Laser Cooling of Antihydrogen.
The paper to which I'll point in this post is this one: Laser cooling of antihydrogen atoms (Baker, C.J., Bertsche, W., Capra, A. et al. Laser cooling of antihydrogen atoms. Nature 592, 35–42 (2021))
Beyond an inordinate interest in neutrons, I'm not much for deeper particle physics. My kid won a medal in the 5th grade science project for a poster on the subject and some of the other parents of kids in the competition accused me of doing the project, but to be perfectly honest, I actually had no idea what he was talking about.
Nevertheless, chemists are aware that on a molecular level life is asymmetric, and one theory that's floated by my consciousness from time to time is the asymmetry of beta decay of neutrons (the Wu experiment) was involved in the mysterious origins of molecular asymmetry. As I understand it, this has been difficult to prove.
I am also vaguely aware that physicists have wondered why the universe seems to have excess matter, that is, why wasn't as much antimatter created as matter was created.
Antimatter can be prepared in the laboratory, and positrons are actually pretty easy to get: Proton bombardment of stable nuclei yields, upon proton capture, neutron poor atoms that decay by positron emission. When positrons interact with electrons, they are annihilated, releasing intense gamma radiation.
The paper cited at the outset is about isolated anti-hydrogen, a hydrogen atom with an anti-proton with a positron (an anti-electron) in its orbital; not only has it been synthesized, but it has been supercooled, which makes this a cool paper or perhaps an anti-cool paper.
Happily, it is open sourced, and anyone can read it by clicking on the link above.
Some excerpts:
The antihydrogen atom, the simplest example of atomic antimatter, offers unique opportunities in challenging the foundational framework of contemporary physics. Precision comparisons of antihydrogen’s properties with those of the well studied hydrogen atom allow tests of fundamental symmetries such as charge–parity–time invariance and Einstein’s equivalence principle, which underpin quantum field theory and the general theory of relativity. The field of antihydrogen studies has seen tremendous advances in recent years. Techniques have been developed to produce15,16,17,18, confine19,20,21 and interrogate cold antimatter atoms with microwaves13,22 and lasers10,11,12,23. In addition, experiments are being built to measure the gravitational properties of antimatter14,24,25. The finite kinetic energies of the anti-atoms impose substantial limitations on the precision of many of these measurements. Therefore, preparation of antihydrogen at the lowest possible kinetic energies is an important objective in the field.
Doppler cooling, the type of laser cooling used in this work, takes place via the velocity-dependent absorption of near-resonant photons by atoms. The atoms moving towards the photon source are selectively excited by setting the photon frequency slightly below the resonance for the atom at rest (red detuning), resulting in a net force opposing the motion2,3. The spontaneous emission of a photon from the excited atom occurs in a spatially symmetric manner in free space, resulting in a null average recoil force. In the case of (anti)hydrogen26, by exciting the 1S–2P Lyman-? transition, a net velocity change of 3.3 m s?1 can be exerted on average by each 121.6-nm photon scattered. In principle, repeating such scatterings only a few dozen times should slow (anti)hydrogen atoms, initially trapped in a well depth of about 50 ?eV (corresponding to a maximum speed of about 90 m s?1), down to submicroelectronvolt energies.
In practice, however, laser cooling of antihydrogen presents a number of technical challenges. First, generating and transporting radiation at 121.6 nm is difficult. There are no convenient lasers or nonlinear crystals at vacuum ultraviolet wavelengths, and the light is readily attenuated in air and in optical components...
A picture:
Fig. 1: The ALPHA-2 apparatus schematic and antihydrogen energy levels.
The caption:
a, Central parts of the ALPHA-2 apparatus are schematically shown. The field for the magnetic minimum trap is produced by five mirror coils for longitudinal confinement and one octupole coil for transverse confinement. The trap has a depth of about 50 ?eV with an axial length of 280 mm and a diameter of 44.35 mm. The magnetic trap is superimposed on a cryogenic Penning trap (the electrodes are shown in yellow). An external solenoid, not shown, provides a 1-T base field for charged particle trapping and cooling. The solenoids at either end of the trap further boost the field in the preparation traps to 3 T for more efficient cyclotron cooling of electrons, positrons (e+) and antiprotons (p¯), before antihydrogen synthesis. The atom trap is surrounded by a silicon vertex annihilation detector made of three layers of double-sided microstrip sensors. The pulsed Lyman-? light at 121.6 nm, generated in a gas cell immediately outside the ultrahigh vacuum chamber, is introduced through a magnesium fluoride window with an angle of 2.3° with respect to the trap axis to allow particle loading on axis into the Penning trap. The intensity of the 121.6-nm pulse is recorded by a solar-blind photomultiplier (PMT) placed after the trap. A cryogenic optical cavity serves to both build up the 243.1-nm laser light needed to drive the 1S–2S transitions, and to provide the counter-propagating photons that cancel the first-order Doppler shift. Microwaves, used to drive hyperfine transitions, and to perform electron cyclotron resonance magnetometry, are injected through the microwave guide. According to the coordinate system shown, we define the longitudinal kinetic energy to be 1/2mHv2z, and the transverse one to be 1/2mH (vx2+vy2), where mH is the mass of antihydrogen, and vx, vy and vz are the velocity components in the x, y and z directions. b, Magnetic field profile on the axis of the trap. The shaded region illustrates a volume in which the field on axis is uniform to 0.01 T, corresponding to a Zeeman shift of 140 MHz in the 1S–2Pa transition. Immediately before reach run, the magnetic field at the centre of the trap was measured via electron cyclotron resonance and the laser frequencies were adjusted accordingly. The measured magnetic minimum field, averaged over the pre-run measurements, was 1.03270 ± 0.00007 T, where the error is the standard deviation from the set of measurements. c, The energy levels of the antihydrogen in the n = 1 and n = 2 states are depicted as a function of the magnetic field. On the vertical axis, the centroid energy difference, E1S–2S = 2.4661 × 1015 Hz, has been suppressed. The dotted vertical black line represents the field at the magnetic minimum of our trap, 1.0327 T (see above). Details of the energy levels near this field and their state labels are shown on the right of the figure. The first value in the ket notation represents the quantum number of the projection of the total angular momentum of the positron, mL + mS, where L is the orbital angular momentum (L = 0 for the S state and L = 1 for the P state, respectively) and S is the spin (S = 1/2). The double arrow shows the antiproton spin (up or down). Initially, both the 1Sc and 1Sd states are trapped in our magnetic trap. The grey arrow indicates the microwave-driven 1Sc ? 1Sb transition to eliminate the anti-atoms in the 1Sc hyperfine state and prepare a doubly spin-polarized antihydrogen sample in the 1Sd state. The solid and broken red (cyan) arrows indicate the cycling transition for laser cooling (heating) with red (blue) detuning ?? (+??
. The purple arrow represents the probe laser excitation to the 2Pc– level. Note that the 2Pc state at a magnetic field of about 1 T is a superposition of the positron spin-up (mL = 0, mS = +1/2) and spin-down (mL = +1, mS = –1/2) states. Owing to this superposition, upon de-excitation from the 2Pc state, the anti-atom can either go back to the original 1Sd state, or undergo an effective ‘spin flip’ transition to the 1Sa state. In the latter case, the anti-atom is forced out of the trap and detected via its annihilation signal. The black arrows show the two-photon excitation from the 1Sd state to the 2Sd state.
Cool/Anti-cool.
Have a nice weekend, and if you are a Christian, happy Easter.
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