Schrödinger’s cat
If everything is made of particles that can be in superpositions, why is nothing around us ever in two states at once? In 1935 the Austrian physicist Erwin Schrödinger, whose equation governs quantum waves, sharpened the question into a famous thought experiment. Seal a cat in a box with a speck of radioactive material, so little that within an hour one atom may decay or, just as likely, none. If one decays, a hammer smashes a flask of poison. Until the box is opened, quantum theory seems to say the atom is in a superposition of decayed and not decayed, so the cat must be too, both dead and alive. Yet nobody has ever seen a cat that is both. Why not?
Schrödinger meant the example to show how absurd it seemed. This course counts it among the best questions humanity has ever asked, because it concerns the boundary between the microscopic and the everyday world. Quantum theory makes no exception for large things. But a large object is constantly jostled by its surroundings, by air molecules and light and heat. The cat becomes entangled with the atom, then with every molecule and photon that touches it, and these carry the information away. No one needs to look. Physicists call this loss of quantum behavior decoherence. For an object weighing a gram, in air, it happens unimaginably fast: even the shortest flash of light yet made, under 20 billionths of a billionth of a second (as of 2026), is far too slow to catch it.
Laboratories have since made real, if small, cats: a single atom in two places at once (Boulder, Colorado, 1996), a 16-microgram crystal vibrating two opposite ways at once (Zurich, 2023), and clusters of more than 7,000 atoms (Vienna, 2026).
Another kind of cat is a superconducting electric circuit that behaves like a single giant atom. In 1984–85 at Berkeley, John Clarke, Michel Devoret, and John Martinis showed that such a circuit, big enough to hold in the hand, acts as one quantum object, work that won the 2025 Nobel Prize in Physics. Cooled to a small fraction of a degree above absolute zero and shielded from disturbance, the best such circuits now hold a superposition for about a thousandth of a second (as of November 2025). That’s too brief to watch directly, but by combining the measured data with theory, physicists are confident they have seen it. Circuits like these are the building blocks of many of today’s quantum computers.
In this reading, such experiments show that Newton’s way of dividing the world into independent parts, with the observer left outside, no longer works at this level. The physicist David Bohm argued that science now calls for “a new, non-fragmentary world view.”
From idea to tool
The one-atom cat, made at NIST in Boulder in 1996, had a sequel. Its first author, Chris Monroe, went on to build quantum computers from trapped ions, the technique of that experiment. In 2015 he and Jungsang Kim of Duke University co-founded IonQ, starting with licenses from the University of Maryland and Duke and, in 2016, $2 million in venture money. On October 1, 2021, IonQ began trading on the New York Stock Exchange, often described as the first publicly traded company devoted purely to quantum computing. A demonstration at the edge of the quantum world had become a company and a new kind of career. Like every quantum computer so far (as of October 2026), its machines are not yet the large, error-corrected kind the field is working toward.
The scale of perception
The idea this course most wants you to take away is what it calls the scale of perception, 感知尺度. It means the range, from the largest to the finest, that human beings can sense and measure at a given time.
Picture early humans, in this course’s sketch: their world reached perhaps a day’s walk from camp, and their finest work was the point of a needle. Instruments pushed both limits outward. Copernicus put the sun at the center in 1543, 65 years before the telescope; from 1609, telescopes showed what the old cosmos couldn’t accommodate, moons circling Jupiter and Venus showing phases like the moon. The ancients had argued that the earth can’t move, because the stars never shift over the year; the shift is real but too small for the eye, and Friedrich Bessel first measured it in 1838. Around 1900, spectrometers, cathode-ray tubes, and detectors of radioactivity reached down to atoms and electrons. At that edge, once again, the old theory failed and the old paradigm had to be rebuilt. Microelectronics came afterward, built on the new theory.
Two images help. The first compares a slide with a staircase. Going down a slide, your height changes smoothly; going down stairs, it drops in steps. Quantum means steps: inside an atom, for example, energy comes only in certain amounts. The second is about scale. Lie on the ground while a line of ants walks over your arm, and you feel only a vague tickle, a continuous stream. Let a line of elephants walk over you instead, supposing you survive, and you’ll feel every single footstep: down, up, down. Many things in nature come in steps. We just couldn’t feel them until our perception became fine enough.
Some questions can’t even be asked until the scale of perception grows. In this course’s example, as long as heaven was pictured as round and earth as square, nobody could ask why people on the other side of the earth don’t fall off. Only someone who knew the earth was a ball could ask that.
Change your paradigm in time
There is a lesson here about careers. In the last great revolution in physics, the sun-centered cosmos, Giordano Bruno was burned at the stake in 1600, though mainly for theological heresies, and Galileo was forced to fall silent. This time was different. Einstein, Bohr, Heisenberg, and Schrödinger did their key work young, many in their twenties and thirties, and were celebrated as the smartest people alive. Heisenberg created matrix mechanics at 23. And many second-tier physicists, in this reading, did first-rate work because they changed their paradigm early, while some brilliant physicists who stayed with classical physics found only minor problems left to work on.
