Schrödinger's cat improves atomic clocks – entangled, overlapping atoms could make optical time measurement more accurate

Using quantum tricks to reach the quantum limit

But optical atomic clocks are also reaching their limits: since their measuring instruments are atoms, quantum physical phenomena such as quantum fluctuations and the Heisenberg Uncertainty Principle limit the measurement accuracy that can be achieved – atomic clocks are reaching their quantum limit. The exact value of the “ticking frequency” can only be determined after a long period of measurement because individual measurements always differ slightly from each other.

Now American physicists have managed to overcome this quantum limit of optical atomic clocks. To do this, they combined laser-based time measurement with techniques already used in quantum computers and quantum gates. “Interfering programmable atomic arrangements with optical atomic clocks provides a new way to reach measurement accuracy to the Heisenberg limit,” explains the team led by Alec Kao of the Gila Institute at the University of Colorado and the National Institute of Standards and Technology. nest).

Entanglement of enlarged atoms

The first “trick” is to bond the clock's atoms together. This quantum physical coupling makes atoms less susceptible to disruptive quantum effects, because they do not interact individually, but in a group, Cao and his colleagues explain. In the experiment, they achieved this by using laser pulses to convert strontium atoms trapped in the laser grating into Rydberg atoms — atoms with greatly enlarged electron orbitals.

“These enlarged orbitals bring the atoms closer together than usual,” explains lead author Adam Kaufman of JILA. “This allows the electrons of neighboring atoms to feel each other, so to speak, and this leads to a strong interaction between them.” In the experiment, physicists created groups of two, four, or eight clock atoms entangled with each other.

Schrödinger's cat with clock atoms

Then comes the second “trick”: quantum superposition. In this quantum phenomenon, made famous by the “Schrödinger's cat” thought experiment, the state of the particle remains indeterminate until the measurement terminates the superposition. “The creation and use of Schrödinger cats — coherent superpositions of two macroscopically different quantum states — is particularly interesting for more precise atomic clocks,” say the physicists.

The team created this superposition on entangled strontium-Rydberg atoms and then tested how the “chimes” of these units affected the accuracy of timekeeping. “This represents the first application of such states in an optical atomic clock with neutral atoms, and the first time the performance of the quantum limit has been compared using laser measurement,” explain the physicists.

More accurate and faster than common clocks

Comparisons showed that strontium atoms, manipulated through entanglement and superposition, produced a much more accurate time signal and were less disturbed by quantum noise. “This means we can achieve the same accuracy in a shorter time compared to standard strontium atomic clocks,” explains Kaufman. The clock atoms, optimized by Schrödinger's cat, showed frequency stability that exceeded the previous quantum limit, the team reported.

“Our results demonstrate the basic building blocks of atomic clocks, which allow us to approach the Heisenberg limit for the accuracy of optical atomic clocks,” say Cao and his colleagues. However, their approach still suffers from one major flaw: in the experiment, they were only able to maintain the entanglement and superposition of the clock atoms for about three milliseconds. Although this was sufficient for their measurements, it would not be practical for an optical atomic clock.