Tag Archives: rubidium

Newton's laws, atoms.

Researchers detect the smallest force ever recorded

Researchers have detected the smallest force ever recorded – 42 yoctonewtons – using a system of super-cooled atoms.


Newton force quantum scale

Newton’s laws don’t really stand anymore when you get to small enough scales. Mechanical oscillators translate an applied force into measureable mechanical motion. The Standard Quantum Limit is imposed by the Heisenberg uncertainty principle, in which the measurement itself perturbs the motion of the oscillator, a phenomenon known as “quantum back-action.”
Credit: Image by Kevin Gutowski

The Newton, named after sir Isaac Newton, is the international unit of measure for force. 1 Newton is equal to 1 kilogram times 1 meter over 1 second square (1N = 1 kg * 1 m / s^2). A yoctonewton is one septillionth of a newton – or in other words, 0.000000000000000000000001 newtons.

Scientists working at the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley used a system of lasers and a cloud of ultracold atoms to measure the force.

“We applied an external force to the center-of-mass motion of an ultracold atom cloud in a high-finesse optical cavity and measured the resulting motion optically,” says Dan Stamper-Kurn, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the UC Berkeley Physics Department. “When the driving force was resonant with the cloud’s oscillation frequency, we achieved a sensitivity that is consistent with theoretical predictions and only a factor of four above the Standard Quantum Limit, the most sensitive measurement that can be made.”

This is not just a significant breakthrough in itself, but it could have significant results in the future. As scientists study (directly or indirectly) more and more tiny things, more accurate ways of detecting forces and interactions are necessary. For example, at the Laser Interferometer Gravitational-Wave Observatory (LIGO), scientists are attempting to record motions as small as one thousandth the diameter of a proton.

Changing things by measuring them

However, there’s a big problem with measuring tiny things. As you get to smaller and smaller scales, quantum effects start to kick in – and they can really mess things up. According to the Heisenberg uncertainty principle, you change the outcome of a value if you measure it – yes, that’s how quantum mechanic works sometimes. As these measurements of force are done with mechanical oscillators, when you get to the quantum scale, the measurement itself affects the osscilator. This barrier is called the Standard Quantum Limit (SQL). In the past couple of decades, scientists have come up with a myriad of creative (yet imperfect) ways of working around the SQL. However, none of them came even close, failing by over 6 orders of magnitude! Until now, that is.

“We measured force with a sensitivity that is the closest ever to the SQL,” says Sydney Schreppler, a member of the Stamper-Kurn research group and lead author of the Science paper. “We were able to achieve this sensitivity because our mechanical oscillator is composed of only 1,200 atoms.”

In their experiments, they set up two equal and opposite optical fields and applied them onto a gas of rubidium atoms optically trapped and chilled to nearly absolute zero. The response is measured using a probe beam with a wavelength of 780 nanometers.

“When we apply an external force to our oscillator it is like hitting a pendulum with a bat then measuring the reaction,” says Schreppler. “A key to our sensitivity and approaching the SQL is our ability to decouple the rubidium atoms from their environment and maintain their cold temperature. The laser light we use to trap our atoms isolates them from external environmental noise but does not heat them, so they can remain cold and still enough to allow us to approach the limits of sensitivity when we apply a force.”

He believes that they can go even closer to the SQL, though not much closer.

“A scientific paper in 1980 predicted that the SQL might be reached within five years,” Schreppler says. “It took about 30 years longer than predicted, but we now have an experimental set-up capable both of reaching very close to the SQL and of showing the onset of different kinds of obscuring noise away from that SQL.”

Harvard and MIT scientists create photon molecules

Photons and molecules

Mikhail Lukin - image via Harvard.

Mikhail Lukin – image via Harvard.

Scientists managed to ‘trick’ photons (the elementary particles of light and all other forms of electromagnetic radiation) into forming molecules for the first time – a state of matter that until recently had been purely theoretical.

Scientists from Harvard University and the Massachusetts Institute of Technology (MIT) are challenging the current paradigm – they want physicists to rethink what they know about light, and they didn’t have to go in another galaxy to do this.

What happened is that a group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic managed to coax photons into binding together to form molecules. The discovery goes against decades of accepted theories and ideas on light. Photons have constantly been described as massless particles that don’t interact with each other (they are only considered to have a mass when they are moving). Shine two photon lasers at each other, and the rays will simply pass through each other – photonic molecules seems a nonsensical term.“Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other,” Lukin said. “What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules. This type of photonic bound state has been discussed theoretically for quite a while, but until now it hadn’t been observed.

What they did isn’t really a photon laser, but rather a… light saber.

Using the Force

"Photonic molecules" behave less like traditional lasers and more like something you might find in science fiction -- the light saber. (Credit: © Yana / Fotolia)

“Photonic molecules” behave less like traditional lasers and more like something you might find in science fiction — the light saber. (Credit: © Yana / Fotolia)

“It’s not an inapt analogy to compare this to light sabers,” Lukin said. “When these photons interact with each other, they’re pushing against and deflecting each other. The physics of what’s happening in these molecules is similar to what we see in the movies.”

