Quantum criticality could be a boon for qubit designers

August 26, 2019

HOUSTON -- (Aug. 26, 2019) -- Physicists studying the strange behavior of metal alloys called heavy fermions have made a surprising discovery that could be useful in safeguarding the information stored in quantum bits, or qubits, the basic units of encoded information in quantum computers.

In a study in the Proceedings of the National Academy of Sciences, researchers from Rice University and the Vienna University of Technology (TU Wien) in Austria examined the behavior of an intermetallic crystal of cerium, palladium and silicon as it was subjected to extreme cold and a strong magnetic field. To their surprise, they found they could transform the quantum behavior of the material in two unique ways, one in which electrons compete to occupy orbitals and another where they compete to occupy spin states.

"The effect is so pronounced with one degree of freedom that it ends up liberating the other one," said Rice's Qimiao Si, co-corresponding author of the study and the director of the Rice Center for Quantum Materials (RCQM). "You can essentially tune the system to maximize damage to one of these, leaving the other well-defined."

Si said the result could be important for companies like Google, IBM, Intel and others who are competing to develop quantum computers. Unlike today's digital computers, which use electricity or light to encode bits of information, quantum computers use the quantum states of subatomic particles like electrons to store information in qubits. A practical quantum computer could outperform its digital counterpart in many ways, but the technology is still in its infancy, and one of the chief obstacles is the fragility of the quantum states inside the qubits.

"You need a well-defined quantum state if you wish to be assured that the information that is stored in a qubit will not change due to background interference," Si said.

Every electron acts like a spinning magnet, and its spin is described in one of two values, up or down. In many qubit designs, information is encoded in these spins, but these states can be so fragile that even tiny amounts of light, heat, vibration or sound can cause them to flip from one state to another. Minimizing the information that's lost to such "decoherence" is a major concern in qubit design, Si said.

In the new study, Si worked with longtime collaborator Silke Paschen of TU Wien to study a material where the quantum states of electrons were scrambled not just in terms of their spins but also in terms of their orbitals.

"We designed a system, realized in some theoretical models and concurrently realized in a material, where spins and orbitals are almost on an equal footing and are strongly coupled together," he said.

From previous research in 2012, Si, Paschen and colleagues knew that electrons in the compound could be made to interact so strongly that the material would undergo a dramatic change at a critically cold temperature. On either side of this "quantum critical point," electrons in key orbitals would arrange themselves in a completely different way, with the shift occurring solely due to the quantum interactions between them.

The earlier study invoked a well-known theory Si and collaborators developed in 2001 that prescribes how the spins of these localized electrons, which are part of atoms inside the alloy, strongly couple with free-flowing conduction electrons at the quantum critical point. According to this "local quantum critical" theory, as the material is cooled and approaches the critical point, the spins of localized electrons and conduction electrons begin to compete to occupy particular spin states. The quantum critical point is the tipping point where this competition destroys the ordered arrangement of the localized electrons and they instead become completely entangled with the conduction electrons.

Even though Si has studied quantum criticality for almost 20 years, he was surprised by the results of Paschen's latest experiments.

"The new data was completely baffling to all of us," he said. "That is, until we realized that the system contained not only spins but also orbitals as active degrees of freedom."

With that realization, Si's team, including Rice graduate student Ang Cai, built a theoretical model that contains both the spins and orbitals. Their detailed analysis of the model revealed a surprising form of quantum criticality that provided a clear understanding of the experiments.

"It was a shock to me, both from the theoretical model perspective and the experiments," he said. "Even though this is a soup of things -- spins, orbitals that are all strongly coupled to each other and to background conduction electrons -- we could resolve two quantum critical points in this one system under the tuning of one parameter, which is the magnetic field. And at each one of the quantum critical points, only the spin or the orbital is driving the quantum criticality. The other one is more or less a bystander."

Si is the Harry C. and Olga K. Wiess Professor in Rice's Department of Physics and Astronomy.

The study's co-lead authors are Cai and Valentina Martelli, formerly of TU Wien and now with the University of São Paulo in Brazil. Additional co-authors include Chia-Chuan Liu and Hsin-Hua Lai, both of Rice; Emilian Nica, formerly of Rice and currently at the University of British Columbia; Rong Yu, formerly of Rice and currently at Renmin University of China; Mathieu Taupin, Andrey Prokofiev, Diana Geiger, Jonathan Haenel and Julio Larrea, all of TU Wien; Kevin Ingersent of the University of Florida; Robert Küchler of the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany; and Andre Strydom of the University of Johannesburg in South Africa.

The research was supported by the National Science Foundation (DMR-1920740, CNS-1338099, PHY-1607611, DMR-1508122), the Robert A. Welch Foundation (C-1411), the Army Research Office (ARO-W911NF-14-1-0525, ARO-W911NF-14-1-0496), the Austrian Science Fund (P29296-N27, DK W1243), the European Research Council (Advanced Grant 227378), the Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro (201.755/2015), the National Natural Science Foundation of China (11674392), the Ministry of Science and Technology of China (2016YFA0300504), the South African National Research Foundation (93549), the University of Johannesburg and RCQM.

