History of Physics: When Quantum Mechanics Became Huge

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Both the physics and the methods used The list of properties that make up a References for this investigation5 lean very much on superfluid is much longer than reported here 1. Kapitza, P. Nature 141, 74 (1938). 2. Allen, J. F. & Misener, A. D. Nature prior works on polaritons in inorganic though. The out-of-thermal-equilibrium 141, 75 (1938). 4 semiconductors, where scattering-free flow and driven-dissipative nature of polaritons 3. Raman, C. et al. Phys. Rev. Lett. and vortex–antivortex nucleation9 around also sets them apart from other quantum 83, 2502–2505 (1999). a structural defect were observed. The key fluids like liquid helium and ultracold 4. Amo, A. et al. Nat. Phys. 5, 805–810 (2009). 10 5. Lerario, G. et al. Nat. Phys. 13, 837–841 (2017). novelty is that it is now possible to study atomic gases. And it is arguable to what 6. Plumhof, J. D., Stöferle, T., Mai, L., Scherf, U. & Mahrt, R. F. such features at room temperature and in extend they are real superfluids, if only a Nat. Mater. 13, 247–252 (2014). a completely amorphous solid-state system certain subset of superfluid-like behaviour 7. Daskalakis, K. S., Maier, S. A., Murray, R. & Kéna-Cohen, S. Nat. Mater. 13, 271–278 (2014). with comparably high disorder. is observed. Nevertheless, because of the 8. Pitaevskii, L. & Stringari, S. Bose–Einstein Condensation However, the transition to the superfluid- temperature involved, these results will (Clarendon, 2003). like regime here is not sharp, which the certainly stimulate debate. ❐ 9. Nardin, G. et al. Nat. Phys. 7, 635–641 (2011). authors largely attribute to the fact that 10. Keeling, J. & Berloff, N. G. Nature 457, 273–274 (2009). their measurements integrate over a whole Thilo Stöferle is at IBM Research – Zurich, excitation pulse, taking the system from the Säumerstrasse 4, 8803 Rüschlikon, Switzerland. normal to the superfluid regime and back. e-mail: [email protected] Published online: 5 June 2017

HISTORY OF PHYSICS When quantum mechanics became huge

As the age of quantum technologies or the goal of a pre-set agenda”. As a dawns, we’ve grown accustomed to the key moment she identifies London’s idea that quantum-mechanical effects can attendance of an international conference be apparent in practical devices. But long in Amsterdam in 1937, on the centenary before the art of isolating, manipulating of Johannes Diderik van der Waals’s and measuring single quantum entities had birth. Discussion points at the meeting been mastered, the concept of quantum included the λ-transition in helium, as phenomena on the macroscopic scale was well as George Uhlenbeck’s retraction of well established for ensembles displaying his earlier criticisms of Albert Einstein’s collective behaviour, in particular work on boson condensation. London superfluidity and superconductivity. then worked to reformulate Bose–Einstein The ground-breaking realization that condensation — which had, he felt, macroscopic wavefunctions underlie these gained the reputation “of having only effects was introduced by Fritz London a purely imaginary existence” — and (pictured) in 1946. Daniela Monaldi now later connected it to the λ-transition offers a fresh and insightful reconstruction (convinced, after strong initial scepticism, of how London arrived at this unanticipated by László Tisza). concept, and explains its inception Monaldi sees this phase in London’s as a synthesis of ideas assembled career as the turning point in his throughout a career characterized by formulation of concepts that eventually EMILIO SEGRE VISUAL ARCHIVES/AMERICAN INSTITUTE ARCHIVES/AMERICAN VISUAL EMILIO SEGRE LIBRARY PHOTO OF PHYSICS/SCIENCE many changes (Stud. Hist. Philos. M. P. unified superconductors, superfluid helium http://doi.org/cbsk; 2017). occupied him until his untimely death in and Bose–Einstein condensation, to show The list of London’s seminal 1954, at age 54. But London’s life was also that quantum mechanisms are not confined contributions is long and varied, from one of constant dislocation: he was forced to the microscopic realm, but can “directly the quantum-mechanical bond theory of to leave Germany following the rise of the reach into the macroscopic world”. London the hydrogen molecule, formulated with Nazis, moving to Oxford and later Paris, finally presented his idea in July 1946 at Walter Heitler, to the phenomenological before emigrating to the US in 1939. a conference in Cambridge, UK, the first relation between current density and Against this backdrop of changes, major international physics meeting after magnetic field in superconductors, Monaldi describes the emergence the war — reconnecting with the broader developed with his younger brother Heinz. of the idea of macroscopic quantum community from across the oceans. He later worked on measurement theory, mechanisms as the “layered result of a macromolecules and superfluidity, which slow realization, not an early intuition ANDREAS TRABESINGER