1. CMTC’S first twitter thread was on the top-20 theoretical breakthroughs in physics since 1900. Physics, being the interpretation of the natural universe, is an experimental science by definition #quantum #QuantumComputing #physics
1.1 Here we are providing a list of the top-25 experimental physics discoveries since 1900. To atone for our producing the theory list first, this thread contains 25 experimental discoveries between 1900-2020. Let us know if we missed your favorites.
2. Photoelectric effect (1900, 1902, 1914): Established the quantum nature of light
3. Atomic nucleus (1911): Established that atoms are not indivisible and there is intra-atomic structure 
4. Superconductivity (1911): Showed that metals may lose all electrical resistance at low temperatures, took 46 years to understand theoretically
5. Atomic levels (1914): Established compellingly the existence of quantized atomic levels
6. Gravitational bending of light (1919): Established that gravity affects light, and made Einstein a house-hold name
7. Electron diffraction (1927): Established the wave nature of electrons
8. Quantum spin (1922-27): Established that manifestly non-classical quantum numbers exist. Led to the distinction between fermions and bosons
9. Positron (1932): Established the existence of antiparticles in nature, making Dirac famous—the relative abundance of particles over antiparticles in the accessible universe is still a mystery
10. Neutron (1932): Crucial discovery in understanding hadrons as constituents of atomic nuclei
11. Neutrino (1934-56): Almost massless neutral weakly interacting ever-present fundamental particle, which is still somewhat of a mystery, originally hypothesized in beta decay, took more than 20 years for direct discovery
12. Muon (1936): Unstable fundamental particle discovered in cosmic rays leading to the realization of there being different leptons in nature
13. Superfluidity (1938): A new macroscopic neutral quantum coherent state in helium, reminiscent of superconductivity, but involving bosons
14. Lamb shift and anomalous electron magnetic moment (1947): Led to the field theory of QED, the most successful quantitative theory in physics
15. Parity violation in beta decay (1956): A landmark and completely unexpected discovery that weak interaction violates left-right symmetry.
15.1 The discoverer was strangely left out of the lofty prizes awarded for its discovery and explanation, most likely because she, Madame Wu, was a woman and a minority
16. Antiferromagnetism (1957): A macroscopic coherent long-range ordered quantum state where the spins alternate spatially (instead of being ordered in the same direction as in a ferromagnet)—an example of spontaneous symmetry breaking.
16.1 Which could only be observed because of developments in instrumentation involving neutron diffraction (ferromagnets have been known forever, but antiferromagnets for only about 60 years)
17. Big Bang (1964): How the universe began (and perhaps how it would end) ushered in modern cosmology. Its serendipitous discovery and how its historical prediction was basically ignored remains a stark lesson in the history of science
18. Quarks (1967-95): Although originally thought to be ‘invisible’, it turns out that quarks can be seen experimentally by being creative—the discovery of top quark in 1995 was the last gasp of the US-based experimental high energy physics
19. Integer QHE (1980): A completely unexpected discovery of a topological invariant in a dirty Si transistor, eventually leading to the current official definition and standard for the electrical resistance unit Ohm, truly amazing that a Chern number shows up in a MOSFET
20. Fractional Quantum Hall Effect (1982): Superficially similar to IQHE above but a truly deep phenomenon signifying topological ordering induced by interaction—a new phase of matter with no obvious order parameter and no spontaneous symmetry breaking
21. W, Z bosons (1983): Established the electroweak unification in the standard model showing that photons actually have cousins.
21.1 Started the rise of CERN as the leading high energy physics experimental research center although it took Americans 15 more years to admit it and throw in the towel
22. High-T_c Superconductivity (1986): Showed that it is possible to have SC transition temperature in the 35-120K range in highly resistive oxides, the last place any theorist would have ever suggested looking for high-temperature superconductivity.
22.1 In spite of huge buzz and hype it led to little technological impact and managed to destroy the careers of many young theorists as the underlying explanation became contentious with no consensus even now 35 years after the original discovery
23. Dark Matter (1930s & 80s): Originally hinted at in the 1930s, dark matter (i.e. the fact that more than 90% of the ‘stuff’ forming the universe is missing from astronomical observations) became well-accepted in the 1980s, still remains a fundamental mystery in cosmology
24. Dark Energy (1998): One of the most unexpected and most important recent discoveries in all of science. The universe is apparently expanding faster and faster, but the associated cosmological constant, while being nonzero, has 123 zeros in it before being finite.
24.1 Why should something decide to be nonzero after being zero for 123 decimal places? We have no idea. None. Nada. Theories on this topic are mostly fantasies and science fictions, we really have no idea
25. Higgs Particle (2012): Finally discovered in 2012, almost 50 years after its prediction, in landmark experiments in CERN is a last missing piece in the standard model of particle physics
25.1 Of course we still have no idea why the standard model with its many fine-tuned parameters works so well or where it comes from
26. Gravity Waves (2016): This completely expected discovery in the LIGO collaboration came almost 100 years after its prediction, and the amazing thing here is the incredible cleverness going into detecting very faint signals—this is now leading to a new type of astronomy
Postscript 1: The list only contains discoveries, no inventions (e.g. transistors, lasers, etc.).
Postscript 2: The list stops at 25 for no particular reason, other than the fact that all the fun is gone in making such a ‘top-something’ list if one includes 100 items
Postscript 3: Rather amazingly, and not by any conscious design at all, the list is temporally balanced. There are roughly 2-3 items per decade throughout the whole 120 years
Postscript 4: Even more amazing is the balance among the subjects in the list (again, not by design): 7 in condensed matter, 8 in particle physics, 6 in atomic structure and fundamental quantum physics, and 4 in gravity/cosmology
Postscript 5: In our subjective opinion, the most important physics experimental discoveries of the last 50 years are the fractional quantum Hall effect and the dark energy—both were completely unexpected and revolutionized physics in their own unique ways
