Why quantum is in the news
A joke to begin: these days, is it even possible to follow the movie Ant-Man and the Wasp without understanding quantum mechanics? The word is everywhere, and this course sets out to explain what it actually means. It starts with the machine that has put it in the headlines: the quantum computer.
Small quantum computers have been available online since 2016, and companies now say their machines beat ordinary computers on particular scientific problems, claims that others are still testing. None yet does everyday useful work that ordinary computers can’t, but the excitement has a real basis. For some problems, a large, reliable quantum computer could be faster beyond comparison. The famous case is factoring. Multiplying two large prime numbers together is easy; splitting the product back into its two factors is extremely hard, and with the best known methods the effort grows explosively with the size of the number. Public-key codes such as RSA, which help protect online banking, rest on problems like this being hard. In 1994 the American mathematician Peter Shor showed that a large enough quantum computer could factor such numbers quickly. The effects that make this possible, superposition, entanglement, and interference, are what this course explains.
There are other promises. A quantum search method, found by Lov Grover in 1996, could search an unsorted list of a million items in about a thousand steps. And, as the physicist Richard Feynman suggested in 1981, quantum computers could simulate molecules and materials. On an ordinary computer, an exact simulation grows explosively with every atom; a quantum computer could avoid that explosion.
Searching faster
To find one marked item in an unsorted list, an ordinary computer checks items one by one. Grover’s quantum method needs roughly the square root of that. Choose the size of the list.
Grover’s speedup applies to unstructured search. Other problems get bigger speedups, like Shor’s factoring, or none at all.
From idea to tool
How hard factoring is depends on the tools as well as the numbers. Before 2026 the largest RSA challenge number ever factored had 250 digits, a 2020 result that took about 2,700 years of computer-core time. Then, within sixteen days in September 2026, two records fell on ordinary graphics chips, both set by researchers working with AI coding assistants: RSA-260, with 260 digits, on September 3, and RSA-896, with 270 digits, on September 19. Anyone can check such a result by multiplying the factors back together. The keys that guard the web are much longer: by one team’s estimate, the standard 2,048-bit key, 617 digits, is not meaningfully affected (as of October 2026).
Four revolutions
This course places quantum technology in a sequence. The first industrial revolution ran on steam, the second on electricity, which you followed in Course 4. The third, beginning around the 1940s, was a revolution not of power but of computation. In this reading, the shortage was no longer energy but calculation. Think of Kepler repeating one laborious calculation for Mars at least seventy times, or Gregor Mendel tracking some 28,000 pea plants by hand. Computers freed people from that drudgery, and as they grew faster, speed itself became a new capability. A line made famous by Stephen Chow’s film Kung Fu Hustle puts it well: of all the world’s martial arts, none can beat speed (唯快不破). Enough speed turns quantity into quality, and in this reading that is one reason today’s artificial intelligence exists. Even the computer’s basic switch, the transistor (Bell Labs, 1947), is a quantum device.
China lost the Opium Wars (1839–42 and 1856–60) before the age of electricity, missed most of the second revolution, and nearly collapsed around 1900; it caught only the later part of the third. The argument here is that a fourth revolution, in quantum technology, will follow, driven by an even greater hunger for computing.
What the word means
So what is a quantum? Start with the Chinese word, which misleads many readers. 量子 liàngzǐ ends in 子 zǐ, like the names of particles: 原子 atom, 质子 proton, 中子 neutron, 光子 photon, 离子 ion. In names like these, 子 marks a small thing, not the honorific 子 of Confucius, 孔子, and Laozi, 老子, where it means “master.” But a quantum is not a kind of particle. The word comes from Latin quantum, “how much”: a fixed amount, the smallest packet of something, such as energy or light. A photon is one such packet. The rest of this course explains what comes in packets, and why that changes everything.
Two small clouds
By the end of the nineteenth century, classical physics seemed to many nearly complete. Mechanics, electromagnetism, and thermodynamics explained heaven and earth, and the engines of two industrial revolutions. When Max Planck, a Munich student in the 1870s, asked the professor Philipp von Jolly about physics, he was told, as he later recalled, that perhaps “a speck of dust or a bubble” remained to examine in some corner. In 1894 the American physicist Albert Michelson repeated an unnamed colleague’s remark that the future truths of physics were to be looked for “in the sixth place of decimals.” X-rays, radioactivity, and the electron followed within three years.
William Thomson, Lord Kelvin, was one of the greatest physicists of the age; the absolute temperature scale is named after him. On April 27, 1900, at the Royal Institution in London, Kelvin, then 75, said the theory of heat and light was “at present obscured by two clouds.” The first: how could the earth move through the ether that was supposed to carry light? Experiments, most famously Michelson and Morley’s in 1887, could not detect that motion. The second was the rule that heat energy is shared equally among all the ways a body’s molecules can move, which failed for the heat capacities of gases. He never mentioned radiation from hot ovens, though the same rule, it turned out, failed there too.
Kelvin hoped to clear both clouds within the old physics. With hindsight, each hid a door: behind the first, relativity; behind the second, the quantum, by way of Einstein’s 1907 theory of specific heats. Kelvin never called his clouds small; the word came with the retelling, as in the Chinese 两朵小乌云. This course borrows the frog in the well from the Zhuangzi: physicists had been admiring the patch of sky above them. Chasing the two clouds, they climbed out, and saw how vast the sky really was.
