Quantum clocks

Having the right time is unbelievably important for technologies that involve positioning and navigation such as global positioning systems and space navigation or for synchronisation in the world of high-frequency frenzied financial trading. To get it all exactly right, an international team of physicists is proposing a network of atomic clocks linked at the quantum level, which they say would be more accurate and stable than any individual atomic clock on Earth.

Based on a combination of precision metrology (the science of measurement) and quantum networks (atoms entangled at the subatomic level), physics graduate student Peter Kómár, Dr Eric Kessler and Professor Mikhail Lukin from Harvard University in the US, along with colleagues from the University of Colorado, Yale, and the Niels Bohr Institute, Denmark, show that, in theory, a network of atomic clocks sharing quantum entanglement, if distributed around the Earth and on satellites, could maintain and synchronise time standards across multiple parties in real time – a true real-time world clock rather than, as at present, each timekeeping institute having their own clocks that send time signals to the International Bureau of Weights and Measures in Paris, France, which averages them monthly (a so-called paper clock – obviously not in real time) to create the world time standard.

“Furthermore, the internal structure of the network, combined with quantum communication techniques, guarantees security both from internal and external threats,” write the researchers in their published paper. The types of threats the researchers refer to include eavesdropping and deliberate sabotage.

Quantum entanglement, which Einstein described as “spooky action at a distance”, occurs when two or more particles or systems of particles behave in a way that is influenced by the other, even when the systems are separated in space. This means that measurements taken of the quantum state in one system seem to be instantaneously proportional in another. Scientists can now prepare (entangle) various particles or sets of particles in single quantum states. For example, if two particles result from the decay of a single particle with a quantum spin of 0, the two new particles when they move in opposite directions will have a spin of -½ and ½ respectively to conserve the original quantum spin of 0. Even when the particles are separated, they are still entangled in this state of conservation. Physicists have different theories about why this occurs.

A lot of the work for such a network is still theoretical, although some parts of the system have been proven. Scientists have, for example, entangled as many as 14 atoms in a quantum system. Precise atomic clocks, based on measurements of superfast fluctuations in energy states of an atom’s electrons as they jump from one energy level to another, also exist.

The latest standard clock recently commissioned in the US uses caesium atoms, which emit microwaves at such a precise rate per second that the clock is reported to not lose or gain a second in some 300 million years. Apparently, we can do better. The research team reports in their paper that a global quantum clock network, all ticking in unison, would be about 100 times more precise than any individual clock.

The researchers write that a network of such clocks could have important scientific, technological and social applications. “Besides creating a world platform for time and frequency metrology, such a network may find important applications in a range of technological advances for Earth science, precise navigation of autonomous vehicles and space probes (requiring a high refresh rate) and in the testing of and search for fundamental laws of nature, including relativity and the connection between quantum and gravitational physics.”

The research was published in the August 2014 edition of the science journal Nature Physics.