(a) Conventional spatial cloak based on a planar transformation (x, y) —> (x', y'). An observer located to the far right on the x axis does not see the object. (b) Spacetime cloak. An analogous coordinate transformation is used, but now in (x, t) rather than (x, y). The cloak now conceals events near the spacetime origin. The (schematic) intensity distribution for various times is shown on the right, indicating the formation and subsequent evaporation of the intensity null that is fully developed at t = 0. The observer to the right never suspects the occurrence of any non-radiating events near the spacetime origin and sees a uniform intensity for all time. Superluminal propagation (cf. points A and B in the inset) can be avoided in more careful designs.1
Martin McCall’s group at Imperial College have extended the idea of cloaking objects in space to hiding events in spacetime.1,2 The idea is a mathematical extension of the concept of a spatial electromagnetic cloak in which media are designed to caress light around an object, rendering it invisible to a distant observer.3,4 In the new scheme, one spatial variable is replaced by time so that different portions of a ray are sped up and slowed down such that certain events are never illuminated. On reversing the process, the illuminating light is restored to its original uniform condition so that a distant observer will never suspect the occurrence of the un-illuminated events.1
The design of both types of cloaks relies on the principle that Maxwell’s equations appear the same after coordinate deformations. When one reinterprets such deformations as a change of material parameters rather than coordinates, the deformations can be actualized in physical space. By varying the magnetic and electrical responses in step, one can “fool” light into behaving as if the vacuum has been curved and hence no longer travels in straight lines. For the event cloak, the coordinate deformations must embrace time as well as space, and for this, the Imperial group turned to manipulating Maxwell’s equations in their so-called covariant form in which time and space appear symmetrically.
The best event cloak solution turned out to be a design in which the material is engineered to appear as though it is moving. When this effective speed varies for different points on the ray, as well as differing with time, the prescribed light-speed modulation can be achieved. Although such a design is beyond current metamaterial technology, we proposed a method to produce a simple event cloak using programmable nonlinearity in optical fibers. We estimated that a concealment of 3 ns using 100 m of customized optical fiber was possible.
Inspired by Imperial’s theoretical paper, a recent arXiv report filed by Alex Gaeta’s group at Cornell claims an experimental demonstration of a spacetime cloak.5 Instead of the original suggestion of directly accelerating/decelerating light, they used so-called split-time lensing, which combines tailored chirping with dispersive propagation through a fiber. Using this technique, they cloaked an interval of 12 ps.
The applications of event cloaking are likely to focus on optical signal processing, which occurs on precisely the short timescales that are achievable by experimental cloaks. One possibility is the resolution of processing conflicts in which a primary channel, carrying for example a clock signal, converges at a node with a channel carrying occasional priority data. Applying the event cloak to the clock channel allows an “interrupt-without-interrupt” functionality in which, for the external circuit timing, processing on the priority channel is carried out instantaneously. The event cloak concept has opened up a significant new cloaking paradigm that is only just beginning to be explored.
Martin McCall and Paul Kinsler are with the physics department at the Imperial College London, United Kingdom.
References and Resources
1. M.W. McCall et al. J. Opt. 13, 024003 (2011).
2. M.W. McCall and P. Kinsler. Physics World, Special Issue on Invisibility, 24, July 2011.
3. U. Leonhardt and T.G. Philbin. New J. Phys. 8, 247 (2006).
4. J.B. Pendry et al. Science 312, 1780-2 (2006).
5. M. Fridman et al. “Demonstration of temporal cloaking,” arXiv:1107.2062v1 (2011).