A 100-year-old physics problem has been solved

June 22, 2017
A wave-interference and resonant energy transfer from one source to another distant source or object, pertaining to the fundamental concept ofresonances. Credit: EPFL - Bionanophotonic Systems Laboratory

At EPFL, researchers challenge a fundamental law and discover that more electromagnetic energy can be stored in wave-guiding systems than previously thought. The discovery has implications in telecommunications. Working around the fundamental law, they conceived resonant and wave-guiding systems capable of storing energy over a prolonged period while keeping a broad bandwidth. Their trick was to create asymmetric resonant or wave-guiding systems using magnetic fields.

The study, which has just been published in Science, was led by Kosmas Tsakmakidis, first at the University of Ottawa and then at EPFL's Bionanophotonic Systems Laboratory run by Hatice Altug, where the researcher is now doing post-doctoral research.

This breakthrough could have a major impact on many fields in engineering and physics. The number of potential applications is close to infinite, with telecommunications, optical detection systems and broadband harvesting representing just a few examples.

Casting aside reciprocity

Resonant and wave-guiding systems are present in the vast majority of optical and electronic systems. Their role is to temporarily store energy in the form of electromagnetic waves and then release them. For more than 100 hundred years, these systems were held back by a limitation that was considered to be fundamental: the length of time a wave could be stored was inversely proportional to its . This relationship was interpreted to mean that it was impossible to store large amounts of data in resonant or wave-guiding systems over a long period of time because increasing the bandwidth meant decreasing the storage time and quality of storage.

This law was first formulated by K. S. Johnson in 1914, at Western Electric Company (the forerunner of Bell Telephone Laboratories). He introduced the concept of the Q factor, according to which a resonator can either store energy for a long time or have a broad bandwidth, but not both at the same time. Increasing the storage time meant decreasing the bandwidth, and vice versa. A small bandwidth means a limited range of frequencies (or 'colors') and therefore a limited amount of data.

Until now, this concept had never been challenged. Physicists and engineers had always built resonant systems—like those to produce lasers, make electronic circuits and conduct medical diagnoses—with this constraint in mind.

But that limitation is now a thing of the past. The researchers came up with a hybrid resonant / wave-guiding system made of a magneto-optic material that, when a magnetic field is applied, is able to stop the wave and store it for a prolonged period, thereby accumulating large amounts of energy. Then when the magnetic field is switched off, the trapped pulse is released.

With such asymmetric and non-reciprocal systems, it was possible to store a wave for a very long period of time while also maintaining a large bandwidth. The conventional time-bandwidth limit was even beaten by a factor of 1,000. The scientists further showed that, theoretically, there is no upper ceiling to this limit at all in these asymmetric (non-reciprocal) systems.

"It was a moment of revelation when we discovered that these new structures did not feature any -bandwidth restriction at all. These systems are unlike what we have all been accustomed to for decades, and possibly hundreds of years", says Tsakmakidis, the study's lead author. "Their superior wave-storage capacity performance could really be an enabler for a range of exciting applications in diverse contemporary and more traditional fields of research." Hatice Altug adds.

Medicine, the environment and telecommunications

One possible application is in the design of extremely quick and efficient all-optical buffers in telecommunication networks. The role of the buffers is to temporarily store data arriving in the form of light through optical fibers. By slowing the mass of data, it is easier to process. Up to now, the storage quality had been limited.+

With this new technique, it should be possible to improve the process and store large bandwidths of data for prolonged times. Other potential applications include on-chip spectroscopy, broadband light harvesting and energy storage, and broadband optical camouflaging ("invisibility cloaking"). "The reported breakthrough is completely fundamental—we're giving researchers a new tool. And the number of applications is limited only by one's imagination," sums up Tsakmakidis.

Explore further: Group develops technique to shape pulses of intense infrared light

More information: "Breaking Lorentz reciprocity to overcome the time-bandwidth limit in physics and engineering" Science (2017). science.sciencemag.org/cgi/doi … 1126/science.aam6662

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12 comments

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Nik_2213
not rated yet Jun 22, 2017
Could such 'catch' a soliton ??
EmceeSquared
1 / 5 (4) Jun 22, 2017
"the length of time a wave could be stored was inversely proportional to its bandwidth."

