New diamond Quantum computer memory stores information for hours

Storing information in a quantum memory system in a quantum computer is a difficult challenge, as the data is usually quickly lost, but if we are ever going to make functional, commercial quantum computers, the first of which are slated to arrive in 2020, then it’s a challenge we’re going to have to overcome. But despite the fact that researchers have been experimenting with both exotic DNA and Atomic storage technologies now there might be an equally exotic form of memory for tomorrow's quantum computers, diamonds, after a team from TU Wien University (TWU) in Austria achieved a breakthrough and managed to store quantum information for hours using tiny diamonds.

Today quantum information, the information generated by quantum computers and “systems,” can be stored using quantum particles, but those particles only persist for very short periods of time, so as a result, and as you can imagine, this is now an area of intense focus, and it has applications on how we develop other quantum technologies such as extra sensitive Quantum Sensors that could replace GPS, as well as Quantum communication networks like the one recently demonstrated by the Chinese, and, of course Quantum Computers.

There is, however, a significant problem when trying to solve this challenge though - it’s difficult to store information in a “quantum physical system” for a long period of time because the quantum information tends to dissipate within fractions of a second because of interactions with the environment that surrounds it.

At TWU the team, which was led by Johannes Majer, demonstrated it’s possible to store quantum information for hours at a time using special diamonds that made it even more stable than the conventional information stored in the working memory of traditional logic based computers.

The teams research was published in the journal Nature Materials.

"We are using tiny diamonds intentionally seeded with small defects," says Majer.

Normally a diamond is composed only of carbon atoms, but by irradiating the diamond the team found out that it was possible to introduce a nitrogen atom into the diamonds’ structure in place of a carbon atom at certain points, which then leaves an unoccupied point in the crystal lattice next to it. This "lattice defect" is known as an NV Centre or Nitrogen-Vacancy Centre. The nitrogen atom and the empty site can assume different states so this lattice defect site can be used to store a single bit of quantum information.

The decisive issue on whether or not this technique is successful though, as far as the world community is concerned, is how long this information remains stable.

"The time scale in which a quantum bit typically loses its energy and with it the stored information is technologically one of the most important characteristics of such a quantum bit," explained Thomas Astner, the lead author of the publication, "understanding precisely the reason for the energy loss and the speed of this process is therefore crucial."

And the team made progress here too - for the first time the researchers were able to experimentally determine the characteristic period during which the diamond errors lose their quantum information, and they did this by coupling the diamonds to microwaves so quantum information can be written and read. The special microwave resonator used for this purpose was developed by Andreas Angerer at TWU in 2016 and the team used it to figure out, with great precision, how much energy was still being stored in the diamond. The measurements were also carried out at very low temperatures, just above the absolute zero temperature, at 20 millikelvins because heat would otherwise have disturbed the environment and erase the quantum information, and during their research it became apparent that the diamonds were able to store their information over several hours, much longer than was thought possible.

"The information in the D-RAM chip of an ordinary computer memory is much less stable. There the energy is lost within a few hundred milliseconds, meaning that the information then has to be refreshed," said Majer.

Not all diamonds with defects offer the same storage periods though, and the record for retaining quantum information, for over eight hours, is currently held by a special diamond manufactured by the team working with Junichi Isoya at the University of Tsukuba in Japan that was irradiated with electrons over several months to generate as many NV Centre defects as possible.

"Initially we could hardly believe these wonderful results," says Majer.

“Whereas other materials display lattice vibrations that can rapidly lead to the loss of the information stored, the coupling of quantum information to the lattice vibrations is very weak in diamonds and energy can be stored for hours," says Astner.