Pentagon-backed 'time cloak' stops the clock (Update)

January 4, 2012 By Bill Steele
In this 2011 illustration, provided by Cornell University, scientists demonstrate how they have have created, a new invisibility technique that doesn’t just cloak an object _ like in Harry Potter books and movies _ but masks an entire event. It is a time masker that works by briefly bending the speed of light around an event. Cornell scientists explain what they are talking about in this 2011 illustration that shows that if this technique is ever scaled up an art thief can walk into a museum and steal a painting without setting off laser beam alarms or even showing up on surveillance cameras or your eyes. (AP Photo/Heather Deal, Cornell University)

Pentagon-supported physicists on Wednesday said they had devised a "time cloak" that briefly makes an event undetectable.

In movie magic, people and objects can appear or disappear or move from place to place in an instant. Just stop the camera, move things around and start it again. Now, Cornell researchers have demonstrated a similar "temporal cloak" -- albeit on a very small scale -- in the transport of information by a beam of light.

The trick is to create a gap in the beam of light, have the hidden event occur as the gap goes by and then stitch the beam back together. Alexander Gaeta, professor of applied and engineering physics, and colleagues report their work in the Jan. 5 issue of the journal Nature.

Pentagon-backed 'time cloak' stops the clock (Update)
A laser beam passes through a "split-time lens" - a specially designed waveguide that bumps up the wavelength for a while then suddenly bumps it down. The signal then passes through a filter that slows down the higher-wavelength part of the signal, creating a gap in which the cloaked event takes place. A second filter works in the opposite way from the first, letting the lower wavelength catch up, and a final split-time lens brings the beam back to the original wavelength, leaving no trace of what happened during the gap. Image: Gaeta lab

The researchers created what they call a time lens, which can manipulate and focus signals in time, analogous to the way a glass lens focuses light in space. They use a technique called four-wave mixing, in which two beams of light, a "signal" and a "pump," are sent together through an optical fiber. The two beams interact and change the wavelength of the signal. To begin creating a time gap, the researchers first bump the wavelength of the signal up, then by flipping the wavelength of the pump beam, bump it down.

The beam then passes through another, very long, stretch of optical fiber. Light passing through a transparent material is slowed down just a bit, and how much it is slowed varies with the wavelength. So the lower wavelength pulls ahead of the higher, leaving a gap, like the hare pulling ahead of the tortoise. During the gap the experimenters introduced a brief flash of light at a still higher wavelength that would cause a glitch in the beam coming out the other end.

Then the split beam passes through more optical fiber with a different composition, engineered to slow lower wavelengths more than higher. The higher wavelength signal now catches up with the lower, closing the gap. The hare is plodding through mud, but the tortoise is good at that and catches up. Finally, another four-wave mixer brings both parts back to the original wavelength, and the beam emerges with no trace that there ever was a gap, and no evidence of the intruding signal.

None of this will let you steal the crown jewels without anyone noticing. The gap created in the experiment was 15 picoseconds long, and might be increased up to 10 nanoseconds, Gaeta said. But the technique could have applications in fiber-optic data transmission and data processing, he added. For example, it might allow inserting an emergency signal without interrupting the main data stream, or multitasking operations in a photonic computer, where light beams on a chip replace wires.

Pentagon-backed 'time cloak' stops the clock (Update)
Time cloaking doesn't involve a DeLorean, just a kilometer of optical fiber coiled up on a lab table, supervised by postdoc Moti Fridman and research associate Yoshi Okawachi.

The experiment was inspired, Gaeta said, by a theoretical proposal for a space-time cloak or "history editor" published by Martin McCall, professor of physics at Imperial College in London, in the Journal of Optics in November 2010.

"But his method required an optical response from a material that does not exist," Gaeta said. "Now we've done it in one spatial dimension. Extending it to two [that is, hiding a moment in an entire scene] is not out of the realm of possibility. All advances have to start from somewhere."

Fridman's work was part-supported by the Defense Advanced Research Project Agency, or DARPA, a Pentagon unit which develops futuristic technology that can have a military use. Its achievements include DARPANet, a predecessor of the Internet.

