absorption there I now have a depth in absorption that means maximum transmission. So where this one was actually just lifting the population up and getting absorbed now I have the red light actually coming out so transmission happens and this profile which is the conjugate of that gets dispersion profile gets modified with a very steep slope right here. Earlier you are never bothered about this region because everything was getting absorbed so nothing was coming out so light when it does not come out you cannot really probe. Now that I have made this little transparency window I can actually probe this region and come out with some really exciting physics. So what is the real excitement in the dispersion curve if this extreme positive dispersion can lead to slow light. If you look at the group velocity expression how a profile would transmit through such a material is given by the free space light speed divided by the index n. You are all familiar with that for example for air for glass for water 1.33, 1.5 would be typical range of n semiconductors may be 3, may be 6 maximum that is what you have seen. I am talking about an ng of 10 to the power 5, 10 to the power 6, 10 to the power 7 close to 10 to the power 8. So real real slow light moving at the speed of a cycle inside my material so not going that fast. So can we actually use it and can we actually ask the question if I could really slow it down can I actually stop it. If I stop it what would I achieve I would achieve storage of information may be if it is coded in that light. So this is the excitement in this phenomenon called EIT. The moment I have this laser on this you know this change happens the moment I turn this off of course I go back to my original two level system. So this is really is the idea. So yes so I think what I am going to simplify this so this is the probe that is going to come out and this is the control that I am using to induce these changes in the profiles. So this would always happen when the control is on. So I will simplify this further so the physics of EIT would always have this probe laser this is the one I am looking at whether it is coming out or not and this is the one I am using as called controller coupling and essentially you would always have this capital lambda kind of a configuration so it is called a lambda system. Now there are many ways of understanding what the hell is going on because when this is on resonance you would always have absorption if I did not have this coupling beam. So on resonance I am not having absorption anymore I am having perfect transparency. So there are many ways of understanding it you can solve the full problem which I will not get into but a very basic physics way of understanding is this that like in interference if I you know let me take a break it a question yes no. So let me show you what let me just answer one would be a direct one that through B to A I think the one animation got killed probably and there is a way of going from B to A straight or you go B to A via this coupling beam and come back to A. This is a second order process normally you would not bother about it but these two things together actually having the cancellation effect. So if you if you are familiar with double slit experiment young's double slit I have a slit I have just made two holes and I have a source of light I am looking at the interference pattern on the other side suppose I close one hole and let light go through on a particular spot I will actually I am receiving light and then I close the other one I am also receiving light at the same spot now I suddenly open both the slits it could so happen depending on where I am looking that I get no light at all. So light plus light can be no light at all so if you think of it normally you do not bother about it when you see two headlights of cars crossing you do not see such effects but suppose I keep on dimming the light source and I get to the level of say one particle of light photon I mean very loosely speaking at a time that I am shooting from the source one photon going through the slit A hitting the screen one photon going through slit B and hitting the screen both are happening the moment I open both both of them go together but they cancel each other. So our eye is a sensitive two things that we call intensity or power they are square of amplitudes kind and they are only positive definite the minimum possible value for that is 0 when you are in a perfectly dark room intensity is 0 nobody has seen negative light so when two positive definite quantities add up to give you 0 it tells you the way you are adding is not correct right so physics is happening so opposition is happening at a level which is below the sensory power of our eyes okay so that is the electric field electric field is positive in this may be negative in this two of them canceling each other 0 square is 0 so that is what you are seeing you are seeing a dark fringe. So when you go down to very very low light levels dark fringes are actually explained by saying that there are two possible paths of one photon to go through to reach that spot one through slit A one through slit B and these two paths interfere canceling each other. A similar thing is happening here there are two possible ways of lifting population from B to A one directly one indirectly through the coupling these two paths are canceling each other under certain conditions and that is really is the physics of EIT electrolyte yes yes and then reabsorption so it is a third order that is why it is actually the third order process very correct so this is really the indirect path and the direct path combining together essentially canceling the probe absorption along this okay and this happens because this is actually more powerful than that otherwise this order effect will not actually contribute. So this is really the physics of it and essentially as in all interference effects there is some kind of