only lasers measure time ( and distance ) accurately.
Nope. Indeed I know of no clock that is based upon a laser
Don't be so pedantic, I meant: Bose-Einstein Condensates,
including "atom lasers" ( coherent atoms ), lasers, and masers.
Early atomic clocks were based on masers at room temperature.
Since 2004, more accurate atomic clocks
first cool the atoms to near absolute zero
temperature by slowing them with lasers and probing
them in atomic fountains in a microwave-filled cavity.
Today, even the kilogram is measured in terms of lasers,
Planck's constant ( h, e = h*frequency ), and "e/c^2 = m".
Not really. The kilogram was recently redefined in terms of various
constants of nature (including c and Planck's).
But lasers are neither part of the definition nor essential.
Japan Times, May 25, 2019:
NEW YORK - Last week the kilogram got a new
definition — one that rests on a quantity of light.
All electrons, protons, neutrons and photons have
perfectly uniform masses and other properties.
They don’t age or get dented or scratched.
And yes, particles of light — photons —
can have a mass under certain circumstances.
Light is massless when it travels the speed of light,
but you can confine photons in a sort of mirrored box,
so that they bounce back and forth but
have an average speed of zero relative to the box.
In that case, the photons indeed have predictable mass.
The box with the photons will have more mass than the box alone.
The energy of a photon is equal to
its frequency times the Planck constant.
Just as scientists had to set the speed of light at a
fixed number in order to define the meter without the
platinum bar, so on Monday they set Planck’s constant
at a fixed value to get rid of the mutable kilogram.
There were two different kinds of devices that had
been used to precisely measure Planck’s constant,
and both can be used to measure the kilogram,
now that Planck’s constant is fixed.
One device, called the Kibble balance, can measure
either the kilogram or Planck’s constant by balancing
weight against electromagnetic forces.
The other apparatus uses a sphere of precisely measured size
— so precise that scientists can approximate the number
of atoms it contains.