China's quantum heavenly group selected as one of the top ten advances of the year in APS

 

On December 20, the Physics website of the American Physical Society (APS) announced the ten major advances in international physics ("Highlights of the Year") for 2022 [1], reviewing representative scientific results achieved internationally this year. The series of experiments on real number quantum mechanics tests completed by Jianwei Pan and Chaoyang Lu of the University of Science and Technology of China and Jingyun Fan of the Southern University of Science and Technology were also included in the APS.

 

It is worth mentioning that this is the second year that Pan's team has been selected as one of the top ten advances in APS. 2021, the "Zuchongzhi II" and "Jiuzhang II" quantum computing superiority experiments completed by Pan Jianwei, Zhu Xiaobo, Lu Chaoyang and others at USTC were successfully selected as one of the top ten advances. This year's top ten advances were selected.

 

01Quantum mechanics is not a real number

 

The quantum Schrödinger equation is written with an imaginary number i, which means that the solution of the equation can be a complex value. But are complex numbers just mathematical tools? Is it possible to develop quantum theory based on real numbers alone? Some physicists have long indeed argued that real-valued quantum mechanics can give the same predictions as traditional quantum mechanics, but in 2022, the teams of Jianwei Pan and Chaoyang Lu at the University of Science and Technology of China and Jingyun Fan at the Southern University of Science and Technology independently experimentally ruled out the standard quantum mechanics in real-number form.

 

As shown in the figure below, Alice, Bob and Charlie perform a three-way experiment similar to Bell's inequality. Two sources distribute entangled quantum bits between Alice and Bob and Bob and Charlie, respectively. Each party independently chooses from a set of possibilities the measurements to be made on its quantum bits. Since the sources are independent, the quantum bits sent to Alice and Charlie are initially uncorrelated, and Bob receives a quantum bit from both sources and, by performing a Bell state measurement, he generates entanglement between Alice's and Charlie's quantum bits, even though these quantum bits never interact - a process called "entanglement. -This process is called "entanglement swapping".

 

 

Real number quantum theory cannot describe certain measurements on small quantum networks. Two sources assign entangled quantum bits to three observers, who calculate a "fraction" from measurements of the quantum bits. In both experiments, the fractions obtained are incompatible with the traditional quantum mechanical formulation of real values.

 

In short, two sources distribute the entangled quantum bits to three observers, who calculate the "fraction" by measuring the quantum bits. In this theoretical framework, the real number form is bounded at 7.66.

 

At the beginning of 2022, Jing-Yun Fan's team at SUSTech used a "partial" Bell state measurement in their experiment, while Jian-Wei Pan's team at CSU performed a full Bell state measurement. CSU used a superconducting quantum processor in which the quantum bits have independent control and readout functions. SUSTech chose a photonic implementation that more easily achieves this independence. Specifically, polarization-entangled photons generated by parametric down-conversion are used and probed in a superconducting nanowire single-photon detector. However the photonic approach does not allow for a complete Bell state measurement.

 

In conclusion, both experiments provide convincing results: the CSU and SDSU experiments beat the real number theory score by 43σ and 4.5σ standard deviations, respectively, providing convincing evidence that complex numbers are needed to describe the experiments.

 

But the story does not end there. Experimental scientists have been working continuously to close various potential loopholes in order to obtain more precise experimental proofs.

 

To test the objective existence of complex numbers more rigorously, Jianwei Pan, Chaoyang Lu, and Qiang Zhang from the University of Science and Technology of China, in collaboration with researchers from the Jinan Institute of Quantum Technology and other institutions, have tested quantum mechanics in the form of real numbers using spacelike compartmentalized entangled exchange optical quantum networks, closing the loopholes of definiteness, measurement independence, and entangled source independence for the first time in the international arena [2].

 

Based on the spatially separated entanglement-switched optical quantum network, Pan's team uses two independent sources in the network to each independently generate entangled photon pairs, which are distributed to three participants at a distance for high-speed random photon measurement operations, as shown in the figure below. During the experiment, the participants are not affected by the measurement choices and results of other participants, and each independently performs the local random operation.

 

Schematic diagram of the experiment. The portraits show the three tied first authors, from left to right, Xuemei Gu, Dian Wu, and Yangfan Jiang.

 

The experiments are distributed over five locations, each at least 89 meters apart, ensuring that information needs to propagate from one part of the experiment to another at speeds greater than the speed of light in order to interfere with the results. This precaution was intended to help rule out the possibility of unknown mechanisms (at least those allowed by the current laws of physics) affecting the experiments.

 

The experimental results exceed the quantum mechanical predictions of the real form by 5.3 standard deviations, rigorously verifying the indispensability of complex numbers in quantum mechanics. For the first time in the international arena, the loopholes of definiteness, measurement independence, and entanglement source independence are closed. The above analysis also shows that complex numbers are essential to describe such experiments and that real numbers are not sufficient to describe the quantum world as we normally understand it.

 

02Nine other advances in APS

 

In addition to the real number quantum mechanics test experiment, nine other advances have been selected.

