Expanding the Limits of Measurement Accuracy

Scheme of the new method based on a non-classical state adapted to two measurement parameters at once. This will enable precision measurements of molecules which could reveal interactions between conventional and dark matter. (Source: F: Wolf)

For centuries, humans have been expanding their under­standing of the world through more and more precise measurement of light and matter. Today, quantum sensors achieve extremely accurate results. An example of this is the development of atomic clocks, which are expected to neither gain nor lose more than a second in thirty billion years. Gravi­tational waves were detected via quantum sensors as well, in this case by using optical inter­ferometers.

Quantum sensors can reach sensi­tivities that are impossible according to the laws of conventional physics that govern everyday life. Those levels of sensi­tivity can only be reached if one enters the world of quantum mechanics with its fasci­nating properties – such as the phenomenon of super­position, where objects can be in two places at once and where an atom can have two different energy levels at the same time. Both generating and controlling such non-classical states is extremely complex. Due to the high level of sensi­tivity required, these measure­ments are prone to external inter­ference. Furthermore, non-classical states must be adapted to a specific measurement parameter.

“Unfor­tunately, this often results in increased inaccuracy regarding other relevant measurement parameters”, says Fabian Wolf. This concept is closely linked to Heisenberg’s uncertainty principle. Wolf is part of a team of researchers from Leibniz University Hannover, Physika­lisch-Technische Bundes­anstalt in Braunschweig, and the National Institute of Optics in Florence. The team introduced a method based on a non-classical state adapted to two measurement parameters at once. The experiment can be visualised as the quantum mechanical version of a simple pendulum.

In this case, the adapted measurement parameters are the pendulum’s maximum displace­ment (amplitude) and the number of oscil­lations per second (frequency). The pendulum comprises a single magnesium ion embedded into an ion trap. Via laser light inter­actions, researchers were able to cool the magnesium ion to the ground state of a quantum mechanical system, the coldest achievable state. From there, they generated a Fock state of the motion and oscillated the single atom pendulum using an external force. This allowed them to measure amplitude and frequency with a sensi­tivity unmatched by a conven­tional pendulum. In contrast to previous experiments, this was the case for both measurement para­meters without having to adjust the non-classical state.

Using this new approach, the team reduced the measurement time by half while the resolution remained constant or doubled the resolution with a constant measurement time. High resolution is parti­cularly important for spectro­scopy techniques based on changing the state of motion. In this particular case, researchers intend to analyse individual molecular ions via laser irra­diation in order to stimulate molecular movement.

The new procedure will enable them to analyse the state of the molecule before it is disrupted by too intense laser irradiation. “For example, precision measure­ments of molecules could reveal inter­actions between conventional and dark matter, which would be a great contri­bution to solving one of the biggest mysteries in contem­porary physics”, says Fabian Wolf. The measurement concept, which researchers demon­strated for the first time, could also improve the resolution in optical inter­ferometers such as gravi­tational wave detectors – therefore providing more in-depth insights into the dawn of the universe. (Source: PTB)

Reference: F. Wolf et al.: Motional Fock states for quantum-enhanced amplitude and phase measurements with trapped ions, Nat. Commun. 10, 2929 (2019); DOI: 10.1038/s41467-019-10576-4

Link: Institute for Experimental Quantum Metrology (P.O. Schmidt), Physikalisch-Technische Bundesanstalt PTB, Braunschweig, Germany

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