Explore further: Researchers create terahertz invisibility cloak

More information: Nature journal announcement:

Now you detect it, now you don’t (pp 62-65; N&V)

A ‘time cloak’ that makes an event temporarily undetectable, albeit on the picosecond scale, is described in this week’s Nature. The work could represent a step towards the development of spatio-temporal cloaking.

Recent developments in spatial cloaking show that it is possible to hide an object by manipulating electromagnetic waves around it, creating a ‘hole in space’. Such devices currently have limited functionality. Here Moti Fridman and colleagues demonstrate that a related effect, temporal cloaking, can be achieved. They manage to create a ‘hole in time’ for around 40 trillionths of a second (40 picoseconds).

The fibre-based system steers light ‘around’ an event so that no evidence (a change in the temporal or spectral properties of the light beam) of the event is detectable, by speeding up and slowing down different parts of a light beam. This effect is achieved using a split time-lens that breaks light up into its slower (red) and faster (blue) components, thereby creating a tiny temporal gap.

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4.6 / 5 (10) Jan 04, 2012
They took a signal, sent it down a fiber optic cable, split the signal, slowed or speed half, re-joined them, ..that's " cloaking an event in time "..?

..I know I'm pretty dense sometimes, but I don't get it ?
5 / 5 (15) Jan 04, 2012
The "time cloak" label is misleading. This is a pure optical illusion - nothing else. Sure, photon speeds have been adjusted but, other than using time as a measuring stick for the delay, no true "time cloak" exists. A more accurate title for this would be "Light Cloak"
1 / 5 (1) Jan 04, 2012
Time to start perfecting my sleight of hand routines.
3.7 / 5 (14) Jan 04, 2012
This hype has been publisized before.

It has nothing to do with cloaking "time". As the article states, they stopped light from reaching cameras. So this is the same type of light controlling cloak as nearly any other previously proposed.

PhysOrg journalism seems to sometimes be akin to Yahoo journalism.
4.5 / 5 (15) Jan 04, 2012
This article almost made me vomit. It's full of useless analogies intended for idiots, the problem is idiots won't realize they are analogies.

For example, these physicists did not "erase a split second of history"...
3 / 5 (8) Jan 04, 2012
Really all they did was prevent light from reaching a specific point of observation... big deal, I can do that with a curtain, they just did it in a very elaborate way.


They did not change the speed of photons. Photons travel at C. The rate of propagation of the wavefront along a linear trajectory is affected by the properties of the medium. In this case the photons do not take a linear path even though the wavefront does, so the photons can move at a constant velocity (C) even though the wavefront is not propagating at that velocity.

Think of an object on a zig-zag path. The object alternates between two different trajectories, but if you average those two trajectories you get a single trajectory of travel. The objects true velocity will be greater along the two zig-zag trajectories than it will be along the single averaged trajectory. That is the only way to "slow light", to make the photons take an indirect path compared to the average path of the wave.
5 / 5 (1) Jan 04, 2012
Deathclock (interesting handle BTW) - understood. I was typing irritated and that slipped by me before I knew I did it. Must have been a mental cloak or something...:^)
5 / 5 (5) Jan 04, 2012
It works by "bending the speed of light"??? Really? Did I just read that? Bending a speed? Is that like a Beckham?

It's important to point out that this experiment did do something more than just block light like a curtain - it relinked up light from events before the blocked event and after, so that the information received outside was as if that time didn't occur at all. Which is a little more interesting.

Horrid article though.
3 / 5 (2) Jan 04, 2012
Since wild over the top analogies seem fair game in this article I say, its like farting but pinching your nose so you wont smell it, but still i can hear it!
5 / 5 (2) Jan 04, 2012
It has nothing to do with cloaking "time". As the article states, they stopped light from reaching cameras. So this is the same type of light controlling cloak as nearly any other previously proposed.

Firtsly I agree the artcile was poorly written but I think you've missed the point here. At no stage did they stop any light from reaching its destination. What they did was to increase the interval of time between two photons of light without any perceptible change to the receiver. From the viewpoint of the person recieving the signal, nothing has happened. But at the point of the cloak there has been a little "hole" of time created. (hole is a bad word for it, but without anything better I'll use it).