coherence you talk about in Young's level slate very wrongly actually we talk about the coherence of being a condition for seeing the interference a stable interference fringe here also these fragile quantum interference effects are wide as long as you have Raman coherence between the two lower levels or the spins as we call. So roughly that is the idea and we have actually done you can solve this problem I am not going to get through this the real issue is that if I solve the problem solving the problem would mean don't look at the equations not meant for you so for example you do the hydrogen atom problem if you are in first year undergraduate you know then you solve the bare atom problems here the atoms are actually dressed by an optical field so you solve the full problem what is interesting I want to point out is that when you solve that you get an Eigen state which is called the dark state so on two photon resonance this has a zero probability to be excited so even though it is on resonance the system has a state that is always dark it does not see light and as I many descriptions for that and we really don't need to get through that the general EIT scheme is as I said is a lambda system like that and the EIT condition would be a two photon resonance when this delta R is actually zero and when this capital delta is non zero I will consider mostly zero then this is not a very very efficient system so what is the idea if I want to really do this the idea is that can I switch it's like a switch that turns things dark and light dark and bright dark and bright so we can actually do many mixing of that so switch between transparency and absorption at multiple frequency create logic gates and so because it depends on in quantum interference effects it could be constructive or destructive at different frequencies and can alter dramatically so again the control because you have understood it and depending on the control frequency and its power I can actually shift this EIT the way we like to do an experiment of course the first trick is to select a simple usable atomic level configuration I talked about a lambda system no real life system is that simple okay so for an experiment is the best thing is to really look for one and we had one idea and that's what I am going to talk about this is the entire work was done in collaboration with my friends in Orsay in France in a group and very Indian looking friends of mine who were my collaborators for last 10 years what we do is look at metastable helium it's metastable because if you look at this is not the ground state of helium 2 3 s 1 has a lifetime of 8000 seconds as good as you know not decaying at all so it's almost a stable metastable as we call it and we select a lambda system using a transition which is in the infrared 1 micron little above 1 micron and there are tricks up our slaves by using things called polarization circular polarization we turn this into actually a simple lambda system by using right circularly left circularly polarized light in the respective arms so there are many good things about helium for the metastable helium for you can actually use extremely low low power diode lasers that are available of the shelf and there are many nice things about collisions in such helium because when I first started talking about the system many experts have actually told me that this will not work room temperature systems cannot show quantum coherence effects because there will be collisions that would actually kill everything that we wish to do and we did not want to go into low temperature systems because they are not they are expensive I will be always lagging behind the best of labs in the Europe and America and also because they cannot be taken to the field if you wish to work with gadgets that are applicable you can't possibly you try to avoid such sophisticated equipment because essentially you will not be able to carry it in a suitcase and go so this we managed to do and it turned out things that people thought were going to affect it adversely came to our rescue it actually became quite nice and collisions when they do not destroy coherence they help to increase transit time so because rather than having a ballistic motion of the helium atom through the laser beam it does exact like a in like a diffusive motion and therefore the time it takes to cross the laser beam increases therefore your interaction time works so I will not get into the details in velocity changing collisions have a very positive effect this is a tabletop setup that you see the helium beam helium cell actually is only 5 centimeters inside the shield and is a tabletop experiment that you can actually do to set up EIT and lo and behold actually you get to see this is actually shown in transmission so a transmission peak when delta is 0 this is inverse of this absorption that you that you see and you can make it very very narrow you know the width was about 10 kilohertz or so even in a room temperature system this of course we publish there is a lot of excitement then we had to get into the domain of doing the modeling of it because nobody knew how to actually work this out with all the relaxation processes we have and the nice thing about this modeling is that you are not filling with numbers numbers are already known so actually you are fitting the data with all known constants and trying to figure out them excellent understanding came out of this analysis that we did of this EIT the results in the first go I will just show you is something very nice I talked about this transmission but remember the conjugate effect where dispersion also had a very strange looking curve and if I instead of shining a laser which is continuous suppose I now send beams like that I can look at what is the speed at which is going remember I talked about slow light indeed if you do that you will find even in a system which had nothing it was a tabletop experiment with initially was