Waves don't have "bandwidth". I stopped reading at that blabber.
Da Schneib
5 / 5 (2) Jun 23, 2017
Actually, though it doesn't make sense to talk about ideal sine waves having bandwidth, real waves are actually combinations of different frequencies. If these frequencies are close together, then it makes sense to talk about the bandwidth of a wave.

In resonant systems, the ratio of the energy in a waveform to the energy lost in each cycle of the resonator is called the Q factor; this is sometimes interpreted as the "quality" of the resonator. In other words, in a high Q circuit in electronics, little energy is lost at each cycle; but the bandwidth of the frequencies at which the circuit operates is more and more restricted the higher the Q.

What these guys have done is shown that it's possible to make a circuit with a high Q factor, that is, low loss, but with a much wider bandwidth than is normally possible, using an asymmetric resonator.
[contd]
Da Schneib
5 / 5 (1) Jun 23, 2017
[contd]
It's worthy of note, however, that one of the reasons for making high Q circuits is precisely to restrict bandwidth; in radios, for example, it is desirable to discriminate among signals at different frequencies, and this is done using high Q filters. The narrower the bandwidth of the filter, the less nearby waves can distort or interfere with the desired signal. And this of course also applies in applications like microwave wave guides, and optical communications, particularly when it is desirable to use extremely narrow channels in order to maximize the number of channels in a given frequency range.

We'll have to see how this plays out and the article is currently embargoed, so we can't see these asymmetric resonant circuits.
jcromano
5 / 5 (2) Jun 23, 2017
From "A 100-year-old physics problem has been solved," third paragraph:
"The number of potential applications is close to infinite..."

So many questions here. Let's start with: Is the number of potential applications an integer?
Moody
5 / 5 (1) Jun 23, 2017
All waves in the physical universe are wave packets with some bandwidth associated with them. The bandwidth may be very narrow, even "homogenous" in which it is limited by the uncertainty principle but it is still finite. If you don't understand what bandwidth is and how it affects wave mechanics, there are plenty of articles and books about it on the internet that can illuminate the topic for all knowledge levels.
Gigel
4 / 5 (2) Jun 23, 2017
The article is not freely accessible, but one can find similar works by searching for: arxiv "Breaking Lorentz reciprocity"

I wonder if this can be done for quantum systems and what it would mean in that case. No reciprocity would mean a non-hermitian operator is involved; e.g. measurements are non-hermitian.
swordsman
not rated yet Jun 23, 2017
The "Law of Superposition" has just been repealed. Electromagnetic field waves in space not longer pass through one another without interaction other than add and subtract vectorially. It is no longer possible to send controlled signals through these regions without interference. Lets see them create a similar situation here on earth. It will change much of the laws of physics.
Dingbone
Jun 23, 2017
This comment has been removed by a moderator.
Da Schneib
5 / 5 (1) Jun 23, 2017
@Gigel, asymmetric oscillators are not well explored either mathematically or experimentally. Virtually every experiment and every hypothesis probes unknown territory, and not least because we thought we knew everything because of Lorentz reciprocity in symmetric oscillators.

So far this is not a quantum effect; it is a classical effect discovered by Lorentz (as evidenced by its name). This will be an interesting field to watch as it develops. From such niches are important theories derived after proper research. But this one needs a lot more, and I find the claims in the article rather grandiose, as it appears you do.
Dingbone
Jun 24, 2017
This comment has been removed by a moderator.
pluj
not rated yet Jun 26, 2017
The K. S. Johnson's law is a result from the linear time-invariant theory, and under certain constraints there exists reciprocity. Obviously the system the authors proposed is not a time-invariant system, and thus does not necessarily obey the K. S. Johnson's law. Besides, their system dose not "overcome the time-bandwidth limit", which clearly states that the duration and the bandwidth of a pulse (or a wave packet) response cannot be small at the same time, i.e., ∆t∆f>~1, not that the bandwidth cannot be large for a long-lasting response. Maybe my understanding is wrong, but this article does not make much sense.

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