If you can recognize when you’ve reached the edge of your own scale of perception, and rebuild your first principles in time, this course argues, you gain the advantage. In business, Andy Grove, who led Intel, called such a moment a “strategic inflection point” and argued for moving while the old business still works. In 1985, with Japanese rivals undercutting its prices, Intel left memory chips, its founding business, for microprocessors. The turn cost a year of losses and thousands of jobs, and Grove later admitted that they had moved slowly. In the long run the bet paid off.
Two quantum revolutions
The twentieth century brought the first quantum revolution. It began with discovery: Planck, Einstein, Bohr, and the famous debates around the 1927 Solvay conference in Brussels. It ended by transforming technology, with nuclear energy, the transistor that underlies modern electronics, the laser, and precise standards of measurement such as the quantum Hall effect. The first revolution was about understanding the quantum world and testing that understanding. The second, now under way, is about controlling individual quantum systems to compute, communicate, and sense.
In this reading, seen from China, the country came late to the first quantum revolution and caught only the tail end of the computing revolution, though Chinese physicists helped test the new theory: Ye Qisun (叶企孙) remeasured Planck’s constant in 1921, and Wu Youxun (吴有训) helped confirm the Compton effect. The hope, as this course puts it, is that this time, with its present strength, China will master the core technologies of the second quantum revolution.
What physics teaches
Now look back over the whole story. Physics began with forces you could see and touch. Then came forces you could feel but not see, then fields and energy you could only imagine, then laws of laws, and finally quantum theory, which even the imagination struggles to grasp. In this course’s telling, only the most gifted can build the models, and the rest of us follow. What never changed was the search for deeper, more general, simpler, more beautiful principles. This course hopes that anyone, in any profession, can borrow from physics’ clear and objective way of thinking to widen their own paradigm.
Physics stripped the world of Aristotle’s purposes and of theology. Yet great physicists like Einstein and Planck kept something like a religious feeling. It wasn’t reverence for a god who answers prayers, but awe before ultimate laws. Compared with prehistoric people, this course suggests, we have a few more tools and a few more laws, but before nature we may not be so different. The last word goes to Einstein. In 1936 a sixth-grader named Phyllis, from a Sunday school in New York, asked him whether scientists pray. Not in the usual sense, he replied, since a scientist sees every event as following natural laws. Yet anyone seriously involved in science, he added, comes to feel humble, convinced that
…some spirit is manifest in the laws of the universe…
Albert Einstein, letter to Phyllis Wright, January 24, 1936, in the translation edited by Alice Calaprice (2002)
What does this course mean by the “scale of perception”?
Closer to home
Much of the first quantum revolution’s technology was born in the United States. The transistor was invented at Bell Labs in New Jersey in 1947, and the first laser was built at Hughes Research Laboratories in Malibu in 1960. Andy Grove, a Hungarian refugee who arrived in New York in 1957, built Intel into the company whose microprocessors run on transistors. In the 1950s, as Course 1 told, Chinese-born physicists working in America, Tsung-Dao Lee, Chen-Ning Yang, and Chien-Shiung Wu, made some of the century’s deepest discoveries about the quantum world. The transistor also has a quieter sequel: in 1966 one of its inventors, William Shockley, published a physics textbook, Mechanics, with Walter Gong, a son of Chinese immigrants who taught science at San Jose State. Gong’s Learner/Teacher method made every student a teacher, so that teaching became a way of learning.
The second quantum revolution is being contested openly. China launched Micius, the world’s first quantum communication satellite, in 2016. Since then Google (2024) and a team in Hefei (2025) have shown that adding qubits to an error-correcting code can make it more reliable, not less. Every claim to have beaten the best ordinary computers has been challenged, IBM’s of July 2026 within weeks. Governments have begun replacing the encryption a future quantum computer could break. As of October 2026, no large, useful, error-corrected quantum computer exists.
Another view
Two cautions. First, decoherence explains why a dead cat and a live one never show quantum interference, but not why a measurement gives one result rather than the other, and physicists still disagree about that. No mind is needed: the idea that consciousness causes collapse was always a minority view, and Eugene Wigner, its best-known champion, later gave it up. Second, the scale of perception is this course’s own idea, akin to Gerhard Vollmer’s “mesocosm” and Richard Dawkins’s “Middle World,” and quantum behavior is not only a matter of size: superconductors and lasers are quantum on a large scale.
What to keep
- Large objects become entangled with their surroundings and lose their quantum behavior almost instantly (decoherence), so we never see a cat both dead and alive; no observer’s mind is needed. The best isolated circuits keep it for about a millisecond (as of 2025).
- The scale of perception, this course’s idea: new phenomena, and new worldviews, appear at the edges of what we can measure. Recognize when you’ve reached your edge, and change your first principles in time.
- The first quantum revolution was about understanding the quantum world and gave us transistors and lasers. The second is about controlling quantum systems; as of October 2026 it has not yet produced a large, useful, error-corrected quantum computer.