a very very short helium cell we could achieve a group velocity of around 3 kilometer per second remember the free space speed of light 3 times 10 to power 8 meters per second from there we got into 10 to power 3 per second and without having to do much so this was quite remarkable that you can do this so what I am going to show you again a cartoon yes please yes no no I think you need much higher power we are actually not using that kind of power we are using a trick in in the steepness of the dispersion curve so even low power is capable of doing it because we are very close to the zero window it's a very narrow window but yeah if I could put in more power and have a way of looking at it more power would also have some effect on collisional things that I have not talked about but yes it is in that domain but I am not seeing it okay so let me show you a very brief cartoon of how what I mean by quantum memory so how do I store and retrieve information based on such NEI protocol supposing this is the material one atom inside that material is shown in this three level structure and this material when the probe comes the probe is all over okay sorry the control comes the control is all over this is a powerful control now comes a very weak probe pulse which is goes from a to b and it's coming in it's coming in going in and at this point if I now turn off the control the blue one that was there then the state of light is stored in the Raman coherence of the two lower lower states nothing comes out of this material I turn on the control then the retrieval starts happening the probe pulse comes out in the original with all the original parameters now of course there are many many issues in here how slowly can you turn off on the control field what is the impact of spontaneous decay and can we actually apply it in a single atom trapped in optical cavity we try to address all of these things in subsequent papers but the excitement was that starting in 1999 so it's quite old nature cover had this that in an ultra cold gas this was really really the coldest spot in the universe kind of a gas of sodium atoms you people showed 17 meters per second of velocity okay it was not completely stopped and a lot of work had happened after that in sodium and rubidium system we are talking about the helium but essentially what I'm talking about sophisticated light switches that control not just light but information suppose I have coded some information in my incoming light and that can be changes a modulation frequency amplitude phase what have you normally we play with these three so when the light stops that information is stored just like information stored in electronic memory of this computer right so it's a stable state unless like a switch it's by stable on is on off is off it's a basic coding device right till I actually disturb it it remains that memory okay so essentially these are very complicated light switches that I'm talking about and to access the information you turn on a control laser and off it comes so this is essentially the idea of stopping a restarting light so we talked about this and then some really crazy things came out of it and I'm going to today I'm just telling you the story I'm not going to the details of it but there are many offshoots that are puzzling very very fundamental questions so in the interest of time I'm just going to go through this giving you the idea and then we can talk later I talked about this expression whether you understood or not this has some approximation in it I've assumed only first order second order dispersion theory first order dispersion theory right now and what we have seen is that the group velocity is given by the free space speed of line given a by the group index okay very simple now so you can understand what I talked about that this group velocity will be much smaller than C if this one is much larger than 1 okay just about 1 about 3 about 6 you are familiar with now I'm talking about much larger than 1 that is 10 per 6 10 per 7 of that order now what happens if I have this one is less than 1 then Vg would become greater than C or I can actually have a backward light where group velocity would become negative okay now when you saw it there is nothing that we have done here that violates Maxwell or anybody essentially talks about fast light not just slow light that a counter intuitive but it does not violate Einstein's causality you know I say special theory of relativity of comes that that speed of information cannot be greater than the speed of light if that happens then I can go to my past till my grandfather and so I would not be born so you can actually violate all kinds of principles of causality that we believe in in physics if you could transmit information faster than the speed of light because then you can go to the past alter things in there so your present would be affected so causal cause and effect relationships would all get violated and that's sort of something that we do not allow there's no first proof of principle I believe you know the experts would correct me to prove this so time and again you find mostly Italian papers try to violate causality and showing some experiment where you would find always something wrong in the explanation saying they're violated causality so here the point is that when I was in high school I was told nothing can move faster than the speed of light by the time I was in the US to do my PhD people have actually shown experimentally very nice experiment that phase velocity that is the velocity of a frequency component of an envelope can be faster than speed of light nothing was violated even then now I am saying group velocity can actually exceed the speed of light and textbooks should be rewritten because group velocity does not contain information a smooth profile Gaussian if you look at a packet which extends from minus infinity to plus infinity the problem of how we define speed you need to