RCQM leverages global partnerships and the strengths of more than 20 Rice research groups to address questions related to quantum materials. RCQM is supported by Rice's offices of the provost and the vice provost for research, the Wiess School of Natural Sciences, the Brown School of Engineering, the Smalley-Curl Institute and the departments of Physics and Astronomy, Electrical and Computer Engineering, and Materials Science and NanoEngineering.
-end-
High-resolution IMAGES are available for download at:

http://news.rice.edu/files/2018/05/0119_HEAVY-Si-bookV-lg-15jp9sj.jpg?CAPTION: Qimiao Si (Photo by Jeff Fitlow/Rice University)

http://news.rice.edu/files/2014/09/0929-RCQM-purple1-lg.jpg?CAPTION: The Rice Center for Quantum Materials is a multidisciplinary effort to solidify Rice University's leadership in the exploration of high-temperature superconductors and other exotic materials. (Image courtesy of Rice University)

Links and resources:

The DOI of the PNAS paper is: 10.1073/pnas.1908101116

A copy of the paper is available at: https://doi.org/10.1073/pnas.1908101116

Qimiao Si: qmsi.rice.edu

Silke Paschen: http://www.ifp.tuwien.ac.at/paschen

Rice Department of Physics and Astronomy: physics.rice.edu

Wiess School of Natural Sciences: naturalsciences.rice.edu

Rice Center for Quantum Materials: rcqm.rice.edu

Related research from Rice:

Superconductors due for a tuneup -- Jan. 16, 2019?https://news.rice.edu/2019/01/16/superconductors-due-for-a-tuneup%e2%80%a8/

Theory for one type of superconductor solves puzzle in another -- May 8, 2018?http://news.rice.edu/2018/05/08/theory-for-one-type-of-superconductor-solves-puzzle-in-another/

Rice U. physicists discover new type of quantum material -- Dec. 18, 2017?http://news.rice.edu/2017/12/18/rice-u-physicists-discover-new-type-of-quantum-material/

Physicists probe magnetic fluctuations in heavy fermion -- Sept. 29, 2016?http://news.rice.edu/2016/09/29/physicists-probe-magnetic-fluctuations-in-heavy-fermion/

Evidence mounts for quantum criticality theory -- Jan. 30, 2015?http://news.rice.edu/2015/01/30/evidence-mounts-for-quantum-criticality-theory/

Rice launches Center for Quantum Materials -- Sept. 30, 2014?http://news.rice.edu/2014/09/30/rice-launches-center-for-quantum-materials/

Electron conflict leads to 'bad traffic' on way to superconductivity -- April 4, 2013?http://news.rice.edu/2013/04/04/electron-conflict-leads-to-bad-traffic-on-way-to-superconductivity/

Rice's 'quantum critical' theory gets experimental boost -- Jan. 10, 2012?http://news.rice.edu/2012/01/10/rices-quantum-critical-theory-gets-experimental-boost/

This release can be found online at news.rice.edu.

Follow Rice News and Media Relations via Twitter @RiceUNews.

Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,962 undergraduates and 3,027 graduate students, Rice's undergraduate student-to-faculty ratio is just under 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for lots of race/class interaction and No. 4 for quality of life by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger's Personal Finance.

Rice University

Related Science Articles from Brightsurf:

75 science societies urge the education department to base Title IX sexual harassment regulations on evidence and science
The American Educational Research Association (AERA) and the American Association for the Advancement of Science (AAAS) today led 75 scientific societies in submitting comments on the US Department of Education's proposed changes to Title IX regulations.

Science/Science Careers' survey ranks top biotech, biopharma, and pharma employers
The Science and Science Careers' 2018 annual Top Employers Survey polled employees in the biotechnology, biopharmaceutical, pharmaceutical, and related industries to determine the 20 best employers in these industries as well as their driving characteristics.

Science in the palm of your hand: How citizen science transforms passive learners
Citizen science projects can engage even children who previously were not interested in science.

Applied science may yield more translational research publications than basic science
While translational research can happen at any stage of the research process, a recent investigation of behavioral and social science research awards granted by the NIH between 2008 and 2014 revealed that applied science yielded a higher volume of translational research publications than basic science, according to a study published May 9, 2018 in the open-access journal PLOS ONE by Xueying Han from the Science and Technology Policy Institute, USA, and colleagues.

Prominent academics, including Salk's Thomas Albright, call for more science in forensic science
Six scientists who recently served on the National Commission on Forensic Science are calling on the scientific community at large to advocate for increased research and financial support of forensic science as well as the introduction of empirical testing requirements to ensure the validity of outcomes.

World Science Forum 2017 Jordan issues Science for Peace Declaration
On behalf of the coordinating organizations responsible for delivering the World Science Forum Jordan, the concluding Science for Peace Declaration issued at the Dead Sea represents a global call for action to science and society to build a future that promises greater equality, security and opportunity for all, and in which science plays an increasingly prominent role as an enabler of fair and sustainable development.

PETA science group promotes animal-free science at society of toxicology conference
The PETA International Science Consortium Ltd. is presenting two posters on animal-free methods for testing inhalation toxicity at the 56th annual Society of Toxicology (SOT) meeting March 12 to 16, 2017, in Baltimore, Maryland.

Citizen Science in the Digital Age: Rhetoric, Science and Public Engagement
James Wynn's timely investigation highlights scientific studies grounded in publicly gathered data and probes the rhetoric these studies employ.

Science/Science Careers' survey ranks top biotech, pharma, and biopharma employers
The Science and Science Careers' 2016 annual Top Employers Survey polled employees in the biotechnology, biopharmaceutical, pharmaceutical, and related industries to determine the 20 best employers in these industries as well as their driving characteristics.

Three natural science professors win TJ Park Science Fellowship
Professor Jung-Min Kee (Department of Chemistry, UNIST), Professor Kyudong Choi (Department of Mathematical Sciences, UNIST), and Professor Kwanpyo Kim (Department of Physics, UNIST) are the recipients of the Cheong-Am (TJ Park) Science Fellowship of the year 2016.

Read More: Science News and Science Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.