Cooling ions by pick-pocketing ultra-cold atoms

What do we mean when we say we wish to ‘cool’ something down? In day to day life, we simply mean, that its temperature be reduced or heat from the material should be removed. But what actually happens inside the material being cooled? How much heat can we remove from a material? Can we reach temperatures almost equal to absolute zero i.e. 0 Kelvin? We know that all matter is made up of large number of atoms which are in a state of incessant back and forth motion. This energy due to motion, or kinetic energy, is perceived as temperature. Cooling any material essentially means reducing the kinetic energy of the atoms in the material. When a system is sufficiently cold, the kinetic energy is negligible. In this case, the potential energy, which in dilute gases of atoms is very small, starts to determine interaction among the atoms, and so plays a major role in deciding the properties of the material. As this happens, very basic quantum properties which would otherwise be masked at higher temperatures reveal themselves. Cooling systems to such regimes is therefore of critical importance for many physics experiments, and often the biggest challenge to overcome.

Researchers Sourav Dutta and Sadiq Rangwala at the Raman Research Institute (RRI), Bengaluru have developed a novel method of cooling ions based on a phenomenon called resonant charge exchange (RCE). “What we demonstrate in this work from RRI is a novel and very efficient way of cooling ions by pick-pocketing the electron from the atom. New cooling mechanisms are very rare and have ramifications for future advances,” says Prof. Sadiq Rangwala, who is  an author of this study. Their study, published as a Rapid Communication in the journal Physical Review A, was supported by DST, under the DST-INSPIRE Faculty Award, and Indo French Centre for the Promotion of Advanced Research (CEFIPRA).

As the atoms and ions in the gaseous phase tend to fly apart, even when very cold, chances of studying their interaction is greatly reduced. Thus if one wishes to study an atom-ion interaction for a longer time, some form of geometric confinement of the ions and atoms, in other words “trapping” is required. Usually, trapping is done using a combination of electric, magnetic and light fields. In the RRI experiments, this led to two, co-centred globes of trapped ions and atoms, millimetres in extent, suspended in very high vacuum at the centre of a steel experimental chamber with many windows.

Once the motion of the ion is restricted to the volume of the trap, the ion/atom are conventionally cooled by two methods. The first approach relies on “laser cooling” where, particles of light or photons from a laser are scattered by the atom/ion, undergoing multiple absorption and emission. The multiple collisions result in a recoil for each absorption and emission in a very specific way, leading to atom/ion cooling. The second approach relies on “sympathetic cooling”, where atoms which are already laser-cooled to a lower temperature collide with a higher temperature ion, thus reducing the ion’s kinetic energy. Over the course of multiple collisions, the ion loses most of its kinetic energy. However, both of these processes require multiple collisions since each collision only removes a fraction of the kinetic energy from the ion.

In contrast, in this new cooling method based on RCE, almost the entire kinetic energy of the ion is removed in a single collision between a colliding ion and the parent atom. “Cooling by resonant charge exchange is essentially a ‘swap cooling’ mechanism where an electron is transferred to a fast ion from a pre-cooled parent atom held essentially at rest. This results in an ion that is almost at rest. The mechanism works in homo-nuclear ion-atom systems i.e. when the ion and the atom are of same species, such as the Cesium-Cesium+ combination”, says Dr. Sourav Dutta, who was a DST-INSPIRE funded Faculty fellow at RRI. He is currently an Assistant Professor at the Department of Physics, IISER Bhopal. The swap cooling technique is found to be more efficient than other conventional methods, since a single collision produces a cold ion. Under similar experimental conditions, the per-collision cooling via RCE mechanism was found to be about 100 times higher than cooling via elastic collisions.

Experiments across the world are attempting to study ultracold collisions for the combined ion-atom system which has so far been elusive. According to the authors, this study underlines the importance of RCE as a mechanism in ion-atom systems and is an instrumental mechanism for ion-atom experiments in the ultracold regime.cooling-ions

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Exotic substance in our galaxy?

