Why aluminum should replace cesium as the standard of time

The second is defined as 9,192,631,770 vibrations of a cesium atom and measured in a device known as a fountain clock. These work by cooling a tiny cloud of cesium atoms to a temperature close to zero, tossing it up in the air and zapping it with microwaves as it falls.

Then you watch the cloud to see if it fluoresces. This fluorescence is maximised when the microwave frequency matches a hyperfine transition between two electronic states in the atoms, at exactly 9,192,631,770 Hz.

Various labs around the world use this method to run clocks with an accuracy of around 0.1 nanoseconds per day. That’s impressive but not perfect. Fountain clocks have one drawback: the clouds of cesium tend to disperse quickly and that limits how accurately you can take data.

Now there’s a new kid on the block which looks as if it’s going to be better at keeping time.

Today some chaps from the the University of Nevada in Reno and the University of New South Wales in Sydney outline a new clock that relies on an effect called the Stark shift in which a spectral line is split by an electric field (this is the electric analogue of the Zeeman effect in which spectral lines are split with a magnetic field).

This is a complex phenomenon but the key thing is that the same electric field can influence the split in different ways. In fact, a couple of groups have recently discovered that in certain circumstances these can cancel out each other at specific “magic” frequencies of an electric field. When that happens, the line splitting vanishes.

This should be pretty straightforward to measure. The electric field is supplied by trapping the atoms in a standing electromagnetic wave, otherwise known as a standard optical lattice. Then change the laser frequency while looking at the atomic spectra. When the line splitting vanishes, you’ve hit the magic frequency.

The big advantage of this method is that you can trap millions of atoms easily in an optical lattice and that should make such a clock much more robust than a fountain, while achieving at least the same kind of accuracy.

So what kind of atom should we choose to sit at the heart of these “micromagic clocks”? The Ozzie-American group says that, contrary to previous reports, cesium does not have a magic frequency and so can’t be used in this technique. Aluminum, on the other hand, should be perfect.

The second is dead, long live the second.

Ref: arxiv.org/abs/0808.2821: Micromagic Clock: Microwave Clock Based on Atoms in an Engineered Optical Lattice

Creating random numbers the quantum way

The stream of high quality papers continues from the lab of Andrew Shields at Toshiba Research in Cambridge, UK. Today, his team unveils a new type of quantum random number generator and a fine looking machine it appears to be.

Here’s the idea. Create a stream of single photons are emitted at random intervals that depend entirely on quantum processes–an attenuated continuous wave laser should do the trick. Fire them at a gated photon detector which accurately records their (entirely random) arrival time. The arrival time within a gated time period is then a random number ready for use in quantum cryptography or whatever app you happen to need it for.

The team uses the souped up photodiode that we saw a couple of weeks back to make the photon detections at rates of 4MB/s. And Shields says 100MB/s is possible–that’s two orders of magnitude faster than existing quantum random number generators.

What’s more, the new device is much simpler than other quantum random generators. One popular approach is to send a photon through a beam splitter and see which way it goes. In principle, the outcome is perfectly random but in practice it ain’t because it’s almost impossible to make a beam splitter with perfect 50% probability split.

In practice, the data from these devices needs a certain amount of massaging which can be costly and time-consuming.

So Shields looks to be on a roll.  Exciting times in his lab.

Ref: arxiv.org/abs/0807.4111: A High Speed, Post-Processing Free, Quantum Random Number Generator

Terminator 0.0.1 (alpha)

The French start up Aldebaran-Robotics based in Paris has high hopes for its humanoid robot called NAO.  The device is 57 cm high and weighs 4.5 kilograms (about the size of a 6 month old baby) and you may be about to see a lot more of it. The company has sent a simplified version to 16 teams playing in the Robocup humanoid football league this year.

NAO looks an impressive device, judging by the design, which the company has posted on the arXiv today.   And others clearly agree. Earlier this year, the company picked up  Euros 5 million in venture capital funding to help commercialise the device. The target market is university research labs involved in developing the next generation of software and hardware for robotics.

That’s a smart move because  it could make NAO a de facto standard.

