One hundred years of Solvay, one hundred years of quantum
After five years, the 28th Solvay Physics Conference was finally successfully held recently. For the first time ever, the theme of the conference is "The Physics of Quantum Information".
The Solvay Conference is a conference for discussions in the fields of physics and chemistry founded by the Belgian entrepreneur Ernest Solvay in the early 20th century. In 1911, the first Solvay Conference was held in Brussels, and it has been more than 100 years.
Readers familiar with photon boxes should know that the Solvay Conference is closely related to Einstein's photon box experiments and the entire field of quantum physics. Looking back on the 100-year history of the Solvay Conference, the most famous one is the "Bohr-Einstein Controversy" about quantum physics in 1927, and the "greatest group photo in the history of physics" after the conference. Of the 29 people pictured, 17 won the Nobel Prize in Physics.
1927 Fifth Solvay Conference. Source: International Solvay Institute
Back row from left to right: Picard, Henriette, Ehrenfest, Herzen, De Donde, Schrödinger, Fairchafelt, Pauli, Heisenberg, Fowler, Brie Yuan; middle from left to right: Debye, Knudsen, Bragg, Kramer, Dirac, Compton, de Broglie, Born, Bohr; front row from left to right: Langmuir, Planck, Marie Curie, Lorenz, Einstein, Langevin, Guy, Wilson, Richardson
The post-meeting group photo is a long-standing tradition at Solvay, and this year is special in that it may be the largest gathering of quantum information scientists since the birth of quantum information science in the 1980s. Among them are the pioneers of quantum information science (including Nobel Prize winners), the backbone of the current quantum information science, and the business leaders who promote the implementation of quantum information science, and you must have seen their names in the photon box.
The only regret is that due to the epidemic, domestic quantum information scientists were not able to go. However, there are still many Chinese elements in this conference. In addition to Wen Xiaogang, academician of the National Academy of Sciences, and Jiang Liang, professor of the University of Chicago, Pan Jianwei, an academician of the Chinese Academy of Sciences, also participated in the online panel discussion of Solvay's public lecture on May 22.
28th Solvay Conference 2022. Source: John Preskill
Front row from left to right: Ketterle, Maldecena, Haroche, Henneaux, Gross, Zoller, Wineland, Preskill, Halperin, Wen Xiaogang; second row: Aharonov, Stanford, Engelhardt, Aaronson, Rey, Vazirani, Girvin, Schoelkopf, Blatt , Cirac, Gottesman, Shor, Verstraete; 3rd row: Sevrin, Hubeny, Gambetta, Terhal, Simmons, Khemani, Nakamura; 4th row: Marcus, Bloch, Browaeys, Vidick, Pollmann, Wiebe, Penington; 5th row: Chiang Liang, Fisher, Wall, Harlow, Martinis, Troyer, Farhi, Almheiri, Calabrese, Altman; attended but not in photo: Lukin, Mahadev
From 1927 to 2022, after nearly a hundred years, quantum mechanics developed from a new and controversial theory to the cornerstone of the future information technology revolution. Solvay has witnessed the century-old course of quantum science.
Taking this opportunity, we would like to talk about the origin of the "photon box".
In 1927, in the early stages of development of the theory of quantum mechanics, we generally think that Heisenberg et al.'s matrix mechanics and the Schrödinger equation marked the birth of quantum mechanics. The law of motion of electrons solved from the Schrödinger equation is a "wave function" that permeates the entire space. This conclusion was inconceivable to physicists who grew up in the background of classical mechanics at that time. Because in classical mechanics, the position of a particle at a certain point in time is fixed.
Just as everyone was wracking their brains, Born in 1926 gave a probabilistic explanation. He believed that electrons in quantum mechanics do not have deterministic orbits like classical particles, but appear randomly at a certain point in space. However, the probability of an electron appearing at a particular location is fixed, determined by the wave function solved by the Schrödinger equation.
On this basis, Heisenberg's uncertainty principle and Bohr's complementarity principle were developed later. These series of interpretations made quantum mechanics self-justified for the first time. Since Bohr at the University of Copenhagen was so prestigious at the time, these interpretations were called the Copenhagen interpretation of quantum mechanics and were soon widely accepted by the academic community, except for Einstein.
At that time, two factions were basically formed in the field of physics: the Copenhagen School represented by Bohr, Born, Heisenberg, Pauli and Dirac, and the opposition led by Einstein, Schrödinger and others.
By the Solvay Conference in 1927, the rationality of the Copenhagen interpretation was the main topic of discussion at the conference.
