First, fast, and faster

Apr 06, 2012 By Paul Lett
Figure 1. Schematic diagram of the fast light experiment in PML’s Laser Cooling and Trapping Group. Inset at top right shows the rubidium energy levels relevant to the experiment. Inset at bottom left shows the relationship between frequency detuning and gain.

(Phys.org) -- Scientists in PML's Quantum Measurement Division have produced the first superluminal light pulses made by using a technique called four-wave mixing, creating two separate pulses whose peaks propagate faster than the speed of light in a vacuum.

Laser Cooling and Trapping Group researcher Paul Lett and colleagues report in a forthcoming paper in that this new method of generating “fast ” has resulted in a that travels up to 50 ns faster over the length of a 1.7-cm cell than it would if it were moving through a .

The findings could have a significant impact on optical communications systems in which signal quality may be improved by speeding up or slowing down pulses. In addition, investigation of the quantum-mechanical correlations between the seed and conjugate pulses will provide fundamental insights into quantum coherence, with potential implications for future quantum information-processing.

Many methods of generating faster-than-light pulses involve sending a pulse composed of multiple wavelengths into a non-linear gain medium. The dispersion properties of the medium (that is, the way it changes a wave’s phase velocity depending on its frequency) rearrange the pulse components so that the pulse peak is shifted forward, producing apparent superluminal velocity for the entire group of waves. Conversely, “slow light” pulses can be generated by adjusting conditions so that the peak is shifted backward.

In the PML four-wave mixing experiment (see Figure 1), researchers send “seed” pulses of light into a heated cell containing the gain medium, atomic rubidium vapor, along with a separate “pump” beam at a different frequency from the seed pulses. In the medium, the seed pulse is amplified and its peak is shifted so that it becomes superluminal. At the same time, photons from the inserted beams interact with the medium to generate a second pulse, called the “conjugate” because of its mathematical relationship to the seed. Its peak too, the scientists found, can travel faster than an unaltered reference pulse would in a vacuum. Or it can be tuned to travel slower.

Figure 2. To create fast light, previous experiments have used the double gain features shown in (a.), leading to the linear dispersion region highlighted in red. The NIST experiment uses a gain feature as in (b.), where the region highlighted in red would produce fast light, while the region highlighted in black would produce slow light.

Figure 2a shows the effects of variable dispersion graphically. Sections of the line with a negative slope correspond to conditions in which different frequency components of a pulse “see” a different change in the index of refraction as the pulse moves through a medium. That causes re-phasing, re-forming the pulse ahead of where it would have been. It is that phenomenon that makes it appear that the pulse travels faster than a reference pulse. Researchers can produce this dispersion by pumping a gain medium with the right frequencies. The problem with this method, Lett says, is that the gain features are often so far apart that the dispersion slope is not very steep, resulting in light that is not that much faster than normal.

Instead, the group’s postdoctoral researchers, Ryan Glasser and Ulrich Vogl, used a method more similar to that used to generate slow light, shown in Figure 2b. A single gain feature results in a dispersion with a strong negative slope at the center of a narrow frequency range, causing an increase in the index of refraction for the pulse and slowing it down. By tuning the frequency of the laser off the center of the gain peak, they were able to access a different part of the dispersion curve that has a steep, positive slope, again generating fast light instead of slow light. Because this slope is steeper than those generated by two gain features, the pulses are sped up more.

Figure 3. Previous work with writing quantum information onto a beam of light showed how the image of a cat’s face - produced by sending light through a mask - could be transmitted, resulting in correlated images with entangled photons. The 4-wave mixing amplifies the original image and generates the second image with local intensity and phase correlations to the first image.

Communications researchers have proposed that slow light could act as a sort of controllable delay line, or storage medium, for light carrying quantum information. Alternatively, the fast conjugate pulse that Lett’s group has observed might act as a way to advance the detection of quantum information, should the fast light retain quantum coherence. (NIST scientists emphasized that, while the information detection could be advanced, no information could actually travel faster than the and that, consequently, principles like causality in special relativity were always respected in these experiments.)

The group has already demonstrated the transmission of an image by passing the seed beam through a mask before injecting it into the cell. (See Figure 3.), In the new work, the injected image traverses the cell faster than a reference image, but has distortion in space and time. Lett attributes this distortion to variations in the intensity of the seed beam and the vapor cell itself. “Different regions of the image actually see different advance,” he says.