Our Universe is full of mysteries; the enigmatic dark matter and dark energy, mystifying particles, obscure elements—all of these add up to the puzzle the Universe poses. Now, researchers from the Raman Research Institute (RRI), Bangalore and Lebedev Physical Institute, Moscow, Russia have identified a new piece to this puzzle closer to home. In a study published in the journal Astrophysical Journal Letters, the researchers have interpreted spectral data taken from the Sloan Digital Sky Survey (SDSS), and have pointed at the possible presence of exotic elements in our galactic halo, the roughly spherical component of our galaxy that extends beyond the primary, visible part.

High school physics tells us that when light passes through a prism, it splits up into different colours that make up the spectrum of the light we see. A light spectrum is a vital tool in astronomy as it provides a window into the processes that are putting out the light, as well as the medium through which that light passes. When light from a star or a galaxy passes through (relatively) cold gas on its way to us, atoms in the gas absorb specific frequencies of light, producing gaps in the observed spectrum known as absorption lines. These lines are unique for each element and therefore help astronomers identify the composition of stars and intervening gas.

In 2017, researchers from the University of Arizona, USA analysed spectra of more than 700,000 galaxies in the Sloan Digital Sky Survey (SDSS) dataset, which contains data from 15 years of observations made by an automated telescope that scans the sky. They discovered absorption lines at a wavelength of 6565 Angstroms (10 billion Angstroms is 1 metre), that were produced when light from the distant galaxies passed through gases in our galaxy. Based on the wavelength of the absorption lines that were characteristic of  hydrogen, the researchers claimed to have detected hydrogen gas in our galactic halo. Furthermore, the strength of the absorption lines indicaigs_-_part_10_2_9ted the presence of a large quantity of hydrogen gas that would have helped account for the matter that is known to exist but has not been detected so far.

When Prof. Shiv Sethi of RRI, the lead author of this study, and his colleagues heard about this result, they attempted to model it. The idea was to construct a model of the interstellar medium (ISM) – the space between stars in our galaxy that would show how the newly observed hydrogen gas could exist alongside the other, known components of the ISM and how they would interact.

The researchers found a surprising result. “We realised it could not be modelled because from what we know of the ISM, such a line cannot arise,” explains Prof. Sethi. Producing a hydrogen line with the same strength as was observed would require a sky brightness at ultraviolet frequencies (higher frequencies than visible light) that is much greater than what is seen. As an alternative, the researchers investigated whether the line could have been produced by carbon, nitrogen or oxygen which are the most abundant elements in the ISM after hydrogen and helium. But their calculations showed that the absorbing cloud would have to be larger than the halo of our galaxy thereby ruling out this possibility too.

“Somebody observes something, and we (theorists) usually explain it. But here, it is different – the entire paper was that we could not explain it. Proving a result wrong is hard. It is rare  in astronomy,” quips Prof. Sethi.

The researchers argue that the absorption lines could have been produced by complex molecules known as Polycyclic Aromatic Hydrocarbons (PAH). PAHs are organic compounds containing carbon and hydrogen with the carbon atoms arranged in rings. Laboratory measurements of the spectra of PAHs like pyrene and naphthalene have shown that they produce spectral lines that are close to the observed wavelength of 6565 Angstroms. Using this information, the researchers identify some possible compounds that could have produced the observed absorption lines, but they cannot definitively determine the compound that may be responsible.

PAHs have been detected in the disc of our galaxy, the atmosphere of Saturn’s moon Titan and elsewhere in the Universe. They are of particular interest because they are thought to be the starting material for the earliest forms of life.

“In Astronomy, we see all kinds of exotic phenomena,” concludes Prof. Sethi. However, he cautions that a group of researchers in the US have corresponded with him, and suggested that the observational result may have come about as a result of “calibration errors”. He adds, “We are not sure, and this issue may take some time to be resolved.”