think of in quantum information domain and always information is coded by turning something on so if there is always a discontinuity that you find in that front velocity that is what contains information and not the group velocity this needs little bit more thinking leads a lot more debate and clear understanding of what we actually mean by that but what I am showing here does not violate anything when you have such sharp discontinuity this expression breaks down this is a conjecture actually because what I'm talking about is that when you have such discontinuity the first order theory breaks down right that you need to take care of higher order terms which you are not doing and the entire definition of I'll show you just one reference to our word entire definition of group velocity you have to look at so we did this experiment the same experiment I showed by continuously varying the capital delta which we have not talked about and in one case we could actually go from a group velocity of a plus 10 to power 4 to group velocity of minus 10 to power 4 meters per second so this was never looked at before as I said this near to the 0 of the Raman detuning there was always huge absorption so nobody could actually probe this area now we have made it EIT that is transmitted transmission around that area and we can look at the dispersion curves which have these really sharp kinks and it's possible to see this so essential there is a whole lot that we published on this talking about how this concept of group velocity when you are doing this kind of stuff sort of breaks down so we I was invited to actually do this review on this so there is some details that are given in there and more or less now it's accepted that what I'm saying is that your concept of the decay rate for different frequencies and therefore the group velocity breaks down when you're playing with such kinks and information so I think a separate one that I once discussed with Professor Ray also there's a separate thing should be all quantum information the speed of propagation the entire concept is really crazy is not the way it should be formulated and I think that's where the problem lies that if you believe in the old concepts then what it's saying is negative group velocity is what we saw is that before the beam enters is actually come out of the medium already so essentially says this propagation description that we are giving is actually faulty so again I know I have sort of bombarded with some concept but these are all falling logically from known physics when you have understood we can actually try to use it to control it got us interested in looking at this transmission resonances of different kinds I talked about phase related or coherence related such altered resonances there could be even population related resonances I would not get into the details of it but essentially here they're known atomic coherence in more then it goes by the name of coherence population oscillation so this is also something that we have probed very very deeply in in this helium atomic system having done a three-level system we got excited to talk about what looks like a tripod it's like a double lambda if you like and a double EIT when you have phase distance working for you it's easy to do these kind of calculations because it's much more complicated you're talking about now a four-level atomic system three classical fields and so three Raman coherence is two dark states so lots of complication but it's more fun because now you have more controls in your hand and actually you can play with this thing very very interesting physics and many theoretical papers were there but experimental results were not there and again there's a lot of application you can think of in magnitude optic switches now not just with laser light but using a magnetic field and for quantum information storage we use the helium the same metastable helium but a different transition to go to the upper level and again it's a matter of detail they're all published work you don't need to worry about it very very simple experimental setup again the only thing that's different here is that we have actually put some homemade rectangular coils around our helium cell to be able to control a very weak magnetic field that was needed in this case very simple experiments were done with depending on the configuration of who is being shared in the double EIT in this one is this is the shared one that is shown and then there was nothing exciting in this particular case but when you actually have the coupling beam in on the two arm and the major one is the probe then they're interesting interference effects that you could see and something that was shown as black that was fully transmitting suddenly became an absorption dip and now this is being controlled by the field as well as the magnetic field a lot of what we have done in playing with this funny stuff and essentially you know went on and on and on doing some so the system again be useful as a polarization dependence which in the presence of a magnetic field and recently we have actually done some storage experiments looking at whether we can do light storage what is the capability of these things because the main concern for us was also that when I am using a room temperature system and decoherence because of velocity in collisions are happening then can I really store light for a long enough time for us to use again we worked it out and then came to some applications which again for want of time I would not get into that details but to give you an idea when I am coding some information into the line the information that I am coding can also have very many parameters in it as I said normally you modulate the frequency or the amplitude or the phase or all three or combinations thereof is possible to actually code it with highly non classical highly quantum information into the light one such thing is what we call squeeze light and it is