NAO doesn’t come cheap, however. A single robot will set you back Euros 10K but that is significantly cheaper than most other humanoids. Fujitsu’s HOAP costs $50K, for instance, and Honda hasn’t been able to put price on Asimo.

The company hopes that economies of scale will bring down the price as production scales up. Eventually it hopes to sell NAO to the public for Euros 4K each.

Better start saving.

Ref: arxiv.org/abs/0807.3223: The NAO Humanoid: A Combination of Performance and Affordability

The magnetic magic of liquid mirrors

 

Liquid mirror telescopes are amazing contraptions. They start life as a puddle of mercury in a bowl. Set the whole thing spinning and the mercury spreads out in a thin film up the sides of the bowl.

The result is a fabulously cheap mirror that can be used for a variety of astronomical surveys. If we ever put a telescope on the moon, many astronomers have suggested that it should be one of this type.

It won’t have escaped your attention that liquid mirrors have important limitations. First, they can only point straight up. One or two people  have played with fluids that have a higher viscosity than mercury and so can be tilted a few degrees this way or that but with limited success. And second, they cannot be made adaptive to correct for blurring introduced by the Earth’s atmosphere.

But that may change thanks to some interesting work being done by Denis Brousseau at Université Laval in Quebec et amis.  Their machine controls the shape of the surface of a liquid mirror using a magnetic field. Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

Instead the team have used a suspension of ferromagnetic nanoparticles in oil. A thin highly reflectivity layer of silver particles can then be spread across the surface of the ferrofluid to create a mirror.

Brousseau and co use an array of tiny coils behind the liquid to create a field that deforms the fluid surface as required.  Their tests show this can be done fast and furiously enough to cope with the usual array of optical aberrations that the atmosphere throws up.

However, it may also be possible to use this technique to tilt liquid mirrors further than ever before.  Ferrofluids can easily be made much more viscous than mercury and so combat the deforming pull of gravity. But they can also be deformed in a way that opposes gravity during each rotation of the supporting bowl. That could make them much more tiltable than mercury mirrors.

Of course, such a mirror would be mechanically more complex than the spinning bowls we have today and correspondingly more expensive. And sending one to the moon seems an unnecessary extravagance given the absence of an atmosphere there.

But here on Earth they could be made much more useful.  It’s a combination of new-found utility and value for money that many astronomy projects on a budget will find irresistible.

Ref: arxiv.org/abs/0807.2397: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments

If invisibility cloaks don't work, try the invisibility sheet

When it comes to invisibility cloaks, nobody has done more to advance the field than John Pendry, a theoretical physicist at Imperial College, London. It was he who suggested the idea in the first place and mapped out how one could be built in theory. He even got his hands dirty by  collaborating with the team of engineers who first built a working cloak.

So when he pronounces on the subject, we sit up and listen.

Pendry has clearly been worrying about the limitations  of invisibility cloaks. For a start, they work only in the microwave part of the spectrum and at a single specific freqeuncy. (Optical invisibility cloaks seem as far away as ever because of problems with light absorption.)

The cloaks must be made of exotic materials with properties that vary throughout their structure and are in any case unobtainable in nature and so have to be designed and made by hand.

The resulting cloaks are not perfect and probably never will be. To hide an object completely, the permittivity and permeability of these metamaterials must take infinite values at some points.

So what to do? Pendry argues in a paper on the arxiv that instead of making objects invisible, you can hide them just as well by making them look like a flat conducting sheet. An eminently sensible suggestion.

The advantage of this approach, he calculates, is that it readily works for visible light and over a wide range of frequencies. What’s more, it can be done with ordinary materials that are available today.

All that’s needed is to hide your object under a material that he calls an isotropic dielectric. He’s even done a number of simulations to show how such a material would make anything it covers look like a flat conducting sheet.

Pendry doesn’t bother with the practical details of how to make an isotropic dielectric material. But maybe he doesn’t need to. He wouldn’t by any chance be referring to water, would he?

Ref: arxiv.org/abs/0806.4396: Hiding Under the Carpet: a New Strategy for Cloaking

Simple mod turns diode into photon counter

Counting photons is a tricky business. They’re slippery beasts that arrive silently, often and in packs, in ways that are almost impossible to count.

