A new quantum memory on the horizon

(Nanowerk News) A promising material is lining itself up as a candidate for a quantum memory. A team at the Max Planck Institute for the Science of Light in Erlangen is the first to succeed in precisely locating individual rare earth ions in a crystal and accurately measuring their quantum mechanical energy states. With the aid of ingenious laser and microscopy technology they determined the position of triply charged, positive praseodymium atoms (Pr3+) in an yttrium orthosilicate to within a few nanometres and investigated their weak interaction with light.
The work ("Spectroscopic detection and state preparation of a single praseodymium ion in a crystal") may make an important contribution to the quantum computers of the future – because the ions investigated are suitable for storing and processing quantum information, among other things.
 researchers have addressed individual praseodymium ions in the crystal of an yttrium orthosilicate
Memory candidate with a bright future: Max Planck researchers have addressed individual praseodymium ions in the crystal of an yttrium orthosilicate using resourceful microscopy and laser technology. This opens up the possibility of storing quantum information in these ions, which have several advantages compared to other memory candidates. (Image: MPI for the Science of Light )
Around the globe, numerous researchers are working on components for the quantum computers of the future which will be able to process information significantly faster than today. The key elements of these super-computers include quantum systems with optical properties similar to those of an atom, which explains why many researchers are currently focusing their attention on quantum dots or light-emitting defects (“colour centres”) in diamonds. However, their properties are not optimal for use in quantum computers. “Some of the light sources lose their brightness or flicker in an uncontrollable way,” explains Vahid Sandoghdar, who heads the Nano-Optics Department at the Max Planck Institute for the Science of Light in Erlangen. “Others are greatly affected by the environment into which they are embedded.”
Researchers observe the signals of an individual ion
It has long been known that the ions of rare earths such as neodymium or erbium do not suffer from these problems – which is why they now play a key role in lasers or laser amplifiers. They emit only little light, however, and are therefore difficult to detect. This is precisely what Tobias Utikal, Emanuel Eichhammer and Stephan Götzinger from Sandoghdar’s Group in Erlangen have succeeded in doing: after more than six years of intensive research they were able to detect individual ions of the rare earth metal praseodymium, pinpoint them with an accuracy of a few nanometres, and measure their optical properties with an accuracy never achieved before. Their experiment enabled them to observe signals from an individual Pr3+ ion.
The triply charged, positive ions were embedded in tiny microcrystals and nanocrystals of yttrium orthosilicate (YSO). Their energy varied only slightly depending on their position in the crystal – they reacted to slightly different frequencies. The scientists used this to excite individual ions in the crystals with a laser and to observe how they emit the energy after some time in the form of light. “Because the ions of the rare earths are not strongly affected by the thermal and acoustic oscillations of the crystal, some of their energy states are unusually stable,” says Sandoghdar. “It takes more than a minute before they make the transition into the ground state again – a million times longer than for most of the other quantum systems that have been investigated so far.” Since quantum information can be stored in the different energy states of an atom or ion, the Pr3+ions are suitable as memory for a quantum computer, for example.
The aim is for the signals of the ions to be even easier to observe in the future. Since an individual ion responds with less than 100 photons per second at the moment, the Erlangen-based scientists want to employ nano-antennas and microcavities to amplify the praseodymium signal by a hundred or a thousand times.
Source: Max Planck Institute for the Science of Light