One possible way to improve the conjugate image quality is to lessen the temperature variation within the vapor cell itself. Lett and Glasser, however, think that increasing pump laser power and creating a more uniform beam would remove this distortion.

Additionally, the scientists have to determine how much the reality of the experimental equipment will distract from the quality of the information in the light. High-frequency and low-frequency components of the seed pulse fall outside the frequency range that causes fast light, as shown in Figure 2, and will cause distortion of the pulses. This means that the researchers need to set a condition to define when the relevant pulse-containing information has arrived at their detector.

Their solution is to use smooth pulses that fit into the spectral region, but to look at the signal-to-noise ratio of the spatial information contained in the image. Lett is still hopeful about the result: “We can look at it and see that [we] can push the information detection forward and … say that with a real detector and real pulses, [we] can detect the information arriving earlier.”

One major challenge the experiment faced was the stability of the pump laser frequency. Glasser says they struggled with “tweaking the laser to behave properly and be single-mode across the frequencies of interest. That was probably the most frustrating aspect." Additionally, at these frequencies, they were unable to find a commercially-available laser system that provided them with as much optical power as they would have liked.

One immediate application that the group would like to explore for this system is quantum discord. Quantum discord mathematically defines the quantum information shared between two correlated systems – in this case, the seed and conjugate pulses. It is possible that speeding up or slowing down the light could introduce noise into the system, which would destroy quantum coherences that are required for entangling two systems. By performing measurements of quantum discord between fast beams and reference beams, the group hopes to determine how useful this fast light could be for the transmission and processing of quantum information.

Explore further: Germanium tin could mean better and cheaper infrared cameras in smartphones

More information: “Stimulated generation of superluminal light pulses via four-wave mixing,” Ryan T. Glasser, Ulrich Vogl, and Paul D. Lett, forthcoming in Physical Review Letters.

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User comments : 33

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Modernmystic
1 / 5 (1) Apr 06, 2012
I'm confused, are they making light travel faster than c?
Kinedryl
Apr 06, 2012
This comment has been removed by a moderator.
Lurker2358
1 / 5 (7) Apr 06, 2012
Careful now guys.

When they discover that your instrument is misaligned by a nano-meter, you will be forced to resign and end your careers, just like the guys with the neutrino experiment.

Although photons have no "mass", so who knows, maybe they can be accelerated beyond c in some situations.
Jotaf
5 / 5 (4) Apr 06, 2012
They're just shifting the phase. You still can't use this to transmit information faster than light. You should remember the concept of group velocity from Physics 101...

http://en.wikiped...velocity
Kinedryl
Apr 06, 2012
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Clifton_Richard
Apr 06, 2012
This comment has been removed by a moderator.
pokerdice1
not rated yet Apr 06, 2012
"One major challenge the experiment faced was the stability of the pump laser frequency. Glasser says they struggled with tweaking the laser to behave properly and be single-mode across the frequencies of interest. That was probably the most frustrating aspect." Additionally, at these frequencies, they were unable to find a commercially-available laser system that provided them with as much optical power as they would have liked."

Maybe this type of laser can be perfected, eventually, fo their purposes:

http://phys.org/n...ser.html

This would give them the power and more precise frquency control that they are looking for!?

Burnerjack
1 / 5 (5) Apr 06, 2012
Superluminal? You do know what happened to the last one the used that term, right? Going up against Master Albert can be injurious to both credibility and career.
rbrtwjohnson
3.7 / 5 (3) Apr 06, 2012
Conceptually, a group of phased waves correctly spaced-apart can produce FTL sequences. http://www.youtub...wyr5Udzw
vacuum-mechanics
1 / 5 (4) Apr 06, 2012
By the way, it is interesting to note that we are now discussing about light wave, while according to the conventional concept we do not understand what really it is, why its speed is c (constant) and how it has no rest mass! May be this paper could guide the way.

http://www.vacuum...id=20=en
Cynical1
not rated yet Apr 06, 2012
Maybe - since the actual photon cannot travel faster than light, what IS faster than light is the reference point at which you are observing the photon (ie - leading edge or trailing edge).
Oysteroid
1 / 5 (1) Apr 06, 2012
They're just shifting the phase. You still can't use this to transmit information faster than light. You should remember the concept of group velocity from Physics 101...