Raman Research Institute and ISRO join hands to study the early Universe

MoU_RRI_SACIt’s official! The Raman Research Institute (RRI) and ISRO Space Applications Center (SAC) are teaming up! With the signing of an MoU, RRI and SAC will work together to further our understanding of the evolution of Universe by looking for wiggles in the first light from the very first stars born in the universe. The two institutions will partner to design and develop extremely sensitive radio receivers to detect this light, deploy them in locations free from man-made radio interference, ranging from remote sites on ground all the way to space in lunar orbit. These radiometers will be custom designed to detect signals from the formation of first atoms (Epoch of Recombination) to the formation of first stars (Cosmic Dawn and Epoch of Reionization). The collaboration also aims to design and develop astronomical observatories to look directly at the birth places of stars in our local Universe by studying radiation from molecules. From studying the formation of planetary systems, to molecules that are essential to life, these observatories have exciting science goals. RRI will focus on the science, proof of concept, ground based observing systems, and SAC will apply its expertise towards the space qualification of the experiments for deployment in high-altitude and space, away from the clutter of terrestrial radio frequency interference. Watch out for future updates of this partnership as it takes flight!

Throwing light on dark Matter

All the matter that we see around us is ultimately made up of atoms, which in turn is made up of protons and neutrons. Surprisingly, such ordinary matter constitutes less than 15% of the total matter in the Universe. Most of the Universe is made up of dark matter – matter that does not emit any light but interacts with other objects only through gravity. Although we know dark matter exists, we do not know what exactly it is. Now, a study by researchers from Bengaluru has tried to understand the dark matter better.

Dark matter has never been directly observed or created in a lab. Its existence and properties are inferred from its gravitational effects on objects that we can see. In this study, published in the Journal of Cosmology and Astroparticle Physics, researchers from the Raman Research Institute (RRI), Indian Institute of Science (IISc), and the Indian Institute of Astrophysics (IIA), have examined what future experiments can reveal about the nature of dark matter.

“The first partial evidence for dark matter was seen long ago, in the 1930s. But the main evidence came in the 1970s. It was observed that galaxies are rotating too fast to be held together by the gravity of the visible matter alone. Hence the existence of additional invisible, dark matter was postulated. By the late 1980s, additional lines of evidence had emerged from observations of clusters of galaxies, and from even larger cosmological scales”, says Prof. Shiv Sethi from RRI, who is also an author of the study, talking about our quest to understand dark matter.

Although the exact nature of dark matter is unknown, there are several different models of what the dark matter particle could be, most of them motivated by different particle physics models. The most popular model is “cold dark matter”, where cold refers to the velocity of the dark matter particles in the early Universe. If dark matter particles were ‘hot’, travelling at high speeds, they would rush from high density to low-density regions in the early Universe, thereby wiping out these density differences. In contrast, ‘cold’ dark matter, which moves slowly, would allow density differences in the early Universe to grow into galaxies. Since we can observe galaxies in the present Universe, hot dark matter model is now ruled out.

Scientists probe the nature of dark matter in several ways, including experiments that hope to detect dark matter particles directly. But in this study, the researchers focus on observations of the cosmic microwave background (CMB) — relic radiation left over from the Big Bang. “CMB is the most precise measurement in almost all of astrophysics. Moreover, the physics that goes into it is extremely clean. So when we want to look for something in an experiment, we want to manage these two aspects,” explains Prof. Sethi.

The cosmic microwave background is nearly uniform, but there are small differences in temperature and density. The pattern of variations in temperature and density depends on what particles are present in the early Universe, and their properties. Hence each type of particle leaves behind a distinct signature in the spectrum of temperature and density fluctuations in the CMB.

In this study, the researchers calculate the effects of different dark matter particle models on the CMB spectrum. “We have considered a whole range of models that exhaust nearly all possibilities that are still consistent with current experimental constraints,” says Prof. Sethi. The researchers have tried to answer the question — ‘If dark matter changes its nature below a certain scale, how well can we probe it with future experiments? What kind of constraints can we put on these models?’

