Quest for quantum materials through better measurement of quantum
entanglement

Date:
February 14, 2022
Source:
The University of Hong Kong
Summary:
A research team has developed a new algorithm to measure
entanglement entropy, advancing the exploration of more
comprehensive laws in quantum mechanics, a move closer towards
actualization of application of quantum materials.



FULL STORY ==========================================================================
A research team from the Department of Physics, the University of Hong
Kong (HKU) has developed a new algorithm to measure entanglement entropy, advancing the exploration of more comprehensive laws in quantum mechanics,
a move closer towards actualisation of application of quantum materials.


==========================================================================
This pivotal research work has recently been published in Physical
Review Letters.

Quantum materials play a vital role in propelling human advancement. The
search for more novel quantum materials with exceptional properties has
been pressing among the scientific and technology community.

2D Moire materials such as twisted bilayer graphene are having a
far-reaching role in the research of novel quantum states such as superconductivity which suffers no electronic resistance. They also play
a role in the development of "quantum computers" that vastly outperforming
the best supercomputers in existence.

But materials can only arrive at "quantum state" , i.e. when thermal
effects can no longer hinder quantum fluctuations which trigger the
quantum phase transitions between different quantum states or quantum
phases, at extremely low temperatures (near Absolute Zero, -273.15DEGC)
or under exceptional high pressure. Experiments testing when and how
atoms and subatomic particles of different substances "communicate and
interact with each other freely through entanglement" in quantum state
are therefore prohibitively costly and difficult to execute.

The study is further complicated by the failure of classical LGW (Landau, Ginzburg, Wilson) framework to describe certain quantum phase transitions, dubbed Deconfined Quantum Critical Points (DQCP). The question then
arises whether DQCP realistic lattice models can be found to resolve
the inconsistencies between DQCP and QCP. Dedicated exploration of the
topic produces copious numerical and theoretical works with conflicting results, and a solution remains elusive.



==========================================================================
Mr Jiarui ZHAO, Dr Zheng YAN, and Dr Zi Yang MENG from the Department
of Physics, HKU successfully made a momentous step towards resolving
the issue through the study of quantum entanglement, which marks the fundamental difference between quantum and classical physics.

The research team developed a new and more efficient quantum algorithm
of the Monte Carlo techniques adopted by scientists to measure the Renyi entanglement entropy of objects. With this new tool, they measured the
Re'nyi entanglement entropy at the DQCP and found the scaling behaviour
of the entropy, i.e. how the entropy changes with the system sizes,
is in sharp contrast with the description of conventional LGW types of
phase transitions.

"Our findings helped confirm a revolutionised understanding of phase
transition theory by denying the possibility of a singular theory
describing DQCP. The questions raised by our work will contribute to
further breakthroughs in the search for a comprehensive understanding
of unchartered territory," said Dr Zheng Yan.

"The finding has changed our understanding of the traditional phase
transition theory and raises many intriguing questions about deconfined
quantum criticality. This new tool developed by us will hopefully help
the process of unlocking the enigma of quantum phase transitions that has perplexed the scientific community for two decades," said Mr Zhao Jiarui,
the first author of the journal paper and a PhD student who came up with
the final fixes of the algorithm.

"This discovery will lead to a more general characterisation of the
critical behaviour of novel quantum materials, and is a move closer
towards actualisation of application of quantum materials which play a
vital role in propelling human advancement." Dr Meng Zi Yang remarked.

The models To test the efficiency and superior power of the algorithm
and demonstrate the distinct difference between the entanglement entropy
of normal QCP between DQCP, the research team chose two representative
models -- the J1-J2 model hosting normal O(3) QCP and the J-Q3 model
hosting DQCP, as shown in Image 2.

Nonequilibrium increment algorithm Based on previous methods, the
research team created a highly paralleled increment algorithm. As
illustrated in Image 3, to the main idea of the algorithm is to divide
the whole simulation task into many smaller tasks and uses massive CPUs
to parallelly execute the smaller tasks thus greatly decreasing the
simulation time. This improved method helped the team to simulate the two models previously mentions with high efficiency and better data quality.

========================================================================== Story Source: Materials provided by The_University_of_Hong_Kong. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jiarui Zhao, Yan-Cheng Wang, Zheng Yan, Meng Cheng, Zi Yang
Meng. Scaling
of Entanglement Entropy at Deconfined Quantum Criticality. Physical
Review Letters, 2022; 128 (1) DOI: 10.1103/PhysRevLett.128.010601 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220214111807.htm

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