Tag Archives: laser

Clocks are essential for us to keep track of our time. In the image, spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA

Atomic timekeeping, on the go

New approach may enable more stable and accurate portable atomic clocks.

By Jennifer Chu


CAMBRIDGE, Mass. – What time is it? The answer, no matter what your initial reference may be — a wristwatch, a smartphone, or an alarm clock — will always trace back to the atomic clock.

Clocks are essential for us to keep track of our time. In the image, spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA
Clocks are essential for us to keep track of our time. In the image, spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA


The international standard for time is set by atomic clocks — room-sized apparatuses that keep time by measuring the natural vibration of atoms in a vacuum. The frequency of atomic vibrations determines the length of one second — information that is beamed up to GPS satellites, which stream the data to ground receivers all over the world, synchronizing cellular and cable networks, power grids, and other distributed systems.


Now a group at MIT and Draper Laboratory has come up with a new approach to atomic timekeeping that may enable more stable and accurate portable atomic clocks, potentially the size of a Rubik’s cube. The group has outlined its approach in the journal Physical Review A.


While chip-sized atomic clocks (CSACs) are commercially available, the researchers say these low-power devices — about the size of a matchbox — drift over time, and are less accurate than fountain clocks, the much larger atomic clocks that set the world’s standard. However, while fountain clocks are the most precise timekeepers, they can’t be made portable without losing stability.


“You could put one in a pickup truck or a trailer and drive it around with you, but I’m guessing it won’t deal very well with the bumps on the road,” says co-author Krish Kotru, a graduate student in MIT’s Department of Aeronautics and Astronautics, and a Draper Lab Fellow. “We have a path toward making a compact, robust clock that’s better than CSACs by a couple of orders of magnitude, and more stable over longer periods of time.”


Kotru says such portable, stable atomic clocks could be useful in environments where GPS signals can get lost, such as underwater or indoors, as well as in militarily “hostile environments,” where signal jamming can block traditional navigation systems.


Co-authors of the paper include Justin Brown, David Butts, Joseph Kinast, and Richard Stoner of Draper Laboratory.


A shift in time


The team came up with the new atomic timekeeping approach by making several “tweaks” to the standard method.


The most accurate atomic clocks today use cesium atoms as a reference. Like all atoms, the cesium atom has a signature frequency, or resonance, at which it oscillates. Since the 1960s, one second has been defined as 9,192,631,770 oscillations of a cesium atom between two energy levels. To measure this frequency, fountain clocks toss small clouds of slow-moving cesium atoms a few feet high, much like a pulsed fountain, and measure their oscillations as they pass up, and then down, through a microwave beam.


Instead of a microwave beam, the group chose to probe the atom’s oscillations using laser beams, which are easier to control spatially and require less space — a quality that help in shrinking atomic clock apparatuses. While some atomic clocks also employ laser beams, they often suffer from an effect called “AC Stark shift,” in which exposure to an electric field, such as that produced by a laser, can shift an atom’s resonant frequency. This shift can throw off the accuracy of atomic clocks.


“That’s really bad, because we’re trusting the atomic reference,” Kotru says. “If that’s somehow perturbed, I don’t know if my low-quality wristwatch is wrong, or if the atoms are actually wrong.”


To avoid this problem, most standard fountain clocks use microwave beams instead of lasers. However, Kotru and his team looked for ways to use laser beams while avoiding AC Stark shift.


Keeping time, in miniature


In laser-based atomic clocks, the laser beam is delivered at a fixed frequency and intensity. Kotru’s team instead tried a more varied approach, called Raman adiabatic rapid passage, applying laser pulses of changing intensity and frequency — a technique that is also used in nuclear magnetic resonance spectroscopy to probe features in individual molecules.


“For our approach, we turn on the laser pulse and modulate its intensity, gradually turning it on and then off, and we take the frequency of the laser and sweep it over a narrow range,” Kotru explains. “Just by doing those two things, you become a lot less sensitive to these systematic effects like the Stark shift.”


In fact, the group found that the new timekeeping system suppressed the AC Stark shift by a factor of 100, compared with a conventional laser-based system. Unlike fountain clocks, which shoot atoms more than a meter upwards in order to measure a single second, the team’s apparatus measures time in intervals of 10 milliseconds — an approach that is less accurate than fountain clocks, but much more compact.


“That’s fine, because we’re not trying to make the world’s standard — we’re trying to make something that would fit in, say, a Rubik’s cube, and be stable over a day or a week,” Kotru says.


The stability and accuracy of the system, he says, should be comparable to that of microwave-based atomic clocks on today’s GPS satellites, which are bulky and expensive.


Going a step further, the team tested the system’s response to physical forces. “Let’s say one day we got it small enough so you could put it in your backpack, or in your vehicle,” Kotru says. “Having it be able to operate while you’re moving across the ground is important.”