What became of Kelvin’s two “clouds”?
Closer to home
Both the United States and China treat quantum technology as a strategic priority. Congress passed the National Quantum Initiative Act in 2018; that year, after an American export ban on the Chinese telecom company ZTE, China’s leadership called for self-reliance in core technologies. In Hefei, the University of Science and Technology of China builds two different machines: Zuchongzhi (祖冲之), a superconducting chip, and Jiuzhang (九章), which works with light. The 2025 Nobel Prize in Physics went to John Clarke, Michel Devoret, and John Martinis for experiments at Berkeley in 1984–85 that made a whole electric circuit behave as one quantum object, the root of today’s superconducting qubits. In 2024 the U.S. National Institute of Standards and Technology issued its first “post-quantum” encryption standards, designed to resist future quantum computers. Some adversaries may already be recording encrypted data, hoping to decrypt it later.
Another view
Quantum computing is also heavily hyped. As of October 2026 the best-performing chips have only about 100 to 156 qubits, and all still make errors. A 2025 estimate put breaking a standard 2,048-bit RSA key at under a million noisy qubits running for about a week; a 2026 company preprint claims under 100,000, still far beyond anything built. Claims of “quantum advantage,” from Google in 2019 and a Chinese team in 2020 to IBM in 2026, are contests with classical methods, task by task, on problems chosen for the test; in 2022 a Beijing team did Google’s 2019 task on graphics chips in about fifteen hours, and within weeks of IBM’s claim another team reported simulating its flagship test on graphics chips in under an hour. The real milestone was quieter: in 2024 Google’s Willow chip showed error correction that improved as the code grew. How soon useful machines will come is genuinely uncertain.
A small correction: Kelvin is often quoted as saying that there is nothing new to be discovered in physics. Historians have found no evidence he said it. His real lecture did the opposite, pointing out where classical physics was in trouble, and historians doubt that physicists around 1900 broadly thought their field finished.
What to keep
- Quantum computers could be vastly faster at some tasks, like factoring, search, and simulating molecules, but no useful advantage has been independently confirmed yet (as of October 2026).
- This course counts four technological revolutions: steam, electricity, computation, and now quantum technology.
- A quantum is not a kind of particle but a fixed amount, a packet.
- In 1900 Kelvin named two “clouds” over the theory of heat and light. With hindsight, they pointed to relativity and quantum mechanics.
Think it over
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Sources and further reading
- Lord Kelvin, “Nineteenth Century Clouds over the Dynamical Theory of Heat and Light,” Philosophical Magazine, ser. 6, vol. 2 (July 1901): 1–40, the Royal Institution lecture of April 27, 1900; Oliver Passon, “Kelvin’s Clouds,” American Journal of Physics 89 (2021): 1037, arXiv:2106.16033; Richard Staley, “On the Co-Creation of Classical and Modern Physics,” Isis 96 (2005).
- Max Planck, “Vom Relativen zum Absoluten,” lecture in Munich, December 1, 1924 (published 1925), for Jolly’s advice; Albert A. Michelson, Light Waves and Their Uses, University of Chicago Press, 1903, repeating his 1894 “sixth place of decimals” remark.
- Peter W. Shor, “Polynomial-Time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer,” SIAM Journal on Computing 26 (1997): 1484; Lov K. Grover, “A Fast Quantum Mechanical Algorithm for Database Search,” arXiv:quant-ph/9605043 (1996).
- Richard P. Feynman, “Simulating Physics with Computers,” International Journal of Theoretical Physics 21, 1982.
- Craig Gidney, “How to Factor 2048 Bit RSA Integers with Less than a Million Noisy Qubits,” arXiv:2505.15917 (May 2025); Iceberg Quantum preprint, arXiv:2602.11457 (February 2026), a company claim.
- Google Quantum AI, “Quantum Error Correction below the Surface Code Threshold,” arXiv:2408.13687 (2024), on Willow; Feng Pan, Keyang Chen, and Pan Zhang, “Solving the Sampling Problem of the Sycamore Quantum Circuits,” Physical Review Letters 129, 090502 (2022); IBM Quantum hardware and roadmap pages (ibm.com/quantum, checked October 2026), a company claim; Hidetaka Manabe, Hanfeng Gu, and Feng Pan, “Classical Simulation and Design Frontiers for IBM’s Doped Clifford Sampling Experiment,” arXiv:2608.13110 (August 2026).
- Zuchongzhi 3.0: Physical Review Letters 134, 090601 (March 2025); University of Science and Technology of China, news release on Jiuzhang 4.0 (May 14, 2026).
- Royal Swedish Academy of Sciences, press release for the 2025 Nobel Prize in Physics (nobelprize.org).
- National Institute of Standards and Technology, FIPS 203, 204, and 205, post-quantum cryptography standards (August 13, 2024).
- Eric Lu, “Factoring RSA-260,” Cognition blog, September 9, 2026 (cognition.com/blog/factoring-rsa-260); reports on the RSA-260 and RSA-896 factorizations in PostQuantum (postquantum.com), Crypto Briefing (cryptobriefing.com), and OfficeChai (officechai.com), September 2026.
- Scott Aaronson, Quantum Computing since Democritus, Cambridge University Press, 2013.
- Michael A. Nielsen and Isaac L. Chuang, Quantum Computation and Quantum Information, Cambridge University Press, 2000.