A new quantum memory on the horizon

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 technologies.
This opens up the possibility of storing quantum
information in these ions, which have several advantages
compared to other memory candidates.
© MPI for the Science of Light

Sensitive measurements can be used to detect signals from an individual ion in a crystal

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 performing high-resolution spectroscopy and microscopy on individual rare earth ions in a crystal. 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. In addition to its impact on fundamental studies, the work may make an important contribution to the quantum computers of the future because the ions investigated are suitable for storing and processing quantum information.

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. This is why many researchers are currently focusing their attention on different systems such as light-emitting crystal defects (“colour centres”) in diamond or on semiconductor quantum dots. However, so far there has been no ideal solution. “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 rare earth ions such as neodymium or erbium do not suffer from these problems – which is also why they play a key role in lasers or laser amplifiers. They emit only weakly, 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 to do: after more than six years of intensive research they were able to detect individual praseodymium ions, pinpoint them with an accuracy of a few nanometres, and measure their optical properties with an accuracy never achieved before.

The triply charged, positive ions were embedded in tiny microcrystals and nanocrystals of yttrium orthosilicate (YSO). Their energies varied only slightly depending on their position in the crystal. In other words, 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 form of light. “Because rare earth ions 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.”

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.

http://www.mpg.de/8202685/quantum-ion-crystal

Electrified Diamonds: Basel Physicists on the Trail of Quantum Information

With the help of tiny diamond crystals, physicists at the University of Basel have discovered new possibilities of quantum information: The scientists discovered at specific circumstances electric currents that made it possible to identify defects in the carbon lattice of single diamonds measuring only a few nanometers. 

The results have been published online in the magazine «Nano-Letters».

The team from the University of Basel and the French German Research Institute St. Louis (ISL) investigated diamond crystals of the size of only five nanometers (five millionths millimeter) using scanning tunneling microscopy and atomic force microscopy. 

The physicists then identified the atomic structure of the surface and observed crystalline, hexagonal carbon facets as well as graphitic reconstructions. In doing so, they discovered extra currents at specific voltages when the crystals were illuminated by green light.

These extra currents are related to the presence of defects in the carbon lattice of diamonds, so called Nitrogen-vacancy centers (NV-centers) that are optically active. These centers are promising candidates for future applications in quantum information processing systems, spin-magnetometry sensors or single photon sources. 

Their identification in the range of less than ten nanometers would have been very difficult with conventional methods, which is why the scientists applied a combination of different methods.

«With this study, we are able to show that it is possible to prove, with high resolution, the presence of optical centers in single nanodiamonds», says Prof. Ernst Meyer of the Department of Physics at the University of Basel. In the future, NV-centers could be used in quantum computers that work much more efficiently than conventional computers.

Source: http://www.unibas.ch/index.cfm?uuid=6AA790579D2002EF40FD481B016171E9&type=search&show_long=1

Quantum memory 'world record' shattered

An international team of physicists led by SFU professor Mike Thewalt has overcome a key barrier to building practical quantum computers, taking a significant step to bringing them into the mainstream.

In their record-breaking experiment conducted on SFU’s Burnaby campus, the scientists were able to get fragile quantum states to survive in a solid material at room temperature for 39 minutes. For the average person, it may not seem like a long time, but it’s a veritable eternity to a quantum physicist.

“This opens up the possibility of truly long-term coherent information storage at room temperature,” explains Thewalt.

Quantum computers promise to significantly outperform today’s machines at certain tasks by exploiting the strange properties of subatomic particles. Conventional computers process data stored as strings of ones or zeroes, but quantum objects are not constrained to the either/or nature of binary bits.

Instead, each quantum bit—or qubit—can be put into a superposition of both one and zero at the same time, enabling them to perform multiple calculations simultaneously. For instance, this ability to multi-task could allow quantum computers to crack seemingly secure encryption codes. 

“A powerful universal quantum computer would change technology in ways that we already understand, and doubtless in ways we do not yet envisage,” says Thewalt, whose research was published in Science today. 

“It would have a huge impact on security, code-breaking and the transmission and storage of secure information. It would be able to solve problems which are impossible to solve on any conceivable normal computer. It would be able to model the behaviour of quantum systems, a task beyond the reach of normal computers, leading, for example, to the development of new drugs by a deeper understanding of molecular interactions.”

However, the problem with attempts to build these extraordinary number crunchers is that superposition states are delicate structures that can collapse like a soufflé if nudged by a stray particle, such as an air molecule.

To minimize this unwanted process, physicists often cool their qubit systems to almost absolute zero (-273 C) and manipulate them in a vacuum. But such setups are finicky to maintain and, ultimately, it would be advantageous for quantum computers to operate robustly at everyday temperatures and pressures.

“Our research extends the demonstrated coherence time in a solid at room temperature by a factor of 100—and at liquid helium temperature by a factor of 60 (from three minutes to three hours),” says Thewalt.

“These are large, significant improvements in what is possible.”

Source: http://www.sfu.ca/sfunews/stories/2013/quantum-computers-one-step-closer-to-reality.html