"Atomic clocks keep time by monitoring the natural oscillations of atoms as they move between energy states. Each atom oscillates unimaginably fast. Cesium, for instance, vibrates more than 10 billion times every second. By locking lasers (in optical atomic clocks) or microwaves (in "traditional" atomic clocks) to those frequencies, scientists can measure time down to billionths of a second."
"It involves shining laser light through a cloud of entangled atoms and measuring tiny changes in their collective behavior. When the light passes through the atoms, it briefly nudges them to a higher energy state before they fall back again. As they do, the atoms retain a faint "memory" of the interaction, called a global phase. Scientists had long assumed this effect was irrelevant, but the team at MIT discovered that it actually carries useful information about the laser's frequency."
"Using this information, they were able to more precisely stabilize the laser used to measure the atomic clock's ticking, effectively doubling its accuracy. "The laser ultimately inherits the ticking of the atoms," said first study author Leon Zaporski. "But in order for this inheritance to hold for a long time, the laser has to be quite stable.""
A technique called global phase spectroscopy reduces quantum measurement noise in optical atomic clocks by probing entangled atomic ensembles with laser light and reading their collective global phase. Probe light briefly excites atoms, which retain a faint memory — the global phase — that encodes information about the laser frequency. Measuring that collective phase yields a more precise error signal for laser stabilization, allowing the probe laser to inherit atomic oscillations with greater fidelity. Applying this approach doubles clock precision by quieting quantum projection noise, addressing the stability challenge posed by optical clocks' vastly higher operating frequencies compared with microwave clocks.
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