But have you noticed what the article says? It's this: "In the new work, the injected image traverses the cell faster than a reference image" -- So, unless it's been badly misreported, what they say is that they pass information faster than the reference speed of light.
Urgelt
1.7 / 5 (6) Apr 06, 2012
Boy, talk about self-contradictory statements.

If you can detect information faster than it can be delivered at the speed of light, then information is traveling faster than C.

But the researchers hasten to add that no information is actually being transmitted faster than C.

You can't have it both ways.
Bowler_4007
not rated yet Apr 07, 2012
I think both the article writers and readers (not all) need to learn more about science and how to talk about it without being ambiguous
Tennex
1 / 5 (3) Apr 07, 2012
I think both the article writers and readers (not all) need to learn more about science and how to talk about it without being ambiguous
The critics should learn, how to write comments in less fuzzy and ambiguous way too. What is ambiguous with some above post for example?

The whole subject is ambiguous from certain perspective. The relativity clearly says, the spreading of information with transverse light waves can be never superluminal. But does it apply, when the information is spreading with longitudinal waves? At the water surface definitely not, because the longitudinal waves tend to spread in much faster way at the water surface, than the transverse ripples and the vacuum shouldn't behave differently in dense aether model, being composed of smaller particles too.

After then we can put the question, whether the information spread with mutual interference of the waves coming from opposite directions is really the pure transverse way of information spreading.
Tennex
1 / 5 (2) Apr 07, 2012
Because the longitudinal waves are way faster, than the transverse waves, they do appear like random noise for every observer at the water surface, coming from all directions at the same moment. It's similar to spreading of sound along water surface during underwater nuclear explosion - the surface of water will be suddenly covered with noise uniformly emerging from nowhere in that case. Does this way of information spreading violate the causality of special relativity?

Yes and not. It apparently can serve for sending of information at distance with superluminal speed. But not, the connection between sender and receiver is not defined here. Because this information comes from many direction at the same moment, you can be never sure with its origin. And whole the character of this information is of indeterministic noise - it doesn't manifest like the harmonic light wave with well defined speed and phase.
Tennex
1 / 5 (2) Apr 07, 2012
The Gurther Nimtz has done experiments with tunnelling of photons between large glass prism at the decimetre distance. Because the light is bounced from inner walls of these prism, only evanescent waves can penetrate the gap and mediate the information across it. In dense aether model this experiment corresponds the spreading of ripples across swimming pool, the surface of which is divided into two halves with wooden board. The surface ripples cannot spread across this obstacle, so they're bouncing from it. But because the water is elastic and slightly compressible, a subtle portion of energy can travel after the board via underwater sound waves and reconstruct itself into transverse waves at the other side again.
This spreading will be faster, than the surface ripples - but we will lost the information about exact position of the original source in this way, because whole the surface of board will change into antenna. Such way of wave spreading therefore doesn't violate relativity.
Tennex
1 / 5 (2) Apr 07, 2012
The special relativity actually doesn't prohibit the motion of objects in superluminal speed - it just prohibits us in seeing them during it. For example, if you flash the laser spots at the different areas of Moon fast, you can say, these spots travelled across surface of Moon with superluminal speed - but we didn't see them during it. In this way even the superluminal neutrino will not violate the special relativity in strict sense, if it would do it with quantum oscillations: i.e. when it will disappear and emerge somewhere else. Because we didn't see the neutrino during this, such a jump actually doesn't violate the special relativity, no matter, how fast it really was. It's still strange, but the relativity has nothing to say about it, because you can be never sure, these two neutrinos are actually the same. This neutrino is actually recreated from quantum fluctuations during it. It's not the same neutrino, like the original one, it can even differ with spin from the original one.
Infinite Fractal Consciousness
not rated yet Apr 07, 2012
It's an optical illusion, not true super-luminous motion, but still very cool.
thingumbobesquire
1 / 5 (1) Apr 08, 2012
Paradoxically, the only "thing" which can travel faster than the speed of light is human thought! And human thought is in no way equivalent to that misguided and risible theory of "information."
Tennex
1 / 5 (2) Apr 08, 2012
Albert Einstein considered already, that the human stupidity may be bigger than the whole Universe: "Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.". If the Universe is expanding with speed of light and the human stupidity is still bigger, it would indicate clearly, that the human thoughts can be superluminal.

