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Optical atomic clocks are the most accurate tools available for measuring time and frequency. It is on them that the maintenance of International Atomic Time (TAI) and, by extension, Coordinated Universal Time (UTC) is based. The synchronization of two of these clocks makes it possible to probe the space-time variation of the fundamental constants, but the maneuver lacks precision due to the disturbances generated by the measurements. Physicists at the University of Oxford have found a way around this difficulty through quantum entanglement.
The accuracy of an atomic clock is that it is based on the resonant frequency of atoms – the frequency of electromagnetic radiation emitted by an electron as it passes from one energy level to another – which is by definition immutable. Atomic vibrations are in fact the most stable periodic events that scientists can observe. Their frequency is very precisely measured by means of lasers. Therefore, the second is historically defined as the exact duration of 9,192,631,770 oscillations of the transition between the hyperfine levels of the ground state of the cesium-133 atom.
Even more precise, the optical atomic clocks, developed in the 2000s, are based on atoms whose energy transitions occur at optical frequencies (aluminum, strontium, mercury, etc.). Seconds should also be redefined according to these clocks when they reach maturity. Methods to reliably and accurately compare different optical watches around the world must first be demonstrated. The task is particularly difficult, because their measurement causes disturbances. The researchers then committed to intertwining two optical atomic clocks so that only a single measurement was required.
Measurement uncertainty reduced by a factor of two
As a reminder, the quantum entanglement (or entanglement) of two systems implies that any change in one instantly affects the other. This intrinsic link is therefore likely to facilitate synchronization of watches. ” Measurements on independent systems are limited by the standard quantum limit; measurements on entangled systems can exceed the standard quantum limit to reach the maximum precision allowed by quantum theory: the Heisenberg limit “, Explain the researchers in Nature.
Local entanglement experiments, at microscopic distances, had already shown that the approach allowed to reduce measurement uncertainties and therefore to increase the precision of optical atomic clocks. In 2020, MIT scientists developed a clock that measures the oscillations of entangled atoms (about 350 ytterbium atoms). A first laser was used to quantum entangle the atoms, then a second laser was used to measure their average frequency. They achieved the same accuracy as an untangled atomic clock, but four times faster!
In this new experiment, the team used not one, but two atomic clocks, each made using a single strontium ion (88sr+), two meters away. Using a laser, they excited the strontium ions to emit blue light. This was then directed via an optical fiber into a Bell state analyzer, which designate the states of maximum quantum entanglement of two particles; the two ions were then entangled via a photonic bond.
From then on, measuring one watch immediately gave access to measuring the other. For frequency comparisons between ions, the researchers report an uncertainty of about 7% (compared to 28% if the clocks are not entangled). ” We find that entanglement reduces the measurement uncertainty by almost √2, the predicted value for the Heisenberg limit the researchers write. According to the laws of quantum physics, it is impossible to measure the clock frequency with perfect accuracy, but this experiment shows that it is possible to get close.
Extreme precision that could help solve many mysteries of physics
Current optical clocks are generally limited by the offset of the probe laser. In this experiment, entanglement reduced measurement uncertainty by a factor of 2 compared to conventional correlation spectroscopy techniques, the researchers point out.
” This two-node network could be extended to other nodes, to other species of trapped particles or, through local operations, to larger entangled systems. “, They add. In particular, they cite the possibility of choosing an ion whose transition has a reduced sensitivity to the magnetic field, a narrower line width or a greater sensitivity to fundamental constants. On the other hand, the use of local operations to increase the number of entangled ions in each node could further reduce the measurement uncertainty for frequency comparisons.
If this experiment can be repeated with more distant clocks, such as in two separate laboratories, or with multiple clocks, it could really advance studies of dark matter or gravitational waves. Indeed, a shift of dark matter between the two intertwined clocks, or small changes in the force of gravity, would immediately induce a difference between their “ticking” frequencies.