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Deeper understanding of quantum chaos may be the key to quantum computers

University of Leeds

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IMAGE: Quantum systems can exist in many possible states, here illustrated by groups of spins, each pointing along a certain direction. Thermalization occurs when a system evenly explores all allowed configurations.... view more 

Credit: Zlatko Papic, University of Leeds

New research gives insight into a recent experiment that was able to manipulate an unprecedented number of atoms through a quantum simulator. This new theory could provide another step on the path to creating the elusive quantum computers.

An international team of researchers, led by the University of Leeds and in cooperation with the Institute of Science and Technology Austria and the University of Geneva, has provided a theoretical explanation for the particular behaviour of individual atoms that were trapped and manipulated in a recent experiment by Harvard University and MIT. The experiment used a system of finely tuned lasers to act as "optical tweezers" to assemble a remarkably long chain of 51 atoms.

When the quantum dynamics of the atom chain were measured, there were surprising oscillations that persisted for much longer than expected and which couldn't be explained.

Study co-author, Dr Zlatko Papic, Lecturer in Theoretical Physics at Leeds, said: "The previous Harvard-MIT experiment created surprisingly robust oscillations that kept the atoms in a quantum state for an extended time. We found these oscillations to be rather puzzling because they suggested that atoms were somehow able to "remember" their initial configuration while still moving chaotically.

"Our goal was to understand more generally where such oscillations could come from, since oscillations signify some kind of coherence in a chaotic environment - and this is precisely what we want from a robust quantum computer. Our work suggests that these oscillations are due to a new physical phenomenon that we called 'quantum many-body scar'."

In everyday life, particles will bounce off one another until they explore the entire space, settling eventually into a state of equilibrium. This process is called thermalisation. A quantum scar is when a special configuration or pathway leaves an imprint on the particles' state that keeps them from filling the entire space. This prevents the systems from reaching thermalisation and allows them to maintain some quantum effects.

Dr Papic said: "We are learning that quantum dynamics can be much more complex and intricate than simply thermalisation. The practical benefit is that extended periods of oscillations are exactly what is needed if quantum computers are to become a reality. The information processed and stored on these computers will be dependent on keeping the atoms in more than one state at any time, it is a constant battle to keep the particles from settling into an equilibrium."

Study lead author, Christopher Turner, doctoral researcher at the School of Physics and Astronomy at Leeds, said: "Previous theories involving quantum scars have been formulated for a single particle. Our work has extended these ideas to systems which contain not one but many particles, which are all entangled with each other in complicated ways. Quantum many-body scars might represent a new avenue to realise coherent quantum dynamics."

The quantum many-body scars theory sheds light on the quantum states that underpin the strange dynamics of atoms in the Harvard-MIT experiment. Understanding this phenomenon could also pave the way for protecting or extending the lifetime of quantum states in other classes of quantum many-body systems.

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Download image from: https://goo.gl/BnBC3e
Caption: Illustration of quantum system. Quantum systems can exist in many possible states, here illustrated by groups of spins, each pointing along a certain direction. Thermalisation occurs when a system evenly explores all allowed configurations. Instead, when a "quantum scar" forms (as shown in the figure), some configurations emerge as special. This feature allows scarred systems to sustain memory of the initial state despite thermalisation.
Credit: Zlatko Papic, University of Leeds

The paper Quantum many-body scar is published in Nature Physics 14 May 2018 (DOI: 10.1038/s41567-018-0137-5)

Please contact University of Leeds press officer Anna Harrison at a.harrison@leeds.ac.uk or +44 (0)113 34 34196 for any additional information.

Reference for the 2017 experiment at Harvard University: H. Bernien et al., Nature 551, 579-584 (2017). https://www.nature.com/articles/nature24622

Support for this research was provided by EPSRC grants EP/P009409/1 and EP/M50807X/1, and Royal Society Research Grant RG160635 and Swiss National Science Foundation.

University of Leeds

The University of Leeds is one of the largest higher education institutions in the UK, with more than 33,000 students from more than 150 different countries, and a member of the Russell Group of research-intensive universities.

We are a top ten university for research and impact power in the UK, according to the 2014 Research Excellence Framework, and are in the top 100 for academic reputation in the QS World University Rankings 2018. Additionally, the University was awarded a Gold rating by the Government's Teaching Excellence Framework in 2017, recognising its 'consistently outstanding' teaching and learning provision. Twenty-six of our academics have been awarded National Teaching Fellowships - more than any other institution in England, Northern Ireland and Wales - reflecting the excellence of our teaching. http://www.leeds.ac.uk

IST Austria

The Institute of Science and Technology (IST Austria) is a PhD-granting research institution located in Klosterneuburg, 18 km from the center of Vienna, Austria. Inaugurated in 2009, the Institute is dedicated to basic research in the natural and mathematical sciences. IST Austria employs professors on a tenure-track system, postdoctoral fellows, and doctoral students. While dedicated to the principle of curiosity-driven research, the Institute owns the rights to all scientific discoveries and is committed to promote their use. The first president of IST Austria is Thomas A. Henzinger, a leading computer scientist and former professor at the University of California in Berkeley, USA, and the EPFL in Lausanne, Switzerland. The graduate school of IST Austria offers fully-funded PhD positions to highly qualified candidates with a bachelor's or master's degree in biology, neuroscience, mathematics, computer science, physics, and related areas. http://www.ist.ac.at

University of Geneva

The University of Geneva, Switzerland, was founded in 1559 by Jean Calvin and Théodore de Bèze and ranks amongst the top 60 best universities in the world. It enjoys worldwide recognition and develops an ever-strengthening international network. The University of Geneva welcomes about 17'000 students in its nine faculties teaching Sciences, Medicine, Humanities, Economics and Management, Social Sciences, Law, Theology, Psychology and Educational Sciences, as well as Translation and Interpreting. The University of Geneva fulfils three missions: education, research and knowledge-sharing. It is a member of the League of European Research Universities since 2002 and is a founding partner of Campus Biotech, the life science hub for the Geneva Lake region. http://www.unige.ch

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