Think of it this way:

Normally photon 1 and photon 2 have a time interval between them of X. At the point of the cloak the scintists have increased this to X Y. Yet the photons are still recieved at the detector with a time interval between them of X.
4 / 5 (4) Jan 04, 2012
It must be that stolen technology from the aliens hiding on the moon.
2.6 / 5 (5) Jan 04, 2012
If I throw a bag over your head and then remove it a minute later guess what I just did the same thing.
1 / 5 (3) Jan 04, 2012
Can't wait for Tom Cruise's "Mission Impossible 4" ...
With 230million in '3's first week, the incentive is there!!
Not to construct the plot but the gimmick!
1 / 5 (3) Jan 04, 2012
...Photons travel at C.

In a vacuum.
The rate of propagation of the wavefront along a linear trajectory is affected by the properties of the medium.

Umm. Like big words do we? The speed of light is dependent on the medium.

You all need to pay attention to the authors of articles. This one is sourced from the Associated Press. Physorg is just running it, the same as CNN, Wired, NYTimes, etc.

tkjtkj, Ghost Protocol is MI4. I'd be surprised if they let the franchise sleep another 5 years before MI5 comes out.
3 / 5 (2) Jan 04, 2012
Great piece of work but the reportage of "Another way to think of it is as if scientists edited or erased a split second of history" is totally misleading.
1 / 5 (3) Jan 04, 2012
tkjtkj, Ghost Protocol is MI4. I'd be surprised if they let the franchise sleep another 5 years before MI5 comes out.

ahh..tnx for correction! I had fun at 'ghost' so apparently lost track of episodes!
Also,supporting your conjecture, the studio's are under the 'pressure' of a thing called 'the ageing process' .. Cruise already is showing the years .. not so badly as did Sean in his last episode: a ninja would dance around him like Mohammed Ali, stinging like a 1000 bee's.. So let's hope its less than 5 yrs!!

1 / 5 (2) Jan 04, 2012
If I throw a bag over your head and then remove it a minute later guess what I just did the same thing.

No you didn't. Thats not even the same thing.

To use your analogy of the bag over the head thing, lets say you had a "Time Cloak" which could block out events for over a minute. You could engage your cloak and put a bag over my head. You could then remove the bag from my head after a minute and disengage the cloak. From your point of view you see me standing there for a minute with a bag on my head. From my point of view I would never have seen the bag at all.

Of course, that raises all sorts of strange questions, such as would I have felt a bag on my head for a minute without being able to see it etc?
2.3 / 5 (3) Jan 04, 2012
Can I use this to manipulate the stock market?
2 / 5 (8) Jan 04, 2012
...Photons travel at C.

In a vacuum.


The rate of propagation of the wavefront along a linear trajectory is affected by the properties of the medium.

Umm. Like big words do we? The speed of light is dependent on the medium.

No, the speed of the wavefront of light is dependent on the medium due to varying degrees of absorption and re-transmission of the photons. The photons always travel at C.
4 / 5 (2) Jan 04, 2012
...Photons travel at C.
In a vacuum.


Nope. Light doesn't need to be absorbed and re-emitted to be slower than C.

The speed of a photon is 1/sqrt(epsilon*mu)
where epsilon is the electric permittivity
and mu is the magnetic permeability

C is the special case for permittivity and permeability of free space (vacuum)
1.5 / 5 (8) Jan 04, 2012
This would not stop scalar waves, which are detectable with non-Western equipment.
3 / 5 (2) Jan 04, 2012
Okay, so to continue with the silly analogy-- If the museum had pressure sensors in the floor, would they not still trip the alarm? To bystanders it might seem as if the doors all slammed shut and red lights started flashing for no 'apparent' reason, but the thief would be caught, just the same. Am I missing something obvious, or just stating something obvious?
1 / 5 (3) Jan 05, 2012
If you have a steady stream of anything and you slow down a part of the stream and then let it go there is an absence of anything downstream WHEN it is slowed. Later when let go there is a downstream abundance.