In this sense may be significant, that if human brains could be entangled in some version of telepathy, then their entanglement would appear superluminal too.
antialias_physorg
4.7 / 5 (3) Apr 08, 2012
I'm confused, are they making light travel faster than c?

No they are not. Pulse fronts faster than c are a well established phenomenon.
Phase velocities faster than c:
http://en.wikiped..._above_c
I hasten to add that this cannot be used for information transmission faster than light, since no photon in the pulse is moving faster than light.

Paradoxically, the only "thing" which can travel faster than the speed of light is human thought!

If you want to be that whimsical then the only thing that travels faster than light (except for bad news) is darkness. Since wherever light gets, darkness has gotten first.
Tennex
1 / 5 (2) Apr 08, 2012
If you want to be that whimsical then the only thing that travels faster than light (except for bad news) is darkness. Since wherever light gets, darkness has gotten first.
Such an insight has its robust physical reasoning too: the dark-state solitons (i.e. the solitons formed with longitudinal waves preferably, like the neutrinos) are considered superluminal in AWT in similar way, like the Falaco solitons are moving along water surface with the speed, which is slightly higher than the speed of surface waves. They're dark, because they're dumping surface ripples at the place, where they're residing.

Actually, no matter how big BS you'll try to invent, the AWT has included it already as a theorem - after all, this is, how the true TOE is supposed to behave...
antialias_physorg
5 / 5 (1) Apr 08, 2012
considered superluminal in AWT

Yes, and rainbows are considered to come out of unicorns' rear ends. Makes a lot more sense, too, than your 'theory'.
Tennex
1 / 5 (1) Apr 08, 2012
But Falaco solitons do exist, unicorns are extinct already (they could survive up to medieval times, though)..
Cynical1
5 / 5 (2) Apr 08, 2012
Paradoxically, the only "thing" which can travel faster than the speed of light is human thought! And human thought is in no way equivalent to that misguided and risible theory of "information."

Actually, since everything in our bodies runs on electricity, your statement is wrong.
The only thing faster is what we imagined/cogitated - at slightly less then speed of light (because most of us do not have a vacuum in our heads).
Tennex
1 / 5 (2) Apr 08, 2012
In aether model not everything electric is luminal. At the water surface every surface wave is followed with some underwater pressure wave, no matter how subtle it its. More detailed analysis of Maxwell's theory supports the existence of scalar waves, too. In linearized, EinsteinMaxwell theory on flat spacetime, an oscillating electric dipole is the source of a spin-2 field. Spin-2 field is effectively the graviton field, i.e. the longitudinal gravitational waves of vacuum in dense aether model. We even needn't spend too much money in its searching, because every common photon has its curvature of space assigned. In this sense every photon is behaving like the soliton of transverse and longitudinal waves, too. The radiowaves of CMBR frequency can be source of not only photons, but scalar waves too at the moment, when electric and magnetic vectors will compensate mutually, for example inside of Caduceus coil.
Tennex
1 / 5 (2) Apr 08, 2012
The interesting aspect of human brain is, it's represented with oriented motion of charged ions along complex neural network, so it could behave like the Caduceus coil too. It doesn't radiate EM waves at distance, but it could exchange the scalar waves. IMO the existence of scalar waves could explain many so-called psychic phenomena. These waves are doing vacuum more dense, they're essentially a flux of low energy neutrinos. And they're spreading in the faster speed, the less organized they're are.

These scalar waves manifest itself with CMBR noise, in this sense it's nothing very new. Because the contemporary physics is strongly oriented to strictly deterministic transverse waves of vacuum, i.e. the light and Lorentz symmetry, the predictions and study of the longitudinal scalar waves belongs into most important branch of dense aether model - it's sorta hidden dual part of contemporary science, because it's nearly completely indeterministic from perspective of formal physics.
DamienS
3.7 / 5 (3) Apr 08, 2012
have you noticed what the article says? It's this: "In the new work, the injected image traverses the cell faster than a reference image" -- So, unless it's been badly misreported, what they say is that they pass information faster than the reference speed of light.

