Science- Chinese team achieves first fractional quantum anomalous Hall state of photons

Pan Jianwei, Lu Chaoyang, Chen Mingcheng and others from the University of Science and Technology of China used a self-developed Plasmonium superconducting high anharmonic optical resonator array to achieve nonlinear interactions between photons, and further constructed an equivalent magnetic field acting on photons in this system to construct an artificial gauge field, achieving the anomalous fractional quantum Hall state of photons for the first time in the world.

This is an important progress in the field of quantum physics and quantum information science, and the relevant research results were published online in the journal Science.

Schematic diagram of the results. The microwave photons trapped in the 16 nonlinear "photon box" array strongly interact to form a fractional quantum anomalous Hall state.

The Hall effect refers to the phenomenon that when current passes through a material placed in a magnetic field, the electrons are acted upon by the Lorentz force, generating a voltage perpendicular to the direction of the current and magnetic field inside the material. This effect was discovered by American scientist Hall in 1879 and has been widely used in the field of electromagnetic sensing. In 1980, German scientist von Klitzing discovered that under extremely low temperatures and strong magnetic fields, the Hall effect exhibits an integer quantized conductivity platform. This new phenomenon goes beyond the description of classical physics and is called the integer quantum Hall effect, which provides a standard for accurate measurement of resistance. In 1981, Chinese-American scientist Tsui and German scientist Stömer discovered the fractional quantum Hall effect. The discovery of the integer and fractional quantum Hall effects won the Nobel Prize in Physics in 1985 and 1998, respectively.

In the more than 40 years since then, the fractional quantum Hall effect has received particular attention. Due to the interaction of degenerate electrons at the lowest Landau level, the fractional quantum Hall state exhibits non-trivial multi-body entanglement, and the theoretical results derived from its research, such as topological order and composite fermions, have gradually become the basic model of multi-body physics. At the same time, fractional quantum Hall states can excite localized quasiparticles, which have strange fractional statistics and topological protection properties and are expected to become the carrier of topological quantum computing.

The anomalous Hall effect refers to the observation of related effects without the need for an external magnetic field. In 2013, a Chinese research team observed the integer quantum anomalous Hall effect. In 2023, research teams in the United States and China independently observed the fractional quantum anomalous Hall effect in double-layer angle-shifted molybdenum telluride.

Traditional experimental studies of the quantum Hall effect use a "top-down" approach, that is, based on a specific material, the existing structure and properties of the material are used to prepare the quantum Hall state. Usually, an extremely low temperature environment, extremely high purity of two-dimensional materials, and an extremely strong magnetic field are required, which places relatively high demands on the experiment. In addition, the traditional "top-down" method makes it difficult to independently manipulate and measure the microscopic quantum state of the system at a single point, which to some extent limits its application in quantum information science.

Constructing artificial gauge fields in nonlinear photonic systems to realize fractional quantum Hall states of photons

In contrast, artificially constructed quantum systems have clear structures, are flexible and controllable, and are a new paradigm for studying complex quantum states from the bottom up. Its advantages include: no external magnetic field is required, and an equivalent artificial gauge field can be constructed by changing the coupling form; by performing high-precision addressable manipulation of the system, comprehensive measurement of the microscopic properties of highly integrated metrological subsystems can be achieved, and further controllable utilization can be achieved. This type of technology is called quantum simulation, which is an important part of the "second quantum revolution" and is expected to be used soon to simulate quantum systems that are difficult to simulate classically and achieve "quantum computing superiority."

Observation of topological correlations and topological photon flow in fractional quantum Hall states

Previously, some quantum simulation work on synthesizing topological states and studying topological properties has been carried out internationally. However, due to the limitations of coupling forms and nonlinear strength in previous systems, people have not been able to construct artificial gauge fields for photons in two-dimensional lattices.

To solve this major challenge, the team independently developed and named a new type of superconducting quantum bit Plasmonium internationally, breaking the constraints between the coherence and anharmonicity of the current mainstream Transmon quantum bit, and using higher anharmonicity to provide stronger repulsion between photons. Furthermore, the team constructed an equivalent magnetic field acting on photons by AC coupling, so that the flow of photons around the lattice can accumulate Berry phases, solving the two key problems of realizing the fractional quantum anomalous Hall effect of photons. At the same time, such an artificial system has the advantages of addressability, independent control and reading of single-point bits, and strong programmability, providing new means for experimental observation and manipulation.

Incompressibility and fractional Hall conductance of quasiparticles observed

In this work, the researchers observed the unique topological correlation properties of the fractional quantum Hall state and verified the fractional Hall conductance of the system. At the same time, they tracked the generation process of quasiparticles by introducing the method of local potential field and confirmed the incompressible nature of quasiparticles.

Reviewers of Science magazine highly praised this work, believing that this work is "a significant advance in quantum simulation with interacting photons", "a novel form of local control and bottom-up approach", and "potentially open new pathways for realizing non-Abelian topological states, which have been extremely challenging to probe in two-dimensional electron gases".

The artificially constructed quantum system has a clear structure, flexible and controllable, and is a new paradigm for studying complex quantum states "from the bottom up". "The advantage of this method is that it provides greater flexibility and controllability. Researchers can precisely control each component, thereby better understanding and manipulating quantum systems." Lu Chaoyang, a co-corresponding author of the paper and a professor at the University of Science and Technology of China, said that this type of technology is called quantum simulation and is an important part of the "second quantum revolution". It is expected to be applied in the near future to simulate quantum systems that are difficult to calculate classically and achieve "quantum computing superiority".

Nobel Prize winner in Physics Frank Wilczek commented that this "bottom-up" approach to building Hamiltonians with artificial atoms is a "very promising idea" and a very impressive experiment, which is an important step for quantum information processing based on anyons. Wolf Prize winner Peter Zoller commented that "this is an outstanding achievement, both scientifically and technically," and "achieving such a goal is one of the holy grails of quantum simulation that the world's top laboratories have competed for many years.

Reference:

[1]https://mp.weixin.qq.com/s/guHatgc4VtBH6v8CzzTAuw 

[2]https://mp.weixin.qq.com/s/z8KRM_3QCpqWNLX1IZypYA 

[3]https://mp.weixin. qq.com/s/qWxCPqSgflkGkKDV1AiL-A