Not content with shielding objects in space, physicists have come up with a new way of concealing events in time. The research involves punching a series of temporal holes into a stream of optical data at gigahertz frequencies using commercially available equipment, and could lead to applications in telecommunications and computing that involve hiding or dividing up information.
Spatial invisibility cloaks are shields made up of artificial “metamaterials” that bend light waves around an enclosed object as if neither the object nor the cloak were present – just as a stream of water would flow around a boulder. A number of such devices have been built and successfully demonstrated, and now physicists are turning their attention to “temporal cloaks”, which hide events during specific periods of time.
Slow down and speed up
The basic idea is to take a portion of a travelling wave, speed up the front half and at the same time slow down the back half, so creating a gap in time at a specific point in space during which the wave does not pass. By then slowing down the front and speeding up the back of the wave, any event taking place during the temporal gap would be invisible to a person receiving the wave – the wave arriving as an undisturbed signal of constant intensity with no trace of the event.
In 2012 Alexander Gaeta and colleagues at Cornell University in the US reported having built the world’s first temporal cloak. They did so by sending a beam of infrared light through a “time lens” that changes the colour of light as a function of time – the effect being to introduce a very sharp transition within the wave from blue to red. They then passed that light through an optical fibre, along which light of different wavelengths travels at different speeds, in order to introduce a time lag between the (faster) blue and (slower) red light, within which an event could be hidden. To restore the uniform wave and so cover up the evidence for such a lag, the light was then passed through a second fibre, which slowed down the blue and sped up the red light, and then through a second, opposing time lens.
The Cornell researchers showed that the signal associated with a light pulse from a second laser fired during the temporal gap was reduced by a factor of 10 as a result of introducing the gap, so demonstrating that their device could indeed cloak events. However, they were only able to do so at frequencies of up to tens of kilohertz, which is far below the gigahertz frequencies typical of today’s broadband data transmission. According to Joseph Lukens of Purdue University in the US, the limiting factor is the very high rates at which the time lenses need to be switched if the incoming wave is to experience the necessary, almost instantaneous, change in frequency.
In the latest work, Lukens and his Purdue colleagues Andrew Weiner and Daniel Leaird overcome this problem by using a temporal version of a phenomenon known as the Talbot effect. First observed in 1836, the Talbot effect is the repetition of the image of a diffraction grating beyond the plane of the grating when crossed by a plane wave. It is caused by interference among all of the wave’s diffracted components. The temporal version employed by the Purdue group involves sending a beam of infrared light through a “temporal” phase diffraction grating and then directing the resulting light along an optical fibre to disperse it. Lukens explains that the different frequencies “move through each other” and that their interference creates gaps in time. Crucially, no abrupt frequency change is needed in the incoming wave. “The hard work is done by the Talbot effect and not by constructing exotic time lenses,” he adds.
Using ordinary phase modulators and a single continuous-wave laser, Lukens and co-workers were able to produce time gaps at frequencies of over 10 gigabits a second, even if the length of each gap was slightly shorter than those of the Cornell group (36 as opposed to 50 picoseconds (10–12 s)). Lukens speculates that in future the military could use this technology to prevent eavesdroppers intercepting secret messages, the idea being that the interceptor would be unaware of any data transmitted during the temporal gaps. More realistically, he says, it might be used to avoid conflicts between different signals in an optical routing system or to ensure separation between distinct channels in high-bandwidth Internet connections.
The next major hurdle for researchers of cloaking devices is to make a “space–time cloak”, which would combine spatial and temporal capabilities in a single instrument. Such a device would allow events occurring in a particular volume of space and within a certain time interval to go unnoticed, and in theory might allow bank robbers to remain hidden from view while they remove the contents of a safe, even though they are under constant video surveillance. However, actually building a space–time cloak would be “very daunting”, according to Lukens, and he does not “see that happening any time soon”.
Martin McCall of Imperial College in London, whose group put forward the concept of the space–time cloak in 2011, agrees. Referring to its potential for use in optical routing systems, he says that the latest work “is certainly a step towards making that functionality a reality”. He adds that their “original space–time cloaking concept is unlikely to assist in the ultimate bank heist, but I do think it opens possibilities towards making current devices work better”.
The research is published in Nature.