October 25, 2021

I, Science

The science magazine of Imperial College

'Time cloaks' not only sound cool, but also offer exciting new technological possibilities...

At just under 300 million meters per second, the speed of light in a vacuum is one of the most important numerical constants of physics. In everyday life however, the speed of light is actually less than this and depends on what it’s travelling through, whether that’s air, water, glass, or something else. The speed of light is therefore defined by a thing called the refractive index, a unitless number that describes how electromagnetic radiation (light) propagates through a medium. Different media have different refractive indices, for example air is n=1 and water n=1.33. A larger refractive index means the light moves slower and experiences more refraction (a change in direction) – this is why if you put an object such as a spoon or straw in a cup of water it will appear to bend at the boundary between the air and the water, and why a glass prism splits white light into a rainbow.

Different colours of light travel at different speeds depending on their wavelength/frequency, a phenomenon of optical physics called dispersion. Dispersive media, materials in which this occurs, can therefore be used to control the movement of light.

Physicists have now extended the theory of spatial cloaks and started researching temporal cloaks, devices that control electromagnetic waves not just in space but also in time. Researchers at Cornell University recently demonstrated a working device dubbed a ‘time lens’ that was able to hide an event from detection, building on theoretical work by a team at Imperial led by Martin McCall. The work was published in Nature.

Consider a simple scientific experiment, where a laser beam is projected in a straight line to a detector. If anything is placed in the path, thereby breaking the continuous beam, the detector will easily notice this has happened. This process is called an ‘event’.

The researchers developed a device that splits a beam of light into a spectrum of wavelengths and then speeds up and slows down different parts. The first optical fibre increases the frequency of the laser beam, towards the blue end of the spectrum, and then decreases it to the opposite red end. The higher frequency bluer part travels faster than the lower frequency redder part, creating a gap in the beam. The beam then moves, with the gap maintained, out and along to the next fibre. The second fibre slows down the first part of the beam, allowing the second part to catch up and close the gap. The laser beam reaches the detector exactly as it left the emitter, with the manipulation of the beam undetectable.

This setup allows for an object to move through the laser beam, without the detector registering that has happened. So the temporal devices have been able to cloak an event from detection. Unfortunately the device was only able to achieve a gap of 50 picoseconds. That’s 50 trillionths of a second (0.000000000050 seconds). It should be possible to increase that time slightly, perhaps up to a few nanoseconds (billionths of a second), but scattering and dispersion effects will make this a big challenge. Considering the current setup, to hide 1 second of time from detection would require a device with emitter and detector about the same distance apart as the Earth from the Sun. Also, the device needs to be developed into 3D to allow cloaking from all directions; trickier to do than the current 2D experiment.

The practical demonstration of concealing events from detection has profound implications for future science and technology. Using time lenses, data can be manipulated and then restored, so the applications of temporal cloaking go beyond simply hiding single events; the process could be used to allow the insertion of something into a continuous beam without causing disruption. This phenomenon could be incorporated into optical devices, with multiple interlaced – but non-interacting – beams allowing much faster data processing and the development of quantum computing. Whilst time cloaks are still very much in their infancy, they promise to have significant involvement in future and allow us to have a better understanding of the spacetime around us.

IMAGE: JSmith Photo, flickr.

DIAGRAMS: James Keen.