One of the most widely used of devices that can spot the arrival of a single photon is the avalanche photodiode. These cheap and easy to use devices rely on the ability of diodes to allow the flow of electrons when the voltage across them is in one direction but prevent that flow when the bias is reversed. But  if the reverse bias is increased beyond a specific threshold then a breakdown occurs and a reverse current suddenly starts to flow.

Choose the right material for your photodiodes and this breakdown can be triggered by a the arrival of a single photon  smashing into an electron which goes on to hit other electrons causing a chain reaction. The result is an avalanche of current that signals the arrival of your photon.

Avalanche photodiodes are widely use to detect single photons but have an important limitation: they cannot distinguish between the arrival of a single photon and the arrival of two or more photon’s simultaneously.

But that is set to change. Today, our old friend Andrew Shields, at Toshiba’s research labs in Cambridge UK, explains how to soup up a bog-standard avalanche photodiode so that it can count photons as they arrive. That’s like turning a Fiat 500 into a Ferrari.

He says that the trick is to measure the characteristics of the avalanche current in the very first instants that it forms. At this early stage, say Shields and friends, the avalanche current  is proportional to the number photons that have struck.

Simple really but with enormous potential. The ability to count photons is one of the key enabling technologies for optical quantum computing. A number of schemes are known in which it is necessary to count the arrival of 0,1 or 2 photons at specific detectors.

Various people, including Shields himself, have  come up with complex, cooled devices that can count photons. But this is a major step forward. Avalanche photodiodes are cheap, widely available and easy to use. With such a cheap detector now available (as well as decent photon guns), we could see dramatic progress in this field in the coming months.

If you haven’t quite seen the significance of this, imagine overclocking your calculator and matching the performance of a workstation. Or polishing up the 3 inch reflector in your attic and outclassing Hubble with your images.

Impressive stuff.

Ref: arxiv.org/abs/0807.0330: An Avalanche-Photodiode-Based Photon-Number-Resolving Detector

Let the SPIT wars begin

If SPAM arrives in your inbox at 4am, the chances are your antispam software will catch it. But even if it doesn’t, you won’t lose much sleep over its arrival.

But it’ll be a different story with SPIT (spam over internet telephony). Junk phone calls at 4am are going to drive you mad because the chances are that antispit software won’t be able to intercept the call.

Today, Andreas Schmidt and pals from the Fraunhofer-Insitute for Secure Information Technology in Darmstadt Germany explain why intercepting SPIT is so much harder than spotting SPAM.  The main difference between junk calls and junk email is that the email arrives at your mail server before you access it. This gives the server time to analyse its content and filter out the junk before it gets to you.

Internet telephony, on the other hand, goes straight through to you in (more or less) real time, giving your server little or no time to analyse its content.

There are still a number of strategies that could be employed to filter out SPIT. For example, white lists that allow only calls from predetermined callers, Turing tests such as audio CAPTCHAs that make a caller prove he or she is human and payment-at-risk services where the caller makes a small payment in advance and is refunded immediately if the receiver acknowledges the call as legitimate.

But Schmidt and pals don’t seem confident that these techniques will work. They happily point out the disadvantages of each strategy, showing how most are either impractical or easily  circumvented by a determined spitter.

They have even created a program that implements all of these attacks. Their idea is to use the program as a benchmarking tool against which people can test antispitting strategies.

Spitting is a problem that is likely to get worse. Much worse, if the estimates are correct that as much as 90 per cent of email traffic is SPAM .

So to all you computer security guys out there: hustle, hustle, hustle. I need my sleep.

Ref: arxiv.org/abs/0806.1610: Spam over Internet Telephony and How to Deal With It

First test of exotic space thruster ends in explosion

In 2006, Mason Peck at Cornell University in Ithaca dreamt up with an entirely new way to control satellites orbiting planets that have a magnetic field. The idea is based on the Lorentz force: that a charged particle moving through a magnetic field experiences a force perpendicular to both its velocity and the field.