At the formal meeting stage, the interpretation of quantum theory by Bohr and the Copenhagen School prevailed. Einstein's questions are usually raised outside the formal meeting, and the debate and confrontation between the two factions mostly take place at the dinner table before and after the meeting every day.
Einstein's starting point is the three assumptions in classical mechanics - conservation law, certainty, locality. Generally speaking, there is little controversy about conservation laws. But the uncertainty principle proposed by Heisenberg violated the assumption of certainty, which Einstein could not tolerate.
Heisenberg's 1925 discovery that the motion of an electron actually has no trajectory to speak of, because the electron's position and momentum cannot be determined at the same time: the smaller the uncertainty of the position, the greater the uncertainty of the momentum, and vice versa. Heisenberg came up with the uncertainty principle.
Einstein's view can be summed up in his famous saying "God doesn't play dice" that the nature of the world is not random, consistent with the view of classical mechanics. Those random phenomena that seem unexplainable are because there are "hidden variables" that have not yet been discovered. Once we find these hidden variables, the randomness ceases to exist.
However, the Copenhagen School believes that the randomness of the microscopic world is inherent and essential, and there are no hidden variables hidden deeper, but only the probability of "collapse of the wave function" to a certain eigenstate.
In the end, until the end of the meeting, the two factions still held their own opinions, and neither was persuaded by the other.
At the sixth Solvay Conference three years later, the two factions once again discussed the sword in Huashan. Einstein proposed his famous "photon box" thought experiment. The experimental device is a sealed box filled with luminescent substances. A small hole is opened in the box. The mechanical clock in the hole can precisely control the opening time of the shutter. Meanwhile, the box is suspended from a precision spring balance to measure its mass.
At the beginning of the experiment, the mass of the box was measured once, and then the shutter was controlled to open for a short time to let a photon escape. When the shutter was closed, the mass was measured again. Let the mass reduced by the box be m, and the energy of the photon is E=mc2.
Einstein believed that in this experiment, the time was measured by a mechanical clock, and the energy of the photon could be obtained by measuring the mass difference through a spring balance. In this way, the uncertainty principle that time and energy cannot be accurately measured at the same time is not valid, the viewpoint of Bohr's school is incorrect, and quantum mechanics is not self-consistent.
Einstein's photon box experiment left Bohr speechless on the spot. But only one night later, using Einstein's own theory of general relativity, Bohr pointed out the flaws of the photon box experiment.
Bohr pointed out: After the photon runs out, the mass of the box hanging on the spring balance becomes lighter, that is, it moves upward. According to the general theory of relativity, if the direction of gravity of the clock shifts, the speed of the clock will change. In this way, the time read by the mechanical clock in the box will change as the photon escapes. In other words, with this device, if the energy of the photon is to be measured, the moment at which the photon escapes cannot be precisely controlled.
Einstein was stunned by Bohr's counterattack, and has since abandoned the idea of
attacking quantum mechanics from the perspective of the uncertainty principle. "Quantum theory may be self-consistent," he said, "but at least incomplete."
Bohr was indeed disturbed by Einstein's "photon box" problem that night, and he still brooded over it for many years to come. It is said that when Bohr died in 1962, Einstein's photon box was still drawn on the blackboard in his studio.
Nearly a hundred years ago, the Boyle-Loss dispute and the photon box experiment at the Solvay Conference laid the foundation for the 100-year glory of quantum mechanics. And this year's Solvay conference may also point the way for the future of quantum information science.
In the words of theoretical quantum physicist Preskill, "This conference promotes exchanges in quantum matter, quantum gravity, quantum hardware, and quantum computer science, and can inspire new ideas and insights to guide our exploration of the intractability of complex, highly entangled systems. elusive character."
Preskill highlighted two themes discussed at this meeting. One is that quantum information physics provides unified concepts and powerful techniques for controlling and exploring complex multi-particle quantum systems; the other is seeking convincing evidence that quantum error correction can extend quantum storage time and improve the performance of quantum gates. Error rate.
Finally, Preskill summarizes several noteworthy issues discussed at the 28th Solvay Physics Conference that may have implications for the future development of quantum information science:
· What can classical machine learning teach us about quantum phases of matter?
· To discover a new phase of quantum simulators, what should we be looking for?
· Is there a smooth path through error mitigation from noisy intermediate-scale quantum (NISQ) devices to fault-tolerant quantum computing?
· Will the recently discovered "good" quantum low-density parity-check codes (eventually) drastically reduce the overhead of fault-tolerant quantum computing?
· What are the fundamental limitations of quantum machines?
· What problem has a huge quantum speedup?
· What should experimenters and theorists do to help people understand quantum gravity?