to do with the fact that classically you know two quantities like the position and the momentum of a harmonic oscillator you can measure with absolute certainty nothing says that you know if I know X for sure I cannot know V for sure but for a quantum particle these two conjugate variables you can never measure simultaneously with absolute precision so if I am a quantum particle if I know if you know my position very very accurately then my velocity would be a completely fuzzy one you would not be able to do so what we understood is that there is an uncertainty principle that's about the product of these two uncertainties right it says that must be greater than a certain fundamental constant okay what we realize that the uncertainty principle is about a product it doesn't say anything about individual uncertainties so can I be smart enough to squeeze the noise out of one at the cost of course increase noise in the other maintaining Heisenberg but using the one that has less noise for all optical measurement purposes and that's what squeeze lines try to do and we have actually shown this to also be a nearly perfect squeezer in this system so essentially you do by creating quantum sources that are like twin brothers so one has noise of this kind the other one would be nullifying it all the time if I use them together I can actually create nearly perfect squeeze like in this so this is a very very involved experiment that we did last year and this year I don't know whether you have seen earlier this year you have come up with a theory and come some nice names of popular return new kind of quantum excitons that you can think of in a system of this so I I stop here and just quickly go to the summary saying that a room temperature system which is in the bulk I'm not talking about effect with one helium atom I'm talking about the bulk and I'm actually using it in the room temperature gas it's an attractive system even to do such quantum coherence and quantum mechanical effects and we designed a very very clean lambda system thanks to polarization selection and then been able to talk about slow light and very interestingly backward light group velocity of few thousand meters per second was actually seen even in a cell that was only 2.5 centimeter long we have increased it now and we have actually done further so quantum storage experiments based on such a electromagnetic induced transparency and coherence population oscillation actually could be demonstrated matter of principle right now but it would it would be quite robust if you could use CPO and not EIT because EIT is essentially very very sensitive because it depends on quantum coherence effects so quantum information system of course what people have been thinking about for a long time is that photonic qubits you use appropriate for communication over distance you want to send an information qubit is quantum bit bit is binary digits that zeros and ones that you use in a classical computer a qubit is a quantum computer which is combinations of zeros and ones more like not yes not no but may be and it has infinite information content in them and so if I can transfer those over a long distance without knowing what it is normally use photonic qubits and to store you use atomic systems these photons are difficult to localize and that's why I talked about this particular experiment so the information is getting stored as memories in atomic qubits so when I transfer from something that is carrying the signal to something that is going to store the signal and again take it out this would mean an atom photon network and at every level when you transform in information from the photon to the atom atom to the photon information that is quantum information that you don't know what it is you know it's a combination of zero and one what are the amplitudes of the two and this work you don't know so this is information transfer problem that we have worked heavily in our group to look at how do you actually transfer this quantum information and make an atom photon network that's the future of quantum information system must acknowledge most of my work was funded by DST a lot of this exchange programs could happen because of generous funding from CEPRA in France I was an invited professor and then my students were supported by CSIR I end with the moral of the story what I'm talking about is that evolution of physics and physics is the theory of science you know it happens when theory and experiment go hand in hand and very naturally some applications come out right but if you are always concentrating on market value of the research and think of what is going to come out tomorrow then you are going to kill this evolution and if you kill this evolution then this would die its natural death so when 1960 the laser was invented probably you know everybody said oh great fundamental tool what is the use by the time I landed in the US that question was never asked every supermarket had a laser for a reader and so if there is a time scale which biotechnologies and information technologies normally do not understand because in instantaneous transfer to the technology domain but in physics and most of science there is a time lag from when you are doing this necessary gambling of theory and experiment you're actually excited about probing a question and then trying to find out why is an and actually as I said how things happen in nature as a result these applications come out and will it be that one day we all would know all the secrets of nature and this is to become jobless I doubt that would ever happen we'll keep on pushing the frontier of knowledge using the methodology that I talked about as scientific methods and that would continue so I am thrilled and I must extend my personal congratulations to the winners and to all the participants and to the we have a center for doing such a wonderful job of bringing in people who would just keep on celebrating what I call