Slashdot Mirror
Temporal Cloak Erases Data From History
ananyo writes "Electrical engineers have used lasers to create a cloak that can hide communications in a 'time hole', so that it seems as if they were never sent. The method is the first that can cloak data streams sent at the rapid rates typically seen in telecommunications systems. It opens the door to ultra-secure transmission schemes, and may also provide a way to better shield information from noise corruption (abstract). The researchers manipulated laser light in time to create regular periods with zero light intensity (a Talbot carpet) in which to hide data. Unfortunately, the current set up erases the data-adding event entirely from history. Though they are confident that future modifications will allow them, or others, to send secret messages successfully, the more immediate use of the technology will be to cut down crosstalk when multiple data streams share the same fibre." Also at Slash Datacenter.
Maybe it's just that it's late, but I have no idea what that summary was trying to say.
Isn't that what happens when you touch the crack in the universe caused by an exploding Tardis?
How does the recipient get the message, if it's so thoroughly erased?
Does this mean they've proven the past is mutable?
Yes. And I used to have already had told you that, but now I haven't yet not failed to have told you. That's why you now no longer know. You've now never known!
Here's proof that it works. I used it to cloak my First Post. Go ahead and check. You won't see it there.
When our name is on the back of your car, we're behind you all the way!
As far as I can tell, they are mostly just doing a twist on something that was known a long time ago: quadrature modulation.
The way color TV transmission worked in the past (not anymore, it's all digital now, but I digress) was that they crammed 3 signals in the space original meant for black-and-white TV by basically converting RGB into Y (an approximation of the Black and White signal), and two color difference signals. These color difference signals were modulated to a high alternating frequency pattern (so that old B & W sets wouldn't see them very much) and then put into quadrature with each other (each signal getting about 1/2 the spatial frequency bandwidth, and essentially interleaving them in time). In some sense in quadrature modulation, you are hiding one signal in the "nulls" you create in the other signal.
In this so-called "cloak" technique, the modulation is more complex and instead of trying to transmit two equal bandwidth signals together, they are exploiting the fact that their is no reason that the split has to be equal...
The below example is overly simplified single-split case, but illustrates what is going on...
Original: Signal ~ sin(wt+kx) ... Psignal ~ sin(wt+kx)*cos(p)
Phase Modulated signal with simple small split phase shift "p": Psignal ~ 0.5*sin(wt+kx+p)+0.5*sin(wt+kx-p)
And using the magic of trigonometry
If the transmitter controls the phase just right, the "cosine" modulates the original signal and creates periods of time where the amplitude is really low (not really zero except at a point), yet back to their nominal amplitude at the receiver. Since the transmitter know when these "nulls" will be, it can put in short bursts of another covert signals that looks nominally like the original signal (same base frequency), but won't really be visible at the receiver (presumably the transmitter would pick the phase parameters of this covert signal so that they were "null" at the receiver).
In practice, a single 50-50 split with complementary phase shift isn't really that great, you have to more harmonics approximate more interesting signal envelopes. Think about making a square wave out of harmonics and you can see how you might make the apparent "null" times much longer.
Thus to the outside observer it looks mostly like the original signal (same nominal frequency for symbols being transmitted since the phase shifts are small, it would just look like jitter), but the transmitter was able to finesse the transmission so that it could transmit relatively normally so that the receiver would still pick out the signal. During the "null" times, the transmitter could transmit a covert signal which isn't picked up by the normal receiver but looks like a plausible innocuous signal, so a simple cursory observation of the channel looks as if only generic transmission is going on.
We don't have to call this stuff using descriptions like temporal cloaks and erasing information from history. Except perhaps analog color tv transmission (may it RIP)