Atomic, Molecular and Optical Physics
Single atoms, molecules and light, controlled with lasers at the highest precision, and ultracold gases used to imitate other systems.
arXiv: physics.atom-ph, physics.optics, cond-mat.quant-gas
26 topics
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In a strongly disordered material, waves scattered along many paths can interfere so that they stop spreading and stay trapped, an effect called Anderson localization. In three dimensions this needs disorder above a threshold, and for light it has never been shown beyond doubt.
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Light pulses lasting a few attoseconds (billionths of a billionth of a second) can time how electrons leave atoms, move inside molecules, and respond to strong laser fields in solids. These experiments test whether our quantum calculations of electron motion are correct on the fastest time scales that exist in chemistry.
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Simple atoms can be both measured and calculated to more than ten digits, so any mismatch reveals either a mistake or new physics. Results on the proton size, the Rydberg constant (the basic energy unit of atomic levels), and helium and positronium energy levels test this.
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Casimir and van der Waals forces
5 problemsQuantum fluctuations of the electromagnetic field make neutral objects attract each other, from atoms and molecules (van der Waals force) to metal plates a micron apart (Casimir force). Precision measurements and calculations of these forces still disagree in several places.
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Photons normally pass through each other, but inside mirrors or atomic clouds they can be made to collide and organize. Atoms in cavities also feel forces from shared light, creating long-range interacting matter.
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Atoms close together emit light as a team, sometimes in a fast burst (superradiance) and sometimes holding it far longer (subradiance). The same teamwork can turn a cloud of atoms into a laser that needs almost no mirror stability.
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Numbers like $\alpha$ and the proton-to-electron mass ratio are assumed fixed, but many theories let them drift slowly or differ across the universe. Comparing atomic clocks over years and reading light from distant quasars tests this.
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Atoms such as dysprosium act like tiny bar magnets, and a gas of them can form self-holding droplets and supersolids, states that are crystals and frictionless fluids at the same time. Molecules with electric dipoles now join these magnetic atoms.
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Efimov states and three-body universality
6 problemsThree atoms that barely interact in pairs can still bind into an infinite ladder of larger and larger molecules, each about 22.7 times bigger than the last for identical atoms. Experiments found that where the ladder starts is set by a simple atomic length, and why is only partly understood.
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An electron, neutron or nucleus with a built-in electric dipole (a slight separation of its charge along its spin) would break the symmetry between time running forward and backward. Experiments with molecules, atoms and slow neutrons look for this tiny effect.
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The fine-structure constant $\alpha$ (about $1/137$) sets the strength of electric forces, and it can be measured in several independent ways. The best measurements, made with atoms recoiling from laser light and with the magnetism of a single trapped electron, should all agree, but some do not.
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Fermionic atoms in a grid of laser light hop between sites and repel each other, copying the simplest model believed to describe copper-oxide superconductors. Microscopes that image every atom let experimenters read off magnetism, holes and pairing directly.
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Physics is believed to look the same in every direction and at every speed, and antimatter is believed to be an exact mirror image of matter. Precision measurements on atoms, antihydrogen and trapped antiprotons test both ideas.
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Light can push and cool tiny mirrors, membranes and floating glass beads until their motion is quantum. The aim is to put ever heavier objects into superpositions of two places at once.
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Parity violation in chiral molecules
5 problemsThe weak nuclear force does not treat left and right equally, so the two mirror-image forms of a chiral molecule should have very slightly different energies. This difference is predicted to be tiny and has never been measured.
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Muonic atoms and muonium spectroscopy
3 problemsA muon is a heavy cousin of the electron; when it replaces an electron in an atom it orbits about 200 times closer to the nucleus, and when it replaces a proton (as an antimuon) it makes muonium, an atom with no proton or neutron inside. These atoms test the theory of light and charges and measure nuclear sizes in ways ordinary atoms cannot.
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Measuring Newton's gravitational constant G
3 problemsG sets how strongly masses attract, and it is the least precisely known fundamental constant. Careful experiments disagree with each other by far more than their stated errors.
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Entangled atoms or photons can in principle measure a quantity with error falling as $1/N$ instead of $1/\sqrt{N}$ for $N$ particles. Noise usually destroys this gain, and it is not fully known when and how it can be kept.
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Atoms that are driven by lasers and lose energy or particles to their surroundings never settle into ordinary equilibrium. Engineered loss can instead create new steady states, phase transitions and one-way (non-Hermitian) behaviour that closed systems cannot show.
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The second is defined by a microwave transition in cesium, but clocks using optical transitions are now about 100 times more accurate. Choosing what replaces cesium, and pushing clocks further, involves both physics and international agreement.
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Einstein's equivalence principle says all objects fall the same way, whatever they are made of. Atom interferometers test it with matter waves and atoms in quantum superpositions, and antimatter experiments test whether antiatoms fall like ordinary atoms.
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Single atoms held in laser tweezers can be placed in any pattern and excited to giant Rydberg states that block their neighbours. This makes programmable magnets, gauge theories and unusual states of matter.
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Analog quantum simulators copy one physical system with another, without error correction. Once they outrun classical computers, a method is needed to tell whether their answers are right.
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Molecules can now be cooled to a millionth of a degree above absolute zero, where chemical reactions follow quantum rules. Their electric dipoles also make them strongly interacting quantum gases.
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If dark matter is made of extremely light particles, it behaves like a wave that makes the constants of nature or atomic spins wobble slightly, which clocks and magnetometers can detect. The same instruments look for new forces between electrons, protons and neutrons.
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Two-component fermionic atoms tuned to the strongest interaction allowed by quantum mechanics form a nearly perfect fluid. How viscous it is, how fast spin spreads and whether atoms pair above the superfluid temperature set benchmarks that also apply to quark matter and strange metals.