It is confusingly put. Information transfer does not happen >c here, as already pointed out.

I take it to mean that an image was transmitted using both 'fast' light and 'slow' light pulses where the wave fronts have been appropriately adjusted. So a portion of the information packet arrives >c and a portion at c part and extract meaningful information without waiting for the remaining retarded pulse, which means overall the image information travelled at c.

The emphasis of the research is to improve signal quality in optical comms systems by speeding up or slowing down pulses, along with investigating QM correlations.
DamienS
5 / 5 (4) Apr 08, 2012
Sorry, last post garbled by use of less than sign...

"In the new work, the injected image traverses the cell faster than a reference image" So, unless it's been badly misreported, what they say is that they pass information faster than the reference speed of light

It is somewhat confusingly put. Information transfer does not happen FTL here, as already pointed out.

I take it to mean that an image was transmitted using both 'fast' light and 'slow' light pulses where the wave fronts have been appropriately adjusted. So a portion of the information packet arrives faster than c and a portion at slower than c, all within a single light pulse whose group velocity is still c. You cannot just use the faster than c part and extract meaningful information without waiting for the remaining pulse, which means overall the image travelled at c.

The emphasis of the research is to improve signal quality in optical communications systems by speeding up or slowing down pulses and QM research
ram420
not rated yet Apr 09, 2012
pl try to understand that you are seeing superimposition of waves, which itself is a wave! it appears to go faster than light but it cannot.
take 2 plastic scales and move them one over the other.the mm marks appear to move much faster but cannot move faster than light, as each molecule creating scale movement will be moving slower than light.
By no means you can make communication ftl using light itself.
rpaul2012
1 / 5 (3) Apr 09, 2012
How many federal dollars are wasted on research which may or may not produce worthwhile knowledge?!? How many federal salaries am I supporting?

The free market should decide what ideas are worth pursuing. No more social welfare for science that doesn't help the economy!

Stop draining my wallet!
antialias_physorg
5 / 5 (4) Apr 09, 2012
How many federal dollars are wasted on research which may or may not produce worthwhile knowledge?!?

Erm...you do know that the point of research is to find things out? If we knew in advance what worked and what didn't then that wouldn't be research (it would be development and/or engineering)

No more social welfare for science that doesn't help the economy!

Fundamental resaerch does eventually turn up in products. If you're too short sighted to see that then those products would not turn up at all (E.g. what do you think we would do without lasers? That was pretty basic research back in the day without immediately obvious applications. According to you such research should have never been funded)

Stop draining my wallet!

Now say this 100 times about the military THEN you may say it ONCE about science. Get your priorities straight!
Jotaf
5 / 5 (3) Apr 09, 2012
How many federal dollars are wasted on military spending which may or may not produce worthwhile security?!? How many federal salaries am I supporting?

The free market should decide what ideas are worth pursuing. No more social welfare for wars that don't help the economy!

Stop draining my wallet!
Eoprime
5 / 5 (1) Apr 10, 2012
Ripplippo:
Actually, no matter how big BS you'll try to invent, the AWT has included it already as a theorem


Erm, do you say: "doesn't matter how much bullshit you invent, AWT has it already included"
So its a BS-compilation? seems reasonable to me :)
Kinedryl
1 / 5 (1) Apr 10, 2012
The correspondence principle requires, every more advanced theory should contain the predictions of less advanced theories as its particular theorems (providing these theories are otherwise logical and formally correct, indeed).

For example, in AWT the space-time is modelled with water surface, which can be perceived from perspective of both transverse, both longitudinal waves of itself. These perspectives correspond the particular perspectives of relativity and quantum mechanic theories.

The trick is, the perspective of transverse waves is always orthogonal/singular to perspective of longitudinal waves, so it cannot be reconciled at the strictly rigorous level, only at the qualitative level. You're indeed not required to describe the water surface from perspective of its own waves only (which enables the AWT to become more general) - nevertheless, as you can see, it contains both the above theories as a two of its predictions - although these theories are mutually inconsistent.
Kinedryl
Apr 10, 2012
This comment has been removed by a moderator.