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Unique quantum inference effect captured by cosmic X-ray polarimeter - New physics experiments opened up by a state-of-the-art space observation instrument

An unanticipated polarization of high-energy X-rays emitted when highly charged ions capture high-energy electrons has been discovered experimentally for the first time. The measurement used a high-energy X-ray polarization detector that has been previously developed for high sensitivity space observations with the electron beam ion trap Tokyo-EBIT experiment at the University of Electro-Communications. Estimates from atomic physics had previously suggested that the electron state transition would emit unpolarized radiation, but the measured X-rays turned out to be highly polarized. Polarization measurements of high-energy X-rays are considered valuable in atomic physics research, but their use has been hindered in the past due to the lack of detectors capable of accurate measurements.

These experiments were performed by Associate Professor Watanabe Shin (JAXA Institute of Space and Astronautical Science, ISAS) who joined the experimental group led by Professor Nakamura Nobuyuki (Institute for Laser Science, University of Electro-Communications) and Professor Takahashi Tadayuki (Kavli IPMU, University of Tokyo). These results were achieved by combining two state-of-the-art instruments and technologies: the EBIT-CC high-energy Compton X-ray polarimeter developed for space observations and adapted for this research principally at the JAXA Institute of Space and Astronautical Science (ISAS), and the Tokyo-EBIT world-leading multiply-charge ion generator and experimental instrument at the University of Electro-Communications.

The experimental result of the unexpectedly large degree of polarization was followed up by theoretical analysis which revealed the observed polarization was the result of a quantum inference effect, in which waves of quantum mechanical probability interfere with one another. Normally, two waves must have the same initial state for interference to occur, but the observed polarization in this experiment was caused by two waves with different total angular momenta. In other words, this unusual interference effect was caused by two waves with different initial states. The theoretical study was undertaken by Associate Professor Xiao-Min Tong (University of Tsukuba), Specially Appointed Researcher Xiang Gao (Institute of Applied Physics and Mathematics, Beijing), and Associate Professor Kato Daiji (National Institute for Fusion Science) using an analysis that avoided traditional assumptions.

The results of this work are therefore a good example of how cutting-edge observation instruments developed to meet the needs of the space science community can become the seeds for new discoveries in other research fields. This research was published in the US scientific journal, “Physical Review Letters”.

When atoms and ions transition from a high energy state to a low energy state, they emit electromagnetic waves with wavelengths that correspond to the energy difference between the two states. These electromagnetic waves carry a variety of information about emitting atoms and ions. For example, by examining the different wavelengths of emitted electromagnetic waves (that is, spectroscopy), it is possible to know the structure of the atoms and ions. It was this that led to the development of quantum mechanics. For observation in space, investigating such electromagnetic waves allows the exploration of the state of atoms and ions in space, which tackles challenges in astrophysics.

One important piece of information contained in the electromagnetic waves is their degree of polarization. Electromagnetic waves are waves that are transmitted through periodic vibrations of electricity and magnetism. The degree of polarization is the extent to which the direction of these vibrations is biased. Light from an incandescent bulb vibrates in random directions. That is, the light from the bulb is non-polarized. On the other hand, light from a laser vibrates in a single direction, which is a typical example of polarized light. By examining the degree of polarization, it is possible to obtain information about the “direction” in which the electrons in the atoms and ions that emitted the light were moving. This makes the polarization important not only for investigating the properties of atoms and ions in detail, but also for understanding the anisotropy of substances and environments that contain those atoms and ions.

This study investigated the degree of polarization of high-energy X-rays emitted by special ions known as highly charged ions. An atom has an equal number of positive charges in its nucleus and the number of negative electrons circling outside the nucleus. The atom is therefore neutral (no charge). But if one electron is removed, an ion with a positive charge is created. Special ions that are made by removing more than one electron are multiply-charged ions. Many of the electromagnetic waves emitted by multiply-charged ions are high-energy X-rays, and obtaining knowledge about the degree of polarization of these X-rays is useful for understanding the anisotropy of high-temperature plasmas, such as celestial bodies or experimental nuclear fusion reactors where multiply-charge ions are abundant.

However, measuring the degree of polarization of high-energy X-rays is a very difficult technique. The light we can see (visible light) and X-rays are both forms of electromagnetic waves, but the degree of polarization of visible light can be relatively easily obtained using a polarizing plate (available for research via mail-order sites). But for high-energy X-rays, the wavelength of the electromagnetic wave is smaller than the size of an atom, and in principle, a polarizing plate cannot be made. It is therefore necessary to apply a method called Compton polarimetry.

A Compton polarimeter measures the scattering angle of X-rays that undergo Compton scattering (a process by which the energy of the scattered X-ray becomes lower than the incident X-ray) in the detector. The degree of polarization can be determined from the property that the distribution of scattering angles from many incident X-rays depends on the degree of polarization of the incident X-rays. JAXA’s Institute of Space and Astronautical Science (ISAS) has developed a Si/CdTe semiconductor Compton camera that also functions as a Compton polarimeter, aimed at achieving high-sensitivity, high-energy X-ray observations. It was originally developed as a soft gamma ray detector (SGD) for the ASTRO-H Hitomi satellite. Then, we have successfully detected polarized X-rays from the Crab Nebula in SGD test observations, and have clarified that the X-ray emission is synchrotron radiation between high-energy electrons and the characteristic magnetic field of the Crab Nebula. Thus, in the field of space observation, we have been working to elucidate physical phenomena in celestial objects by observing polarized X-rays.

This new finding discovered through the combination of the ISAS Compton polarimeter and Tokyo-EBIT which revealed that polarization is greatly affected by quantum interference effects, is expected to be utilized to further understand these new astronomical observations of X-ray polarization. It also impacts other fields. For example, polarization of X-rays emitted by multiply-charged ions is important for verifying the most accurate theories in physics such as in quantum electrodynamics. Precise measurement of the degree of polarization of X-ray transitions in highly charged ions using the methods of this study allow the possibility of conducting experiments that can be said to approach the essence of physics by capturing the wave-nature of virtual photons that mediate quantum electrodynamic theory. These challenges are underway in this joint research team.

This research is the result of research and development in space observation instrumentation, which could not be realized elsewhere. It has become the seeds for discoveries in other fields, and the creation of further research investigation paths.

Journal: Physical Review Letters
Paper title: Strong Polarization of a J=1/2 to 1/2 Transition Arising from Unexpectedly Large Quantum Interference
Authors: Nobuyuki Nakamura, Naoki Numadate, Simpei Oishi, Xiao-Min Tong, Xiang Gao, Daiji Kato, Hirokazu Odaka, Tadayuki Takahashi, Yutaka Tsuzuki, Yuusuke Uchida, Hirofumi Watanabe, Shin Watanabe, and Hiroki Yoneda

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