scientific methodology for want of a better name and this celebration should continue for years to come so thank you again very much for your kind of attention thank you some questions yes please yeah the questions from the audience the students maybe I will start myself so can you tell us of course this is contrary to the spirit you just described about the applications but I understand that quantum information quantum computing is very much in the news nowadays and so can you explain in a language that all of us can understand about how these things can actually lead to computation or storage and transmission of information yes thanks you know the quest has been on the the major problem here well first of all for quantum information processing the kind of quantum computers we are thinking of they are not going to totally replace your classical computers this gadget uses many things that are quantum mechanical but the algorithm it uses is classical okay so quantum computers based on certain quantum algorithms would be able to solve certain kinds of problems that you would be unable to solve or take zillions of years to solve in a classical way okay and a factorization being one that was talked about which is essentially the security key for all your transactions including atm so it depends on your inability classical computers inability to factorize a huge number so the way this system works is that the system is if it is essentially quantum mechanical it works under quantum dynamics so it has its own thing and all the qubits I'm talking about should come under the same kind of framework of dynamics that's the major problem actually for an experiment list to make sure they're all subjected to the same dynamical transformations now having done that if the result could be the result is when you observe the moment you observe a quantum bit it becomes a classical one okay so if it is a combination of unknown combination of zeros and one the moment it's like the schooner cat how many of you have heard of it so I'll just give you that example if you don't know it's a nice story I mean I'm an animal lover I don't like this much but anyway the story is the following that Trindy knew this and he talked about this as a post-mortem physics or some such thing that supposing in a box I put a cat and a radioactive nucleus okay and assume both of them are quantum mechanical so if the radioactive nucleus decays it triggers a poison and the cat dies something like that okay and if it doesn't decay then the cat is alive so before I open the box the cat is in a state of being dead and alive together okay in our real life we have never seen a cat which is both dead and alive but the moment I observe it collapses to either the dead state or the other life state okay so it's like atom being excited electron in of an atom being excited by the laser beam the electron could be either upstairs or downstairs if I could single out two levels okay in between it could be spinning between the two so the state of the system is not upstairs not downstairs but a mixture of up and down anything any mixture okay so this is what we exploit in a quantum dynamics but the moment you observe it loses its charm of being quantum so it's very very fragile it collapses being a classical information either dead or alive either upstairs or downstairs okay so this is really the catch and this is where people have been looking for systems of not just one qubit but you need and I mean millions of qubits to to do the processing that you'd like so there are many candidates for this now and what I'm talking about is more on the global level of the platform of decoding it in a in a from a photonic system to an atomic system and then recovering that system without knowing which combination of dead and alive the photon is so that's really is the trick and so there been many success stories recently using silicon the people could get a huge number of such quantum dots to talk together but the idea of manipulation still beats you know what we need to do so I don't know whether I'll see a quantum computer in my lifetime I hope so at least you know some bits three qubits to seven qubits to some more have been actually demonstrated in the labs already but we need of course to scale it up much more questions yeah yeah I think this is probably a little provocative question this is about again group velocity being faster than speed of light so I understand that you don't we don't expect quality to be violated but one is not just using method equation here they're quantum mechanics at play and we really don't understand quantum mechanics that well so what I wanted to ask is that though while there are attempts going on to understand the meaning of group velocity and what it means to transfer energy and things like that but as an experiment list once this is there you have group velocity that is it like are there any attempts to actually try to see if experimentally you can communicate information faster and fit like yeah actually yes I think that's what we did and that's what my conclusion in that paper was essentially there will be so much of distortion in the information because we have not done the theory properly so the group velocity definition here to be looked that's the point really the definition of group velocity is breaking down when you go group velocity essentially have many frequencies in it in a cavity when you are talking about the decay rates are all different so when you superpose all such exponentially decaying rates it it's a different beast altogether so and that's what people have to look at and so we had a meaning of this but it's nothing to do with the coding that we had done so students either it is very clear or totally unclear or you're too sleepy after a heavy breakfast all right I'm available I mean you can you can write to me later if you feel too shy today too are there any questions okay let's thank Professor Ghosh for