The study finds that previous and current observations of the CMB are not sensitive enough to distinguish between these dark matter particle models. However, many experiments are being proposed in the next decade to observe the CMB, such as the space-based PIXIE (Primordial Inflation Explorer) mission by NASA and the ground-based CMB S-4 (Stage 4) experiment at the South Pole. Based on the expected sensitivity of PIXIE, the researchers expect these experiments to be able to distinguish between at least some of the dark matter particle models, thereby shedding light on the enduring mystery of what dark matter is.img_20180222_152835

What’s cooking in the ‘LMC X-4’ binary star system?

In a new study, researchers from the Raman Research Institute (RRI), Bengaluru, present observations about the binary star system ‘LMC X-4′ in the neighbouring Large Magellanic Cloud satellite galaxy. Their findings, published in the New Astronomy Journal, uses data from receivers aboard spacecraft XMM-Newton launched by the European Space Agency, and the Rossi X-ray Timing Explorer satellite from NASA. The findings hold important clues in the mechanics behind X-ray emissions seen from this binary system of stars.

The Large Magellanic Cloud (LMC) is a satellite galaxy that orbits our own, much larger, Milky Way. LMC X-4 is a two-star system consisting of a pulsar – a highly magnetised neutron star beaming X-rays – and a companion star. Scientists believe that such pulsars are formed after a supernova explosion, when a dying star spectacularly shoots material and expanding shockwaves into space, before being reborn as a compact star. The neutron stars are so dense that their mass could be twice that of the Sun, with a radius of only 10 kilometres. Its high density makes its magnetic field a trillion times stronger than Earth’s.

In this study, Dr. Aru Beri and Dr. Biswajit Paul from RRI examined characteristics of the X-ray signals emitted from LMC X-4, using signal processing techniques and analysis. The scientists observed three states in the intensity of X-ray emitted by the pulsar: ‘flaring’, ‘persistent emission’, and ‘eclipse’.

Neutron stars evolve through ‘accretion’ – the continuous addition of usually gaseous particles by gravitational attraction. Due to very strong gravitational attraction, the neutron stars accumulate gas in streams from its orbiting companion onto ‘hotspots’ on the surface of the neutron star, which briefly flare with X-rays when the accretion abruptly increases. The ‘flaring state’ of LMC X-4, which happens when there is a sudden increase in the amount of material accreting onto the surface of the neutron star resulting in larger X-ray radiation, showed four distinct flares. These flares exhibit a pattern of pulsation that, when plotted, is broad and sinusoidal in shape. This is followed by a non-flaring or ‘persistent emission state’ where a low signal of X-rays is recorded. The third state of ‘eclipse’ is identified by an absence of X-rays when our view of the neutron star is blocked by its much larger companion.

The researchers focused on the complex structure of the X-ray pulses during the persistent emission state, defined as the region with no flares and no eclipse. They observed ‘dips’ in the pulses during the period of steady, low X-rays, identified by sharp drops in the intensity of radiation. They attribute this to the hotspots becoming obscured by optically dense bands of accreting gas that falls from the companion star into the neutron pulsar star.

The study estimates the timescales involved in the formation of the flares, and the subsequent formation of the accretion column that causes dips, stating that “observed differences in the pulse profiles during and after the flares suggest that significant change in the accretion stream happens during the transition between flares and the persistent state”.

This study takes us a step closer to confirming theories on the nature of light, magnetism, gravity and general relativity. Observing and analysing cosmic phenomena such as pulsars, enables a deeper understanding of some extreme conditions in the cosmos that cannot be created in laboratories. This study gives insight into the behaviour of plasma in extremely high magnetic field and very strong gravity.