 

Laser fusion ignition

 

 

The U.S. National Ignition Facility achieves a long-awaited milestone. Earlier this year, the team achieved ignition, a self-sustaining combustion state in which local self-heating dominates external heating and energy loss to the environment [3]. Last week, the researchers announced that they had taken the next step: demonstrating a laser-induced fusion reaction that produced more energy than it consumed [4]. Although practical laser fusion reactors are still decades away, these results suggest that laser fusion is progressing at a rate similar to that of computer development.

 

Diversity in physics

 

This year Physics magazine launched a new podcast, This is Physics. The first episode reports on the difficulties faced by LGBTQ+ physicists. Recent research shows that LGBTQ+ physicists are often shunned or harassed, and that these experiences of exclusion can greatly impact their careers. The LGBTQ+ scientists interviewed shared their personal struggles as well as their positive experiences. They also emphasized that improving the climate in the physics community can be accomplished by demonstrating support, such as respecting pronouns, providing gender-neutral bathrooms, and being prepared to offer help to those who have been treated unfairly.

 

Observing the black hole of the Milky Way

 

 

In May, scientists published the second-ever image of a black hole. The task was not an easy one, as it involved recovering complete images from snapshots taken by different telescopes and corrupted by the motion of the gas around Sagittarius A*. To solve these problems, the team has developed algorithms that can select from thousands of reconstructed images the ones that best fit the incomplete data.

 

More information about the Higgs boson

 

 

Ten years after the discovery of the Higgs boson, no information about the Higgs boson defies the Standard Model. But particle physicists believe that studying the Higgs particle is more important than ever. Understanding how the Higgs particle interacts with itself and with other particles, or finding other Higgs-like particles, could help physicists decipher the nature of dark matter or explain the dominance of matter over antimatter.

 

The third run of the LHC, which began in July, will double the number of Higgs particles available for analysis [5]. Luca Malgeri, spokesperson for the CMS Collaboration, which co-discovered the Higgs particle, said, "We are really entering the era of precise Higgs physics."

 

Two space science milestones

 

 

On July 12, NASA shared with the world the first images taken by the James Webb Space Telescope, the largest telescope ever launched into space. Exactly three months later, they confirmed the first demonstration of a planetary defense tool that involved crashing a spacecraft into an asteroid and accomplishing what it was intended to do: change the asteroid's orbit. While the revelations from both missions are still being uncovered, this year will likely go down in history as a watershed year for space research.

 

Deciphering protein folding

 

 

In the past few years, the machine learning model AlphaFold has had remarkable success in predicting three-dimensional protein structures from their constituent amino acid sequences. This year, researchers showed [6] that AlphaFold can also reveal the underlying physics that govern the folding process. Any given amino acid sequence can be folded in a large number of ways, and AlphaFold can pick out the feasible ways from all the candidate configurations. In doing so, AlphaFold "learns" physical principles, such as the so-called energy potential of protein folding, the study found.

 

This finding suggests that machine learning can uncover information about complex biomolecular processes that cannot be derived from first principles.

 

Gravitational mass = inertial mass

 

 

In late summer, the satellite experiment MICROSCOPE reported that it validated the equivalence principle with the highest precision to date. According to this principle, gravitational and inertial masses are perfectly equivalent, and discovering the difference between the two can reveal physics beyond the Standard Model related to mysteries such as dark matter and dark energy. Experiments conducted since the time of Galileo have confirmed this principle with increasing sensitivity. Undisturbed by tests affecting the Earth, MICROSCOPE has broken all sensitivity records. By comparing the velocity of the fall of two cylinders made of different materials, it showed that the gravitational and inertial masses, if they differ, are less than 1 in 1015.

 

Mercury mystery solved

 

Mercury, the first superconductor ever discovered, unlocked some of its last remaining secrets when researchers developed a theoretical description that could predict the behavior of metals from first principles.

 

The discovery of superconductivity came in 1911, when physicist Heike Kamerlingh Onnes cooled mercury to about 4 K. Although the superconductivity of mercury was later considered universal, no microscopic theory could accurately describe it. By considering the subtle and often overlooked effects associated with the superconductivity of mercury, the researchers have done just that. The insights gained may help in the search for materials for conventional superconductors by design at conditions close to ambient temperature and pressure.

 

Clearer Quantum Hearing

 

A new technique for quickly measuring quantum-mechanically entangled photon pairs has led to a demonstration of a quantum optical microphone that outperforms classical microphones. In the demonstration, the team encoded a series of words spoken at a low volume into an optical signal carried by entangled photons, which were then detected and converted into a sound recording. Listeners recognized the words in these "quantum recordings" more accurately than words recorded with the equivalent classical technique.

 

The researchers say their demonstration shows the potential of their method for measuring fast, noisy signals, such as those generated by the movement of single molecules in biological cells.

 

Reference links：

[1]https://physics.aps.org/articles/v15/197

[2]http://news.ustc.edu.cn/info/1055/80843.htm

[3]https://physics.aps.org/articles/v15/67

[4]https://physics.aps.org/articles/v15/195

[5]https://physics.aps.org/articles/v15/104

[6] https://physics.aps.org/articles/v15/183