So the plan is to somehow ensure that the spacecraft becomes electrically charged as it moves through the planetary magnetic field which should then generate a force that can alter the orbit or orientation of the vehicle. The big advantage of so-called Lorentz actuated orbit control is that it requires no propellant. That’s a big deal since the amount of fuel a spacecraft can carry is the main factor that determines its lifespan. Propellant-free propulsion could significantly increase their operaitng lives.

Today, Peck along with William Gorman and James Brownridge at the State University of New York at Binghamton present the results of the first experimental trials of the idea. The work was funded by NASA but it has to be said: it doesn’t look entirely promising.

The team tested the ability of various objects to hold a charge in a vacuum while being bombarded with plasma, as would be the case in orbit. To generate the charge on the test object, they attached it to a sample of radioactive Americium-24, an alpha-particle emitter, and applied a voltage. The electric field carries away the positively charged alpha particles leaving the object highly charged.

I’ll let the team take up the tale:

“Microscopic arcing was observed at voltages as low as -300 V. This arcing caused solder to explode off of the object.“

Obviously, a proplusion system that explodes while it is in operation needs some more work.

The early pioneers of experimental propulsion systems such as Robert Goddard and Werner von Braun all had to cope with catastrophic failures, so Peck, Gorman and Brownridge are in good company. And as long as nobody gets hurt, a decent explosion livens up any experiment.

So stick with it fellas. Something tells me that if NASA funds the future development of this system, we’re going to be in for some fun.

Ref: arxiv.org/abs/0805.3332: Experimental Study of a Lorentz Actuated Orbit

The puzzling discovery of a motor made from liquid film

Here’s an interesting effect discovered by a group of Iranian physicists at Sharif University of Technology in Tehran, Iran (it’s not often we hear from these guys).

They placed a thin film of water in a square cell and applied two perpendicular electric fields. One was an external electric field. For the other, they used two copper electrodes to generate a voltage across the cell like an electrolysing cell (although no chemical reaction took place).

So they had a pair of electric fields at right angles acting on this thin film.

The unexpected result is that the film of water begins to rotate. The team has a number of movies of the effect on its website. They call it a liquid film motor and it’s a quite extraordinary effect. At one point they divide their cell into nine smaller ones and the liquid in each cell rotates in exactly the same way.

The question is: what’s causing the rotation? The team can easily control the direction and speed of rotation by varying the relative angle and direction of the electric fields, which rules out the possibility that convection is causing the rotation (something that is seen when a field is applied to some thin films of liquid crystals). Neither does adding salt to water change the effect, ruling out the possibility that ion movement directs the flow.

The rotation occurs in polar liquids but not in non-polar ones so the intrinsic dipole moment of the molecules seems to be crucial. People have been observing the electrohydrodynamics  of various types of thin films for a good few years but nobody has seen anything like this. Just what’s going on remains a mystery.

But the puzzle shouldn’t overshadow what looks like an important discovery that could have widespread industrial application in microfluidic devices for mixing.

Ref:  arxiv.org/abs/0805.0490: A Liquid Film Motor

Why tiny helicopters are so hard to fly

Tiny remote control helicopters have become all the rage in the last few years as lightweight motors and materials have plummeted in price. But if you’ve ever played with one, you’ll know how hard they are to control.

That’s not the result of poor construction. Small helicopters are harder to control than big ones because of the laws of physics: moments of inertia drop in proportion to the fifth power of vehicle size. This gives small helicopters quicker response times, making them more agile. But the real killer is that the main rotor tip speed in a small helicopter is the about the same as it is for a large helicopter. So the ratio of the rotor moments to the moments of inertia can become huge and unmanageable.

That’s when you need to develop a model of helicopter dynamics so you can design remote control systems or an autonomous flight control system that can manage this agility, say Hardian Reza Dharmayanda and pals at Konkuk University in Seoul, South Korea.

And that’s what they’ve done in this paper: built and tested a control system for a Yamaha R-50 helicopter, which uses a two-bladed main rotor with a Bell-Hiller stabilizer bar. The next step, they say, is to make the helicopter fully autonomous using their model.

These guys may be re-inventing the wheel but it’s interesting to see how they’re doing it.

Ref: arxiv.org/abs/0804.4757: Analysis of Stability, Response and LQR Controller Design of a Small-Scale Helicopter Dynamics