How two ‘quantum Cheshire cats’ exchanged smiley faces

Anyone who has seen the fairy tale Alice in Wonderland will remember the grinned cat, the Cheshire Cat, which can disappear out of thin air, but the smiley face (attribute) still hangs in mid-air. Objects in the classic world have a variety of properties, such as mass, volume, and so on. It is generally believed that an object’s ontobody is inseparable from the properties it possesses, but the situation is different in the quantum world.

How two 'quantum Cheshire cats' exchanged smiley faces

A 2013 theoretical study showed that the physical properties of microscopic particles, such as electrical charge and spin of electrons, polarization of photons, and so on, can be separated from their ontobody, a phenomenon called “quantum Cheshire cat” by Wolf Award winner Yakir Aharonov and others.

A new study by the Chinese University of Science and Technology shows that in the quantum world, there is a more flexible relationship between matter and its physical properties. The properties of the “quantum Cheshire cat” can be stripped not only from their onto, but even captured by another “Cheshire cat”. In other words, two Cheshire cats can exchange smiley faces.

The study was published in the international academic journal Nature Communications. This newspaper on July 14th reported that “I scholars reveal the “quantum Cheshire cat” unique quantum characteristics” caused social concern, around the concerns of everyone, the reporter again interviewed the scientific research team.

Innovative measurement scants

In order to reveal the exchange of “quantum Cheshire cats” smiley faces, the team of Guo Guangcan academicians at the Chinese University of Science and Technology used a technique known as quantum weak measurement.

“What is called weak measurement is relative to the strong measurements in standard quantum mechanics.” Team member Xu Xiaoye said that it is well known that the implementation of strong measurements of the wave function describing the micro-quantum system will randomly collapse the system wave function into a certain state of the measurement operator, a process that will inevitably destroy the state of the quantum system. For example, the polarization state of the photon can be identified using a deflector, although it is ultimately possible to determine the polarization direction of the photon, but it will also completely destroy the initial state, unless the photon is initially in the checker’s indication. This process is called strong measurement because the coupling between the subsystem to be measured and the measuring probe needs to be completed during the measurement process, and the coupling is so strong that it is possible to completely entangle the system and probe.

Quantum weak measurement, on the other hand, considers another situation where the interaction between the system and the probe is so weak that it does not break the quantum state of the system, and the system can continue to evolve after coupling occurs, while weak measurement coupling does not provide useful information about the state of the system. “This concept was a relatively old topic in quantum mechanics until three scientists, including Yakir Aharronov, came to widespread attention in 1988 when they came up with the concept of weak values.” Xu Xiaoye said that, like the expectation sized in quantum strong measurements, the weak valueof of the measurable measurement is defined in the pre-choice state and the post-choice state, which is determined jointly by both. In their original article, the three scientists pointed out that weak values are no longer the values of considerable measurements, and can be far beyond the range of the values, and even recover values.

Although the concept of weak value is widely controversial, it has been widely used in quantum information technology. For example, the research team at the University of Science and Technology of China achieved high-precision time-delay measurements of asecond magnitude in 2013 based on weak values, and recently realized the direct characterization of entangled system wave functions. Because weak measurements minimize damage to the state of the system, the technology is also widely used in exploring the fundamentals of quantum mechanics, such as measuring the bohm trajectory of microscopic particles and observing the momentum transfer of the non-regional area of Yang’s double seams.

Separation of ontome and attributes

In the original version, Yakir Ahalonov and others considered photons as “quantum Cheshire cats” and the spin (polarization) of photons as their smiling faces. The entire system consists of an interferometer in which the photons can select the upper and lower paths after passing through the beam-splitting mirror. “We can reveal the separation of the photon body and its properties by performing two so-called weak measurements of photons, one for measuring the position of the photon body and the other for measuring its spin state, the position of the cheshire cat’s smiley face, and the measurement results are described by the weak value.”

Yakir Akhalonoff and others have discovered that by crafting the right front and rear selection posture, a Cheshire cat can be shown in Alice in Wonderland that separates itself from its smiling face. Specifically, in the upper path of the interferometer, the weak measurementof of the path and condition spin view measurement is given with the corresponding weak values of 0 and 1, that is, there is a smiley face without a cat in the path. Correspondingly, the weak measurements performed in the lower path resulted in weak values of 1 and 0 respectively, i.e., a cat without a smiley face appeared in the lower path. In short, the “quantum Cheshire cat” in the interferometer is separated from its smiling face.

Shortly after the concept of “quantum Cheshire cat” was introduced, experimental physicists from the University of Science and Technology in Vienna, Austria, demonstrated experiments using neutrons. In the neutron interferometer, the quantum state of its spin and path is precisely regulated by the applied magnetic field, so as to realize the preparation of the pre-selection state and the operation of the post-selection. As the results of the experiment were predicted by Yakir Akhalonov and others, in interferometers, the spin of the neutron always appears in the opposite arm of its ontobody position. Then, experimental physicists at the University of Portland in the United States conducted a similar experiment using predictable single photons from the spontaneous parameter conversion process, and also saw the “quantum Cheshire cat” phenomenon.

Non-contact exchange for smiling faces

Li Chuanfeng, Xu Jinshi, Xu Xiaoye and others of guo Guangcan Academician’s team at the University of Science and Technology of China, in cooperation with Professor Chen Jingling of Nankai University, further realized the contactless exchange of two photon polarizations based on the polarization of the photons, and revealed the unique quantum characteristics of the “quantum Cheshire cat”.

“However, it is not easy to observe the smiley-face exchange of the ‘Quantum Cheshire Cat’, which involves the challenge of extracting weak values from multibody quantum systems.” Xu Xiaoye said that the usual practice of extracting weak values is to introduce auxiliary probes, and as the system grows, the required coupling process becomes more and more complex. The need for contactless exchange of photon polarization is bound to involve multiple “quantum Cheshire cats” and the technical challengeof further introducing additional probes into the system.

In the experiment, the research group first prepared a double photon superentanglement state without the classic description through the conversion process under spontaneous parameters. In this state, the polarization and path freedom of the two photons are in the maximum entanglement state, but the two dimensions are in a straight, unrelated state of accumulation. Further by applying spin on certain paths, the double photons are prepared to a specific pattern, and the preparation of the pre-selection state is completed. Then the virtual evolution is introduced in the optical path to achieve weak measurement of photon path and spin. Finally, an operation called a joint Bell state measurement is performed, and the latter selection of the system is completed. By analyzing the efficiency of photon detection in different kinds of microperturbation and evolution time, the path of photons and the weak values of the polarization can be obtained. These weak values show that both photons in the experiment showed the phenomenon of “quantum Cheshire cat” with the separation of ontobody and properties.

The research results show the flexible relationship between matter and its properties in the quantum world, which is an important progress in understanding the basic problems of quantum mechanics, enlightening to explore whether the properties of microscopic particles are realistic before being measured, and will play an important role in promoting the study of the basic problems of quantum mechanics.