Turning down quantum noise in next-generation atomic clocks
Physicists at the Massachusetts Institute of Technology (MIT), USA, improve the stability of next-generation atomic clocks.
These devices rely on faster-ticking atoms of elements such as ytterbium, tracked using higher, optical-frequency lasers. They can track finer intervals of time – up to 100 trillion ticks per second.
The researchers have stabilised the clocks by reducing ‘quantum noise’, a fundamental measurement limitation that obscures the atoms’ pure oscillations.
The MIT method, which researchers call ‘global-phase spectroscopy’, harnesses a laser-induced ‘global phase’ in ytterbium atoms. The effect can be boosted using a quantum-amplification technique.
Today’s atomic clocks work by using lasers oscillating in sync at microwave frequencies to track the ‘ticking’ of caesium atoms – over 10 billion ticks per second – and are the foundation of digital processes like online transactions and navigation.
The new approach reportedly doubles the precision of clocks based instead on optical frequency, enabling them to discern twice as many ticks per second. The team anticipates that the precision of the method should increase steadily with the number of atoms in a clock.
This builds on earlier research, when the same team demonstrated how quantum entanglement of the atoms could make atomic clocks more precise. When atoms are quantum entangled, they redistribute any noise (or uncertainty) in measuring the atoms’ oscillations for a clearer, more measurable tick.
They also devised a solution to the ‘ticking instability’ in the clock’s laser, by finding a way to amplify the difference in the laser-versus-atom tick rates using ‘time reversal’. This involves boosting the signal by entangling and de-entangling atoms.
However, they were still using traditional microwaves, which oscillate at lower frequencies than the optical frequency standards that ytterbium can provide.
The team has now applied their time-reversal method to optical atomic clocks, sending in a laser that oscillates near the optical frequency of the entangled atoms.
‘The laser ultimately inherits the ticking of the atoms,’ says first author Leon Zaporski. ‘But in order for this inheritance to hold for a long time, the laser has to be quite stable.’
To stabilise the optical laser, researchers took advantage of a phenomenon where light is sent through entangled atoms, causing them to jump up in energy and then fall back to their original energy state, retaining a ‘memory’ of their ‘round trip’, thereby creating a global phase.
Professor Vladan Vuletić at MIT says, ‘[The global phase of the atoms] is usually considered irrelevant. But [it] contains information about the laser frequency.’ They realised the laser induces a measurable change in the atoms, and the magnitude of this change depends on the laser’s frequency.
‘Ultimately, we are looking for the difference of laser frequency and the atomic transition frequency,’ explains Co-author Qi Liu. ‘When that difference is small, it gets drowned by quantum noise.’
Their method amplifies this difference above the quantum noise, allowing them to ‘resolve nearly twice as small a difference in the optical frequency, or the clock ticking frequency, without running into the quantum noise limit’, according to Zaporski.
The team envisions that their global-phase spectroscopy technique could eventually enable precise and portable atomic clocks for measuring a multitude of physical phenomena, including predicting earthquakes.
