Illustrated Physics
physics, drawn
Concept

Atom interferometry — where it appears

Splitting an atom's matter wave into two paths with laser pulses and recombining them, so the atoms' arrivals record the phase difference between the paths. Because that phase depends on gravity and rotation, atom interferometers measure g, its gradient and the constant G to parts in a billion.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Two paths through a neutron interferometer at two heights. A neutron interferometer cut from one silicon crystal, tilted by 30 degrees about its incoming beam so that one path runs higher than the other. Left: the two paths, split at the first slab, turned at the second and recombined at the third, enclosing 10.1 square centimetres; the heights are drawn exaggerated. Taken as a rectangle of the same area, the upper path runs 15.8 mm higher for 3.2 cm. There a neutron of wavelength 1.445 Å, moving at 2738 m/s, is slower by 56.5 micrometres per second, so its wavelength is longer by 20.6 parts per thousand million. Over 3.2 cm that accumulates 28.7 radians less phase than the lower leg — 4.6 whole fringes — computed by integrating the local wavenumber along both legs and checked against 2πm²gλA sin α / h².

The fall that leaves the mass in the phase

Every body falls the same way whatever its mass, and a neutron is no exception. But a neutron is also a wave, and the phase that wave accumulates while falling depends on the mass — as its square, at a fixed wavelength. Tilt a neutron interferometer so that one path runs a centimetre higher than the other and the neutrons swing between its two detectors, which in 1975 was the first measurement in which gravity and quantum mechanics both had to be right at once.

quantum · Matter waves The fringes a clock's own ticking washes out. The visibility of the interference fringes of an atom carrying a clock, sent through an interferometer whose two arms differ in height, against the product of height difference and time apart, for three clocks. The upper arm's clock runs faster by gΔh/c², and when the two versions of the clock differ by half a tick they are distinguishable and the fringes vanish: at Δh·T = 10.7 m·s for strontium's optical transition, 2.3 for the thorium-229 nucleus, 670 thousand for a microwave clock. The largest ground interferometers reach about half a metre-second, shaded.

The fringes a clock's own ticking erases

Send an atom through an interferometer whose two paths differ in height, and gravity shifts its fringes, as neutron experiments showed in 1975. Give the atom an internal clock — a superposition of two energy levels that ticks — and something new happens. Proper time runs faster on the upper path, the two halves of the atom come back with different readings, and the fringes fade, because the clock has recorded which way the atom went. For an optical clock the fringes vanish at a height-times-time of about ten metre-seconds, within reach of instruments now being built.

relativity · Simultaneity

Named alongside it

The objects these essays reach for when they reach for this one.

ActionComplementarityEquivalence principleGravimetryGravitational redshiftMatter waveNeutron interferometryPhaseProper timeQuantum interferenceSagnac effectWhich-path information

All concepts