Just short of physically shaking the system, the group “created a displacement between the atoms and the laser beam,” moving the laser beam from side to side as it probed the cloud of atoms. Even under such simulated shaking, the system was able to measure the atoms’ resonant frequency, with a high degree of sensitivity.


The team is now working to reduce the size of other components of the system, including the vacuum chamber and electronics.


“Additional miniaturization could ultimately result in a handheld device with stability orders of magnitude better than compact atomic clocks available today,” Kotru says. “Such a device would satisfy requirements for more technologically intensive applications, like the synchronization of telecommunications networks.”


This research was sponsored by Draper Laboratory.

Source: MIT News Office


Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab at RSPE. Image Stuart Hay, ANU

ANU Physicists build reversible tractor beam

We have seen use of laser tractor beams from space ships catching or repelling space ships, objects and people. Science and technology have not developed that much to achieve such feats but Physicists at the Australian National University have done something amazing to push the boundaries science and technology a bit more and closer to that goal.

ANU Laser physicists have built a tractor beam that can repel and attract objects, using a hollow laser beam that is bright around the edges and dark in its centre.

Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab at RSPE. Image Stuart Hay, ANU
Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab at RSPE. Image Stuart Hay, ANU

It is the first long-distance optical tractor beam and moved particles one fifth of a millimetre in diameter a distance of up to 20 centimetres, around 100 times further than previous experiments.

“Demonstration of a large scale laser beam like this is a kind of holy grail for laser physicists,” said Professor Wieslaw Krolikowski, from the Research School of Physics and Engineering.

The new technique is versatile because it requires only a single laser beam. It could be used, for example, in controlling atmospheric pollution or for the retrieval of tiny, delicate or dangerous particles for sampling.

The researchers can also imagine the effect being scaled up.

“Because lasers retain their beam quality for such long distances, this could work over metres. Our lab just was not big enough to show it,” said co-author Dr Vladlen Shvedov, a driving force behind the ANU project, along with Dr Cyril Hnatovsky.

Unlike previous techniques, which used photon momentum to impart motion, the ANU tractor beam relies on the energy of the laser heating up the particles and the air around them. The ANU team demonstrated the effect on gold-coated hollow glass particles.

The particles are trapped in the dark centre of the beam. Energy from the laser hits the particle and travels across its surface, where it is absorbed creating hotspots on the surface. Air particles colliding with the hotspots heat up and shoot away from the surface, which causes the particle to recoil, in the opposite direction.

To manipulate the particle, the team move the position of the hotspot by carefully controlling the polarisation of the laser beam.

“We have devised a technique that can create unusual states of polarisation in the doughnut shaped laser beam, such as star-shaped (axial) or ring polarised (azimuthal),” Dr Hnatovsky said.

“We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will.”

The work is published in Nature Photonics.

Source : ANU News

World data transfer record back in Danish hands

Researchers at DTU Fotonik have reclaimed the world data transfer record.

By Lotte Krull


The world champions in data transmission are to be found in Lynbgy, where the High-Speed Optical Communications (HSOC) team at DTU Fotonik has just secured yet another world record. This time, the team has eclipsed the record that was set by researchers at the Karlsruhe Institut für Technologie, by proving that it is possible to transfer fully 43 terabits per second with just a single laser in the transmitter. This is an appreciable improvement on the German team’s previous record of 32 terabits per second.

The worldwide competition in data speed is contributing to developing the technology intended to accommodate the immense growth of data traffic on the internet, which is estimated to be growing by 40–50 per cent annually. What is more, emissions linked to the total energy consumption of the internet as a whole currently correspond to more than two per cent of the global man-made carbon emissions—which puts the internet on a par with the transport industry (aircraft, shipping etc.). However, these other industries are not growing by 40 per cent a year. It is therefore essential to identify solutions for the internet that make significant reductions in energy consumption while simultaneously expanding the bandwidth. This is precisely what the DTU team has demonstrated with its latest world record. DTU researchers have previously helped achieve the highest combined data transmission speed in the world—an incredible 1 petabit per second—although this involved using hundreds of lasers.

The researchers achieved their latest record by using a new type of optical fibre borrowed from the Japanese telecoms giant NNT. This type of fibre contains seven cores (glass threads) instead of the single core used in standard fibres, which makes it possible to transfer even more data. Despite the fact that it comprises seven cores, the new fibre does not take up any more space than the standard version.

The researchers’ record result has been verified and presented in what is known as a ‘post deadline paper’ at the CLEO 2014 international conference.

The High-Speed Optical Communications team at DTU Fotonik has held the world record in data transmission on numerous occasions. Back in 2009, these researchers were the first in the world to break the ‘terabit barrier’, which was considered an almost insurmountable challenge at that time, when they succeeded in transmitting more than 1 terabit per second—again using just a single laser. The benchmark has now been raised to 43 Tbit/s.

Source: DTU News