The researchers of the study have plans to further their research on X-rays emitted from other stars. “We are currently making an X-ray polarimeter named POLIX, which is the main scientific instrument onboard XPoSat, a satellite mission of ISRO to be launched in 2019, dedicated for investigation of polarisation properties of cosmic X-ray sources”, shares Dr. Paul, about his future plans. Observations with POLIX will allow scientists to determine the magnetic field structure of neutron stars, the key element in the formation of the flares and dips in LMC X-4 that have been discovered with the current investigation.Untitled

Hermit photons coaxed to interact via one-dimensional confinement!

Photons, the particles of light, do not contain any charge and hence typically do not interact with each other. In fact, they are so aloof that when you shine a beam of light from a torch onto one from another, they cross each other’s path and move on as if the other beam does not exist.

While this property makes them excellent candidates for long distance communication, it also serves as a hindrance for applications in optical information processing and realization of all-optical circuits, where photon-photon interaction at the level of individual photons (for example, controlling the motion of a single photon with another photon) is highly desired. The question now is this: How do we achieve photon-photon interactions at the level of individual photons?

One way would be to tap into what is known as nonlinear optical behavior, which is exhibited by certain materials that change their refractive index as a function of the intensity of incident light. As a consequence, the propagation of light in the interior of such materials can be controlled by another beam. This gives rise to effective interaction between otherwise noninteracting light beams. Intensity-dependent refractive index is routinely employed in many nonlinear optical devices in laboratory optics and in technology. Could this feature of nonlinear optics be used to make individual or at most a few photons interact?

Unfortunately, the electric fields (and hence the intensity) associated with individual photons is tiny; therefore a vast number of photons are required to realize optical nonlinearity in the interior of any known nonlinear optical material. Let us take a minute here to ponder this question: why do we need a large number of photons to generate nonlinear behavior in a material? Answering this question could point one in the right direction towards achieving nonlinearity with a few photons. Let us consider a cylindrical light beam of diameter ‘d’ incident upon a material made up of atoms. The chance that one photon will “see” one atom in the material is given by the ratio of the effective size of the atom as seen by a photon and the transverse area of the beam. The word, “see” is in inverted quotes because of the following: how well a photon sees an atom is dependent on how well the energy of the photon matches the energy levels of the atom; the closer they are more the chance of interaction. When the energy of the photons matches the energy levels of the atom, the effective size of the atom is approximately the square of the wavelength of the incident photon. The beam diameter is usually much greater than the photon wavelength, and hence the chance of one photon interacting with an atom in bulk materials is very very small. Right away, we see that to improve the chances of interaction and hence nonlinearity we need a lot of photons (which we do not want, since we are aiming for nonlinearity with a few photons) or—here is our aha moment—reduce the diameter of the light beam ‘d’ to such a small value, in fact far below the wavelength of photons, and couple it to an artificial atom whose effective size is far greater than any existing real atom.

Enhanced atom-photon coupling with just a few photons, leading to strong optical nonlinearity, is achieved by confining the photons into tiny spaces with dimensions much smaller than the wavelength of the photons. This has been experimentally realized by squeezing the photons along with individual atoms in confined geometries such as waveguides. One-dimensional waveguides such as superconducting transmission lines can confine microwave photons to deeply sub-wavelength sizes in their transverse dimensions. When we couple an atom (a superconducting qubit here) with a large transition dipole (equivalent to a large effective size) to such confined photons, strong optical nonlinearity may be manifested with a just few photons. In such a case, photons are transversely focused to an area comparable with the scattering cross-section of the atom, and their electric field at the atom becomes large enough to excite the atom (which means that atom absorbs energy from the incident photon and goes to a higher energy state) with very high probability.

In a recent colloquium, Dr. Dibyendu Roy from the Raman Research Institute along with international collaborators has discussed the topic of photons interacting strongly when confined to a one-dimensional geometry from experimental and theoretical perspectives.

To know more about this new exciting research field, please visit

https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.89.021001

Colloquim_1Dphotons

Figure caption: A superconducting qubit (indicated in orange) embedded in a one dimensional transmission line waveguide. The waveguide confines light to deeply sub-wavelength dimensions in the transverse direction.

