The power of photons

Why is light the core of our technology?

The QCi core technology is based upon using light as a versatile tool for many tasks including computing, communications, performing measurements, and generating random numbers. We will explain the specific techniques we use later on, but before we do, it is important to discuss why this is the approach we have chosen. While we all interact with light on a daily basis, we usually don’t think about the underlying physics of light. Light is made of electromagnetic fields which can oscillate and move through space, and is composed of particles known as photons. These particles have interesting quantum mechanical behaviors, but also many other important and useful properties, three very important properties being:

High bandwidth

One advantage of light is that it can carry a lot of information. For example, visible photons of light consist of oscillations that occur hundreds of trillions of times per second. Because these happen so fast, we can reliably distinguish between light with a slightly different frequency, since a small difference will add up quickly. These waves also have very short wavelengths, less than a micrometer, about the size of a bacterium. Even if the frequencies can’t be told apart, a lot of light particles can be placed in a small amount of space and still be told apart. This translates to being able to put a lot of information in a small space and time. Light also has other properties such as polarization, which corresponds to which direction fields in the wave are oscillating along, as well as some more exotic ones which are not important here. In contrast, electronic signals on the other hand typically only have an “on” or “off” state corresponding to two different voltages, and therefore carry much less information. These reasons are why light is often used in telecommunications, but they also make light great for computation and sensing. For sensing the advantage comes from the fact that each particle of light can carry a lot of information about the object we want to understand.

Low (but not too low) energy

A single particle of visible light also carries very little energy, this has two consequences. Firstly, if the photons are used efficiently, the power required to run the computer can be much, much lower than conventional computers. Energy economy is also valuable for sensors. Secondly, a more subtle consequence is that less heat will be produced in a device, making engineering easier. While the energy of a single photon is small, it is still much higher than the energy of thermal vibrations at room temperature, meaning that cooling is not required in either computers or sensors. We can think of this as being in a “Goldilocks” energy range, where the energy for each piece of information can be very low, but not so low that we need to protect it from background heat, which generally costs energy. In contrast, superconducting computers or sensors for example need to be cooled substantially, to protect them. In the case of superconducting qubits to a temperature which is one hundred times colder than deep space is required.

Fast processing

Light travels very fast, in fact from special relativity, information cannot travel faster than the “speed of light” in empty space, which corresponds to going around the Earth more than seven times in a second. Since the light in our devices travels in materials it goes a little slower, about 1/3 to 1/2 of the speed of light in empty space, but this is still very fast. This isn’t actually any faster than the speed at which electrical signals travel, which in a way isn’t that surprising, they are also electromagnetic waves, traveling through a metal wire. However, light still has the potential to process information much faster, the reason is that the speed at which the signals can move isn’t usually what limits the rate at which electronics can process information. The true limitations come from that switching a transistor moves a lot of electrons around and releases a lot of heat, and running too fast would create too much heat and damage the system. Light not only operates at a lower energy; it also dissipates very little energy as heat, so much faster processing is possible.

Other important properties

There are many other useful properties of light, too many to make an exhaustive list here. For example, the fact that the natural state of light is to be moving means that it is relatively easy to move light particles into contact with each other. From the perspective of computing this is important, as hard optimization problems usually require a high level of interaction and all-to-all connectivity greatly increases the power of hardware solvers for such problems. Also, communication and transfer of information is a key bottleneck in high-performance computing, while light information is constantly in motion.

A key development which allowed electronics to thrive was the ability to miniaturize the components, moving from large vacuum tubes to billions of transistors on a chip. We believe that the same ability is key for technology based on light. Fortunately, light can be made to move around in a material, using the same principles which guide waves in optical fibers. This means that we can also build chips which process light, with electronics integrated onto the same chips. While some of our devices may start out on a lab bench, they will eventually end up as compact and robust chip-based devices which can be used by anyone anywhere. Furthermore, there are many materials which can be used for non-linear optics. Our current platform is lithium niobate, because it provides the best optical and optoelectronic properties combined while still being compatible with chip manufacturing processes. As technology advances, new possibilities may become available, too.