Unless your cloak can speed up light you can ride the middle of a disruption in your magical cloak but there will always be a leading and trailing edge which signals your deception.
1 / 5 (4) Jan 05, 2012
Actually Deathclock, the article is misleading by a long way... or is Zed123 misleading? realworld or digital?

One can in fact hide digital events for a long period of time; it would simple mean a delay at the receiver's end by a minimum of at least the event period.

send data,
slow data to enable data splitting.
split data along two paths
send trailing data into a loop/or slowgo
insert event after data lead.
when event's finish,
slow lead and event to separate down two paths
loop/or slowgo the lead data recalibrate with trail' data
send reunited data onward to receiver.

limited to hiding digital data though.

Real world lensing? now thats another story.
2.3 / 5 (3) Jan 05, 2012
Welllll... If it gets perfected, That is so awesome its creepy!
4 / 5 (5) Jan 05, 2012
This 'event cloaking' thing happens to me whenever I have one drink too many.
4.3 / 5 (3) Jan 05, 2012
"This is all happening in beams of light that move too fast for the human eye to see."

Got to be the most ridiculous statement ever
3.7 / 5 (3) Jan 05, 2012
Got to be the most ridiculous statement ever

Don't read the comments much, do you? :D
2.4 / 5 (5) Jan 05, 2012
Nope. Light doesn't need to be absorbed and re-emitted to be slower than C.

The speed of a photon is 1/sqrt(epsilon*mu)
where epsilon is the electric permittivity
and mu is the magnetic permeability

C is the special case for permittivity and permeability of free space (vacuum)

One minute you're talking about "light", the next "photons"... let's try to keep the terminology straight please. Light can refer to the wavefront itself.

Photons are mass-less, they must always travel at c as is required by relativity.

Read this:

"Photons may be viewed as always traveling at c, even in matter, but they have their phase shifted (delayed or advanced) upon interaction with atomic scatters: this modifies their wavelength and momentum, but not speed. A light wave made up of these photons does travel slower than the speed of light."

I work in optics/photonics...
1 / 5 (1) Jan 05, 2012
So you take half the light and send it through a long fiber optic cable and reintroduce it later. Is it only me who thinks things will just get dim and then return to full brightness? Or at best two images of the same thing, only one delayed?
1 / 5 (2) Jan 06, 2012
St Andrews did this years ago: http://iopscience...lltext/. They weren't the first, either. But for both journalists, both scientists involved it's both advantageous, both simpler to pretend so. For journalists their story will attract the traffic done with laymans, who are of short memory and for scientists is increases the chance for additional grants. And no one is bothered with review of previous results - in this way, the same research is payed twice with ignorant publics.
2 / 5 (4) Jan 06, 2012
Another links pointing to the same research http://www.techno...v/26992/ The event they are talking about is a highly nonlinear interaction of two light beams that results in what's called 4-wave mixing.

In this experiment, the detection of frequency at 1539nm, caused by the interaction of the two beams within the medium, is what they are defining as the "event." thus by introducing a "chirp" in the pump beam (introducing a phase shift using self focusing and splitting the beam into a red and blue shifted component and thus creating a temporal gap between the red and blue components) and then doing a reverse "chirp" they can create a gap in the pump beam. this is basically the cloaking mechanism. so when there's a gap in the pump beam, you can effectively "hide" the probe beam in that gap, producing no 4-wave mixing and hiding the event
1 / 5 (2) Jan 06, 2012
The sensationalist part of this article is, this is not a phenomenon that happens at normal day to day light intensities. It occurs in material environment, whose refraction index depends on the light intensity. The vacuum refractive index doesn't depend on the intensity of light, so that the radiative time arrow defined with light spreading in vacuum has not been affected. This "cloaking of events" is still quite normal temporal phenomena, when being observed from perspective of light spreading in vacuum. It's rather journalistic description of light induced change in refractive index, which is essentially known for years.
1 / 5 (1) Jan 07, 2012
I was having a lot of trouble figuring it out, but the old tourtise and hare analogy really just pulled it all together for me. Thanks for that.
1 / 5 (1) Jan 08, 2012
Oh man my brain hurts. Normally I can grasp even something about an article, even if most of it goes way beyond my skills, but this time... nothing. I have ZERO idea how this thing really works, like nothing, absolutely nothing. They bend light, and it makes a "time hole", great, nice. Oh well, I guess it's interesting still, atleast from a journalistic point of view.
1 / 5 (2) Jan 08, 2012
The key information is the notion of Pentagon here. Why Pentagon should be engages in such a research? Because its the subject, who is motivated in studying of extra-fast phenomena, like the explosions of nuclear bombs. These bombs are routinely simulated at NIF, a power laser base, where the ultrafast laser pulses are analyzed. For their study a technology of "time microscope" has been developed before some time and this study is just using a pair of "time lens" (convex and concave) for the isolation of fast laser pulse from observation, i.e. it's a byproduct of this military oriented research.
1 / 5 (3) Jan 08, 2012
The "time cloak" is closely related to the metamaterial concept, i.e. the optical system composed of mixture of convex and concave lens of foamy geometry.