But Harvard researchers can’t really rely on “The Force”, so instead, they began by pumping rubidium atoms into a vacuum chamber. After a while, they used lasers to cool the cloud of atoms to just a few degrees above absolute zero (the lowest thermodynamic temperature – −273.15° on the Celsius scale, −459.67° on the Fahrenheit scale). Then, using very weak lasers, they fired single photons into the cloud of atoms. As the photons enter the cloud, they give energy to atoms along their path, which causes them to slow dramatically. As the photons move through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon.

“When the photon exits the medium, its identity is preserved,” Lukin said. “It’s the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together. But when it exits, it’s still light. The process that takes place is the same. It’s just a bit more extreme. The light is slowed considerably, and a lot more energy is given away than during refraction.”

But when Lukin and his colleagues fired two photons into the cloud, they were really surprised to see the results – the two photons came out of the cloud together, as a single molecule. This is the effect of a rather strange and unintuitive effect, called the Rydberg blockade, which means that when an atom is excited, nearby atoms cannot be excited to the same degree. What this means for this case in particular, is that as the first photon excites an atom, it must move forward before the second one can excited nearby atoms. What’s interesting is that they tend to retain this molecule-like behavior even after exiting the cloud.


Photons with strong mutual attraction in a quantum nonlinear medium. Credit: Nature.
Photons with strong mutual attraction in a quantum nonlinear medium. Credit: Nature.

 This could prove to be valuable for developing quantum computers; quantum logic requires interactions between individual quanta so that quantum systems can be switched to perform information processing.

“What we demonstrate with this process allows us to do that,” Lukin said. “Before we make a useful, practical quantum switch or photonic logic gate, we have to improve the performance. So it’s still at the proof-of-concept level, but this is an important step. The physical principles we’ve established here are important.”

The process could be used in the future to create 3D structures, such as crystals, solely out of light.

“What it will be useful for we don’t know yet. But it’s a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules’ properties,” he said.

Physicists create previously thought impossible super photons

Velocity-distribution data of a gas of rubidium atoms, confirming the discovery of a new phase of matter, the Bose–Einstein condensate. Via Wikipedia

A team of physicists from the University of Bonn developed a totally new type of source of light, the so called Bose-Einstein condensate; the results will be published in the upcoming edition of Nature. They managed to achieve this astonishing feat by greatly cooling Rubidium atoms and stashing them into each other, up until the point they become indistinguishable and behave like a single big particle, which researchers call a Bose-Einstein condensate.

Technically speaking, the Bose-Einstein condensate is a state of matter of a dilute gas, of weakly interacting bosons, cooled to a temperature very close to absolute zero (approximately -273 degrees Celsius). Under such conditions, a large fraction of the bosons occupy the lowest quantum state of the external potential, at which point quantum effects become apparent on a macroscopic scale. [Wikipedia]

The apparently unsolvable problem appears when talking about light; the problem is that when photons are cooled down, they just disappear; however, Bonn physicists Jan Klärs, Julian Schmitt, Dr. Frank Vewinger, and Professor Dr. Martin Weitz succeeded where so many others failed.

For a better understanding of the phenomena, we should ask ourselves how warm light really is; for example, if you warm a tungsten filament (standard light bulb filament), it starts glowing, first red, then moves on to yellow and then, finally, blue(ish). It would seem that for every temperature there is a different colour, but the problem is that different metals glow in different colours, so a common starting unit had to be found. In order to achieve this, physicists created a theoretical model, the so called black body. The black body is an idealized object that absorbs all the electromagnetic radiation directed at it. So if you take this theoretical object and theoretically heat it, it will give a common ground for a corelation between light colour and temperature. But what happens when you cool it ?

The creators of the "super-photon" are Julian Schmitt (left), Jan Klaers, Dr. Frank Vewinger and professor Dr. Martin Weitz (right). (Credit: © Volker Lannert / University of Bonn)

If you would cool it, it will at some point stop radiating in the visible spectrum – it will only give out infrared photons which are invisible for the human eye. Also, as you cool it, the radiation intensity will decrease as the number of photons gets smaller (because photons disappear when cooled). The problem seems impossible – how do you lower the temperature of the photons without “killing” them ?

The Bonn researchers used a really inventive system, basically using two highly reflective mirrors and bouncing a beam back and forth between them. What happens is that when light hits the mirrors, the molecules in the mirror absorb the photons and then spit them back, and a whole number of interesting things happen during those collisions:

“During this process, the photons assumed the temperature of the fluid,” explained Professor Weitz. “They cooled each other off to room temperature this way, and they did it without getting lost in the process.”

This should especially please chip designers, because they use laser light for etching logic circuits into their semiconductor materials; just how small and fine these structures can be is limited by the wavelength of light – the smaller the better. A big wavelength is just like writing on a piece of paper with a big paintbrush. In time, this development will pave the way for more performant microchips, which will ultimately affect us all.