Exploring Clean Technology

Erbium may pave the way to a ‘quantum internet’

February 7, 2018

We all generally aware that yttrium (element 39) and everything from lanthanum (element 57) to lutetium (element 71) on the Periodic Table - otherwise referred to as the rare earths elements or REEs - are vital for the advanced technologies that we use every day. REEs uses are wide ranging…for example: neodymium alloys make the strongest magnets used in everything from audio speakers to electric motors; cerium is used in catalytic converters that reduce harmful emissions in car exhausts; and lanthanum is needed in rechargeable nickel metal hydride batteries.

Crystals containing rare earth ions absorb and emit light at a variety of useful wavelengths in the ultraviolet, visible and infrared ranges of the spectrum, and as such REEs are commonly used in lighting. Phosphors containing europium, terbium and cerium are used to create the red, green and blue pixels that make up full-colour plasma screen displays.

One rare earth element, erbium (element 68), has now been shown to play a crucial part in the future quantum internet. Okay, the Quantum Internet?? How about a modest step back then...

As published by Rose Ahlefeldt and her research colleagues at the Australian National University in Nature Physics and her piece in The Conversation (November 6, 2017), researchers are developing quantum computers “which make use of a unique aspect of quantum physics - quantum superposition, where particles can simultaneously exist in two different states - to encode information.” This is compared to current-day computers, which the internet allows to ‘talk’ to each other. According to another commentator, Alasdair Wilkins, “the quantum computer has the potential to revolutionize the 21st century, solving problems so massively complex they would baffle today’s computers. But a single quantum computer is still no match for the more than billion classical computers currently in use.”

We will need a new type of network that can maintain this quantum information, hence the evolving term – “a quantum internet.”

Ahlefeldt notes that “to make the quantum internet, we need to build the quantum analogues of each element in the classical internet. Most global internet traffic travels as light in optical fibres. This allows fast transmission with very low losses at the right wavelength.” (This is understood to be around 1,500-1,600 nanometres; one nanometre being 10-9 metre).

Even with very low loss rate, light leaking out of the fibre is an issue over long distances, meaning the light has to be intermittently amplified. Since erbium absorbs and emits light at 1,550 nanometres (exactly in the middle of the fibre telecom band), it can be used to amplify the light in a device called an Erbium-Doped Fibre Amplifier (EDFA).

Erbium-doped fiber with green light The undersea optical fibres that form the backbone of the internet have EDFAs at approximately every 80 km. The quantum analogue of the EDFAs used as amplifiers in our current undersea optical fibre systems are called quantum repeaters, which require quantum memory, used to store and synchronise information traffic in the network.

Researchers around the world have been working on quantum memories for over a decade, but storing quantum information for even 1/1,000 of a second is challenging.  Ahlefeldt argues that these systems need storage times of at least 1/10 second for the quantum internet. Since erbium interacts with light at exactly the right wavelength, Ahlefeldt has shown that it may be the obvious choice for quantum memory.

There is a significant ‘However’ - erbium is poor at storing quantum information, as it seems to be sensitive to the tiny magnetic field fluctuations that occur in crystals. This rapidly degrades any quantum information it holds. Recently, Ahlefeldt and her team have found that applying a large magnetic field, similar to that inside a hospital MRI machine, quiets the magnetic field fluctuations, thus greatly improving the quantum storage time of certain erbium crystals (e.g. by a factor of 10,000 to more than 1 second). Think about it… bytes traveling at the speed of light need less than a second to circle the globe. 1.3 seconds is already long enough make a global quantum internet feasible.

Using erbium-treated crystals may remove the requirement to build an entirely new network, as the researchers say quantum information transmitted by using such crystals could be integrated with the Earth’s existing telecommunications system. The next steps include building quantum repeaters with this system, and installing them on a test network.

For more from the researchers, I would invite you to check out the following two-minute video --

Until soon… Ian