- Over its history, physics moved from forces you could touch to laws of laws, always seeking simpler and deeper principles.
Think it over
Your notes are saved in this browser.
Sources and further reading
- William Shockley and Walter A. Gong, Mechanics (Columbus, Ohio: Charles E. Merrill Books, 1966); Susan Peterson Gong, Learning Teaching for Exponential Growth: A Three Person Problem (Brigham Young University, 2002), on Walter Gong’s Learner/Teacher method.
- Erwin Schrödinger, “The Present Situation in Quantum Mechanics,” 1935; trans. John D. Trimmer, Proceedings of the American Philosophical Society 124, 1980.
- Wojciech H. Zurek, “Decoherence, Einselection, and the Quantum Origins of the Classical,” Reviews of Modern Physics 75 (2003): 715, arXiv:quant-ph/0105127; Guido Bacciagaluppi, “The Role of Decoherence in Quantum Mechanics,” Stanford Encyclopedia of Philosophy (plato.stanford.edu).
- C. Monroe, D. M. Meekhof, B. E. King, and D. J. Wineland, “A ‘Schrödinger Cat’ Superposition State of an Atom,” Science 272 (1996): 1131; Marius Bild et al., “Schrödinger Cat States of a 16-Microgram Mechanical Oscillator,” Science, April 2023, and ETH Zurich news, April 2023 (ethz.ch); Pedalino et al., Nature 649 (2026): 866, on clusters of more than 7,000 sodium atoms (arXiv:2507.21211).
- Royal Swedish Academy of Sciences, press release for the Nobel Prize in Physics 2025, to John Clarke, Michel H. Devoret, and John M. Martinis (kva.se); Matthew P. Bland et al. (Princeton University), “Millisecond Lifetimes and Coherence Times in 2D Transmon Qubits,” Nature 647 (2025): 343, published November 5, 2025; Fernando Ardana-Lamas et al., “Brilliant Source of 19.2-Attosecond Soft X-ray Pulses below the Atomic Unit of Time,” Ultrafast Science 5 (2025): 0128 (ICFO, Barcelona); Jingsong Gao et al., “Bright Isolated Light Pulses Reaching 18 Attoseconds from High-Order Harmonic Generation by Post-Compressed Yb Lasers,” Nature Communications 17 (2026): 8810 (Kansas State University).
- IonQ, company history (ionq.com/company), for its 2015 founding by Chris Monroe and Jungsang Kim on licenses from the University of Maryland and Duke; IonQ, “IonQ Raises $20M Series B Round Led by NEA, GV,” press release, July 25, 2017, for the $2 million invested in 2016; IonQ, “IonQ Becomes First Publicly Traded, Pure-Play Quantum Computing Company,” press release, October 1, 2021, for its listing on the New York Stock Exchange that day (ionq.com/news).
- F. W. Bessel, “On the Parallax of 61 Cygni,” Monthly Notices of the Royal Astronomical Society 4 (1838): 152–161, his letter to John Herschel of October 23, 1838.
- Gerhard Vollmer, Evolutionäre Erkenntnistheorie, Hirzel, 1975, on the “mesocosm”; Richard Dawkins, “Why the Universe Seems So Strange” (also known as “Queerer than We Can Suppose”), TED talk, TEDGlobal 2005, on the “Middle World.”
- David Bohm, Wholeness and the Implicate Order, Routledge, 1980.
- Andrew S. Grove, Only the Paranoid Survive, Currency Doubleday, 1996; Richard Rumelt on Intel’s 1985–86 figures (rumelt.substack.com); Intel, press release on Andrew S. Grove (1936–2016), 2016 (intc.com).
- William Duane, H. H. Palmer, and Chi-Sun Yeh (Ye Qisun), “A Remeasurement of the Radiation Constant, h, by Means of X-Rays,” Proceedings of the National Academy of Sciences 7 (1921).
- Jonathan P. Dowling and Gerard J. Milburn, “Quantum Technology: The Second Quantum Revolution,” Philosophical Transactions of the Royal Society A 361, 2003.
- Google Quantum AI and collaborators, “Quantum Error Correction below the Surface Code Threshold,” Nature, December 2024 (arXiv:2408.13687); University of Science and Technology of China, Zuchongzhi 3.2 below-threshold error correction, Physical Review Letters 135, 260601 (December 2025); IBM, press release of July 30, 2026, on its claim of quantum advantage (newsroom.ibm.com); Hidetaka Manabe, Hanfeng Gu, and Feng Pan, “Classical Simulation and Design Frontiers for IBM’s Doped Clifford Sampling Experiment,” arXiv:2608.13110 (August 2026); National Institute of Standards and Technology, post-quantum cryptography standards FIPS 203, 204, and 205 (August 13, 2024).
- Albert Einstein to Phyllis Wright, January 24, 1936, in Dear Professor Einstein: Albert Einstein’s Letters to and from Children, ed. Alice Calaprice, Prometheus Books, 2002; Quote Investigator, December 16, 2011 (quoteinvestigator.com), on the letter’s translations.