Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Making Quantum Connections

11.07.2014

The speed of information in a spin network

In quantum mechanics, interactions between particles can give rise to entanglement, which is a strange type of connection that could never be described by a non-quantum, classical theory. These connections, called quantum correlations, are present in entangled systems even if the objects are not physically linked (with wires, for example). Entanglement is at the heart of what distinguishes purely quantum systems from classical ones; it is why they are potentially useful, but it sometimes makes them very difficult to understand.


Image credit: S. Kelley/JQI

(1) Using a laser, the ion spin chain is optically pumped to a spin state that is uncorrelated with respect to the spin-spin interactions. (2) The system is suddenly perturbed by lasers, turning on global spin-spin interaction. (3) After the spin system evolves for various lengths of time (t1…tn), the spin state of each ion is captured with a CCD camera. The researchers can directly observe the spin-spin correlations propagating across the ion chain.

Physicists are pretty adept at controlling quantum systems and even making certain entangled states. Now JQI researchers*, led by theorist Alexey Gorshkov and experimentalist Christopher Monroe, are putting these skills to work to explore the dynamics of correlated quantum systems. What does it mean for objects to interact locally versus globally? How do local and global interactions translate into larger, increasingly connected networks? How fast can certain entanglement patterns form?

These are the kinds of questions that the Monroe and Gorshkov teams are asking. Their recent results investigating how information flows through a quantum many-body system are published this week in the journal Nature (10.1038/nature13450), and in a second paper to appear in Physical Review Letters.

Researchers can engineer a rich selection of interactions in ultracold atom experiments, allowing them to explore the behavior of complex and massively intertwined quantum systems. In the experimental work from Monroe’s group, physicists examined how quickly quantum connections formed in a crystal of eleven ytterbium ions confined in an electromagnetic trap.

The researchers used laser beams to implement interactions between the ions. Under these conditions, the system is described by certain types of ‘spin’ models, which are a vital mathematical representation of numerous physical phenomena including magnetism. Here, each atomic ion has isolated internal energy levels that represent the various states of spin.

In the presence of carefully chosen laser beams the ion spins can influence their neighbors, both near and far. In fact, tuning the strength and form of this spin-spin interaction is a key feature of the design. In Monroe's lab, physicists can study different types of correlated states within a single pristine quantum environment.

To see dynamics the researchers initially prepared the ion spin system in an uncorrelated state. Next, they abruptly turned on a global spin-spin interaction. The system is effectively pushed off-balance by such a fast change and the spins react, evolving under the new conditions.The team took snapshots of the ion spins at different times and observed the speed at which quantum correlations grew.

The spin models themselves do not have an explicitly built-in limit on how fast such information can propagate. The ultimate limit, in both classical and quantum systems, is given by the speed of light. However, decades ago, physicists showed that a slower information speed limit emerges due to some types of spin-spin interactions, similar to sound propagation in mechanical systems.

While the limits are better known in the case where spins predominantly influence their closest neighbors, calculating constraints on information propagation in the presence of more extended interactions remains challenging. Intuitively, the more an object interacts with other distant objects, the faster the correlations between distant regions of a network should form.

Indeed, the experimental group observes that long-range interactions provide a comparative speed-up for sending information across the ion-spin crystal. In the paper appearing in Physical Review Letters, Gorshkov’s team improves existing theory to much more accurately predict the speed limits for correlation formation, in the presence of interactions ranging from nearest-neighbor to long-range.

Verifying and forming a complete understanding of quantum information propagation is certainly not the end of the story; this also has many profound implications for our understanding of quantum systems more generally. For example, the growth of entanglement, which is a form of information that must obey the bounds described above, is intimately related to the difficulty of modeling quantum systems on a computer.

Dr. Michael Foss-Feig explains, “From a theorist’s perspective, the experiments are cool because if you want to do something with a quantum simulator that actually pushes beyond what calculations can tell you, doing dynamics with long-range interacting systems is expected to be a pretty good way to do that. In this case, entanglement can grow to a point that our methods for calculating things about a many-body system break down.”

Theorist Dr. Zhexuan Gong states that in the context of both works, “We are trying to put bounds on how fast correlation and entanglement can form in a generic many-body system. These bounds are very useful because with long-range interactions, our mathematical tools and state-of-the-art computers can hardly succeed at predicting the properties of the system.

We would then need to either use these theoretical bounds or a laboratory quantum simulator to tell us what interesting properties a large and complicated network of spins possess. These bounds will also serve as a guideline on what interaction pattern one should achieve experimentally to greatly speed up information propagation and entanglement generation, both key for building a fast quantum computer or a fast quantum network.”

From the experimental side, Dr. Phil Richerme gives his perspective, “We are trying to build the world’s best experimental platform for evolving the Schrodinger equation [math that describes how properties of a quantum system change in time]. We have this ability to set up the system in a known state and turn the crank and let it evolve and then make measurements at the end. For system sizes not much larger than what we have here, doing this becomes impossible for a conventional computer.”

This news item was written by E. Edwards/JQI.

Emily Edwards | Eurek Alert!
Further information:
http://www.umd.edu

More articles from Physics and Astronomy:

nachricht A blueprint for clearing the skies of space debris
17.04.2015 | RIKEN

nachricht Quantum Physics – Hot and Cold at the Same Time
17.04.2015 | Ruprecht-Karls-Universität Heidelberg

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Astronomers reveal supermassive black hole's intense magnetic field

Astronomers from Chalmers University of Technology have used the giant telescope Alma to reveal an extremely powerful magnetic field very close to a supermassive black hole in a distant galaxy

Astronomers from Chalmers University of Technology have used the giant telescope Alma to reveal an extremely powerful magnetic field very close to a...

Im Focus: A “pin ball machine” for atoms and photons

A team of physicists from MPQ, Caltech, and ICFO proposes the combination of nano-photonics with ultracold atoms for simulating quantum many-body systems and creating new states of matter.

Ultracold atoms in the so-called optical lattices, that are generated by crosswise superposition of laser beams, have been proven to be one of the most...

Im Focus: UV light robot to clean hospital rooms could help stop spread of 'superbugs'

Can a robot clean a hospital room just as well as a person?

According to new research out of the Texas A&M Health Science Center College of Medicine, that is indeed the case. Chetan Jinadatha, M.D., M.P.H., assistant...

Im Focus: Graphene pushes the speed limit of light-to-electricity conversion

Researchers from ICFO, MIT and UC Riverside have been able to develop a graphene-based photodetector capable of converting absorbed light into an electrical voltage at ultrafast timescales

The efficient conversion of light into electricity plays a crucial role in many technologies, ranging from cameras to solar cells.

Im Focus: Study shows novel pattern of electrical charge movement through DNA

Electrical charges not only move through wires, they also travel along lengths of DNA, the molecule of life. The property is known as charge transport.

In a new study appearing in the journal Nature Chemistry, authors, Limin Xiang, Julio Palma, Christopher Bruot and others at Arizona State University's...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

HHL's Entrepreneurship Conference on FinTech

13.04.2015 | Event News

World Conference On Regenerative Medicine 2015: Registration And Abstract Submission Now Open

25.03.2015 | Event News

University presidents from all over the world meet in Hamburg

19.03.2015 | Event News

 
Latest News

Engineer Improves Rechargeable Batteries with MoS2 Nano 'Sandwich'

17.04.2015 | Power and Electrical Engineering

Comparing Climate Models to Real World Shows Differences in Precipitation Intensity

17.04.2015 | Earth Sciences

A blueprint for clearing the skies of space debris

17.04.2015 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>