What these guys are essentially did is, they passed the light through artificial unidimensional metamaterial foam, which is representing space-time foam in Universe at extreme distance/energy density scales.

1 / 5 (2) Jan 08, 2012
So that the sequence of high level logics required for conceptual understanding of this research with laymans should appear something like this:
1) The space cloaks are using spatial metamaterials, the time cloak will use the space-time metamateials
2) The metamaterials are composed of serial system of convex and concave lens, the time cloak will therefore use a serial system of convex and concave time lens
3) The convex and concave time lens were developed for time microscope, enabling to expand the signals from ultrafast phenomena in time
4) The time microscope has been developed for analyse of laser pulses at NIF base, which is dedicated to military research from 80%
5) The military research is usually sponsored with Pentagon, which explains its notion in the above headline

Got it? It's simple, my dear Watson.
1 / 5 (2) Jan 08, 2012
This high level conceptual logics is necessary for understanding of WHY the people are doing some things. But it still doesn't explain, HOW the people are doing it, i.e. WHY they're using just the WAY, which they've just used. Well, let say, we have a bright laser flash, whose intensity is a complex function of time and we need to determine, how this function looks like. But this flash is too brief for to be recorded and analyzed with common digital devices. So we should slow down it somehow in time in analogous way, so we could use the common digital equipment for it. We should simply apply the time lens to this flash. It's analogical to the replay of ultrasound click of bats with slower speed, so we could analyze it in audible spectrum.

One of methods is the four-wave mixing, which is based into superposition of pulse with higher and lower harmonics of signal, which are susceptible to subsequent dispersion and vanishing during their travel through long path of dispersive environment
1 / 5 (2) Jan 08, 2012
It's equivalent to Fourier decomposition of signal into its spectral density function in real time. It's usually done with passing of pulse through nonlinear optical material, whose refractive index depends on the light intensity (although another methods can be used too). If we mix the light pulse observed with the strong light pulse from independent source, then the refractive index of environment will change fast during light pulse, which will lead into its decomposition into spectral function in time.

The spectral function signal will travel trough dispersive environment (realized with optical fiber), which will spread them in time. Now we can apply the same wave mixing to the spectral function signal again in the opposite way - so we will restore the original signal, just expanded in time. We just realized the time lens.
Elwood 812
1 / 5 (1) Jan 08, 2012
The possibility of injecting a hidden data packet or two into that gap is intriguing. You could cloak your key your way inside of a operating system....come out with your booty. On a physical would feel the bag on your skin and the floor sensor would get you. You would have to have the second and third dimension also covered to be a true time completely be an invisible event.
2 / 5 (4) Jan 09, 2012
OK, as a reader who doesn't possess a PhD in physics, I'd appreciate it if PhysOrg didn't try to trick me into reading articles I won't understand by sensationalizing the titles and filling them with retarded analogies.
1.3 / 5 (3) Jan 09, 2012
PO just republishes the Cornell university story like many other news servers. You would be more effective, if you would contact the PR manager of this university directly.

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