November 29, 2016

Progress to overcoming the last obstacle to the creation of all optical computers

Recent developments in nanoscience have led to dramatic decreases in computer components size. The inherent property of such small-size systems is the impossibility of separation of the environment from the system under consideration. For this reason, the investigation of open and, in particular, non-Hermitian systems has been among the main topics of physics over the last decade.

Recent work solves much of the loss problem with photonics. Optical signals travel much faster than electrons — at the speed of light — and are not subject to "resistance." Scientists have already created all the major components needed to create the ultimate all optical computer. nfortunately, the waveguides down which optical signals travel in a photonic computer introduce loses much like the resistance against electrons in copper wires.

Unfortunately, when traversing the twists and turns of photonic circuits, they do loose "intensity," just as less than 100% of light is reflected from a mirror. If this problem was solved it would go long way to realizing the all-optical computer of the future without the need for amplifier or superconductivity.

All-optical computers using photons traveling at the speed-of-light in theory could make the electronics we know today obsolete. All the subsystems are in place, but one key obstacle remained — optical losses. Now the Moscow Institute of Physics and Technology (MIPT) claims it may have cleared that last hurdle.

According to the journal, Nature, a new method that can compensate for losses just by carefully designing dual waveguides to match the wavelength of the light traveling through them. By doing so, the traveling waves can reinforce each other along the way, thus introducing a slight gain that compensates for the normal losses.

“We have learned how to amplify optical waves by periodically changing the distance between their waveguides. Thus by merely configuring their flow of energy we can compensate for the normal losses in a waveguide resulting in a net gain,” professor Alexander Pukhov, a senior researcher at MIPT’s Laboratory of Quantum Information Theory

They demonstrated that when the system is at the exceptional point, any perturbation that changes the parameters of the system leads to increasing eigenmode amplitudes. As a result, the system becomes unstable with respect to such perturbation. This phenomenon is a new effect in optical non-Hermitian systems: parametric instability near the exceptional point (PIEP).

Changing the coupling constant leads to an increase in total power limited only by nonlinear effects. Moreover, we show that the transmission coefficient of such waveguides is larger than in a system with constant parameters

The phenomenon of PIEP may be used in metamaterial, plasmonic, and nanooptic devices whose applicability is substantially restricted by losses. It opens a wide range of applications in optics, plasmonics, and optoelectronics, in which loss is an inevitable problem and plays a crucial role.

While still in the design stage, MIPT's next step is to prove the concept in the laboratory. After optimizing the effect, the last major obstacle to the era of photonic computers will be removed.

Dependency of signal intensity (solid line) and field amplitude (dashed line) depends on the coordinates along in the first and second waveguides.
(Source: MIST)


Nature Scientific Reports - Parametric instability of optical non-Hermitian systems near the exceptional point

In contrast to Hermitian systems, the modes of non-Hermitian systems are generally nonorthogonal. As a result, the power of the system signal depends not only on the mode amplitudes but also on the phase shift between them. In this work, we show that it is possible to increase the mode amplitudes without increasing the power of the signal. Moreover, we demonstrate that when the system is at the exceptional point, any infinitesimally small change in the system parameters increases the mode amplitudes. As a result, the system becomes unstable with respect to such perturbation. We show such instability by using the example of two coupled waveguides in which loss prevails over gain and all modes are decaying. This phenomenon enables compensation for losses in dissipative systems and opens a wide range of applications in optics, plasmonics, and optoelectronics, in which loss is an inevitable problem and plays a crucial role.



SOURCES - Nature Scientific Reports, Moscow Institute of Physics and Technology, EEtimes

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