Science

First, a physicist gets a glimpse of a quantum ghost

Wave functions (an abstract concept used to predict the behavior of quantum particles) are the basis for physicists to build an understanding of quantum mechanics. However, this foundation itself, literally or philosophically, is not completely understood by physicists. Wavefunctions are not something you can hold in your hand or place under a microscope.And confusingly, some of its properties are simply Not real.. In fact, mathematicians openly label them as imaginary numbers. The so-called imaginary numbers, which result from seemingly meaningless feats such as taking the square root of a negative integer, are an important element of the well-proven force of the wavefunction and are experiments in the real world. In short, when we say that a wavefunction “exists”, it does so at the vague intersection between metaphysical mathematics and physical reality.

Currently, researchers at the University of California, Santa Barbara and their colleagues have made great strides in bridging these two areas. For the first time, they reconstructed the wavefunction by measuring how semiconductor materials react to ultrafast optical pulses.Appearance Nature In November, the team’s work could help guide electronics and quantum material design to a new era of fine-tuned understanding and precisely controlled innovation.

For real-world applications such as modern electronics, the somewhat mysterious wavefunctions are the best source of information for physicists about what’s really happening inside new gadgets. To predict the speed at which an electron travels through a material and the amount of energy it can transfer, we need to start with the so-called Bloch wavefunction, named after the physicist Felix Bloch, who devised it in 1929. This is especially important for engineering. Quantum devices are said by Joe Costello, a student at UCSB physics and co-author of recent research. “If you’re thinking of building any kind of device that takes advantage of quantum mechanics, you need to know that. [wave function’s] The parameters really work, “he emphasizes.

This includes the so-called phase of the wavefunction. This is a completely imaginary parameter, but it is often important in the design of quantum computers. “Energy has long been characterized. [of the electrons].. That’s the basis of all electronics, “says Mackillo Kira, a physicist at the University of Michigan. He read an early draft of the study, but was not directly involved in the study. “But now, in quantum information technology, the next level is to go beyond that and finally get these. [wave function] Phase. “

To reach that next level, the team used two lasers and the semiconductor material gallium arsenide. Their experiment consisted of three steps. First, a pulse of near-infrared laser light collided with an electron inside the material. This gives these particles extra energy and begins to pass through the semiconductor quickly. As each negatively charged electron began its competition, so-called holes, like its shadow particles (similar to electrons but positively charged), moved together. The researchers then used another laser pulse to tear the hole and the electron and immediately recombine them. This is a kind of quantum version of Peter Pan, which has lost its shadow and reattached. When the holes and electrons recombine, the extra energy stored while running solo is emitted as a burst of light.

Ten years ago, a team of physicists led by Mark Sherwin at UCSB noticed something strange about these bursts. Those properties were mysteriously sensitive to the properties of the laser pulse that first initiated the execution of the particles. Sherwin and his colleagues have found that there are important and almost unexplored nuances in how semiconductor electrons react to light. “This was unexpected,” he recalls. “But we decided to investigate it further and began to systematically investigate it.” In the new work, a postdoctoral scholar who is a member of Sherwin’s team and co-lead author of the study. A calculation made by a Qile Wu. This clear sensitivity proved to be more than just curiosity, as it can be used to reconstruct the Bloch wavefunction of holes in semiconductors.

The relationship between the absorbed laser light and the emitted flash was revealed by measuring a characteristic called polarized light, that is, the direction in which the light wave oscillates as it travels. In the experiment, the polarization of the laser beam affected the phase of the wavefunction of the running electron and its shadow partner, the hole. When the recombination of the two generated lights occurred at the end of the experiment, the polarization of the flash was determined by the phase of these two wavefunctions. Since such phases are usually represented in physicist equations as imaginary numbers rather than real numbers, associating them with highly realistic and measurable polarization of light is a breakthrough for Wu and his collaborators. It was a real thing. Shambhu Ghimire, a physicist at Stanford University who was not involved in this study, accurately emphasizes this feature of the new study. We used light to get information that was previously considered purely mathematical. “They are [light-based] Methods can be difficult or conceptually difficult, but in most cases they provide access to this imaginary part of a complex number. [wave function] It’s not accessible in any other traditional way, “he says. In addition, the team succeeded in reverse engineering the entire Bloch wavefunction from those same polarization measurements.

Ghimire further states that the type of laser light used by UCSB researchers is more important than polarized light. They adopted an ultrafast laser pulse and struck an electron with only one trillionth of a second of light. Electrons in solids tend to hit atoms rather than move uninterrupted, so teams can control them with such a selection to perform electron and hole Peter Pan and shadow operations. Was important. Otherwise, in any run of the experiment, one of them could hit some atomic obstacle and prevent reunification. Seamus O’Hara, another co-author and PhD in this study. Students from Sherwin’s group acknowledge some of the technical benefits of the team’s use of UCSB’s state-of-the-art free electron laser equipment.

However, the impact of this work can extend beyond specialized equipment and simple semiconductors. For gallium arsenide, Wu’s theoretical studies have shown that for the mathematical reconstruction of the Bloch wavefunction, it is necessary to know the properties of a very small number of re-emitted light. However, other semiconductor materials may require more complete and perhaps elusive knowledge. “This work is fascinating as a very basic demonstration of what you can do if the answer is really well defined,” says Mette Gaarde, a physicist at Louisiana State University who wasn’t involved in the study. .. “But this means that you could use it to learn something about more complex structures.”

The UCSB team already has an ambitious plan for the next step. In the future, researchers are interested in applying the technology to materials where electrons interact strongly with each other and where laser light excites particles that are more exotic than electrons and holes. “We are looking for new materials. If we have semiconductors that people want to see, we are excited to give them a try,” Costello said, what the intangible world of wavefunctions is. I want more opportunities to get a glimpse of it.

https://www.scientificamerican.com/article/in-a-first-physicists-glimpse-a-quantum-ghost/ First, a physicist gets a glimpse of a quantum ghost

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