‘Organic’ material set to make solar energy truly ‘green’

IGs - Part 9 (1)_0Illustrations: Purabi Deshpande

Scientists believe that inorganic semiconductors used in making solar cells may have hit the efficiency ceiling besides being environment hazard.

In my opinion, silicon technology has reached a sort of limit regarding efficiency. It is around 20% efficient right now, but it hasn’t seen a huge leap in efficiency in a long time. Production of electronic grade silicon pollutes the environment significantly and manufacturing silicon wafers indigenously on a large scale is still a challenge,” says Prof. Sandeep Kumar, of the Raman Research Institute (RRI), Bangalore, who specializes in Soft Condensed Matter. 

Going ‘organic’ is the alternative, says Prof. Kumar, who along with his team of researchers, is studying the use of organic materials like polymers or carbon fullerenes in solar cells. “Organic photovoltaic (OPV) research has received a good momentum in the past decade as promising sources of alternative energy due to their low cost, ease of fabrication, roll-to-roll processing, flexibility, and lightweight, he says.

Traditionally, inorganic semiconductors like silicon and its alloys have been used in making solar cells due to their appropriate electrical properties and wide availability. In addition to being an environmental hazard, some scientists believe that silicon-based solar cells may have also hit the efficiency ceiling. The power conversion efficiency (PCE) of OPV’s has crossed the 10% mark and is increasing day by day”, he explains.

In a recently published study, Prof. Kumar of the Raman Research Institute in collaboration with researchers at the Center of Material Sciences, University of Allahabad, and National Physical Laboratory, New Delhi, have used indigenously developed Discotic Liquid Crystals (DLC) to build effective organic solar cells that could become an eco-friendly alternative to conventional solar cells. This study was funded by the Department of Electronics and Information Technology (DEITY) and published in the Liquid Crystals Journal.

“DLCs were discovered at the Raman Research Institute in 1977. They are of fundamental importance not only as models for the study of energy and charge migration in self-organized systems but also as functional materials for device applications such as one-dimensional conductors, photoconductors, light emitting diodes, photovoltaic solar cells, field-effect transistors and gas sensors”, remarks Prof. Kumar.

DLCs are made of stacks of disc-like molecules in a state of matter that lies between those of a liquid and a solid crystal, like powdered sugar, which is made of solid crystals, but collectively behaves like a liquid taking the shape of the vessel in which it is stored.  Earlier studies have shown that DLCs efficiently conduct electricity along their length with very little loss, effectively making them molecular wires.   In this study, the researchers have used DLCs in solar cells to increase their efficiency.

The world is witnessing a shift in its primary energy sources, with fossil fuels giving way to renewable sources like solar and wind. India is well along this line of transformation with 10GW of electricity currently produced from solar energy, compared to just 2.6 GW in 2014. This shift to solar energy across the world has revived the solar cell (also known as the photovoltaic cell) industries, and now increased investments pour into this sector.

A solar cell, in principle, converts the energy of light falling on it, into electricity. The light conversion in OPV cells is based on charge generation at the interface between two different organic semiconductors, followed by their separation and migration toward opposite electrodes. The researchers of the study sandwiched a layer of DLC between an active layer (PCDTBT and PCBM) and a molybdenum trioxide buffer layer.

“We have explored the use of DLC as an additive in classical organic photovoltaic cells and observed significant improvement in conversion efficiency from 1.24% to 5.14%. The enhancement is attributed to the better charge mobility in the ordered system due to the presence of columnar phase of the DLC”, exclaims Prof. Kumar.

Although solar is hailed as a ‘green’ energy source, the mining and the manufacturing process associated with silicon based solar cells releases large amounts of pollutants, overshadowing the eco-friendly status of solar power. Organic solar cells, on the other hand, overcome these shortcomings and with studies like this, they may soon surpass the efficiency of silicon-based cells, finally providing us a true ‘green’ source of power.