Quantum Sensing & Metrology
The world is quantum!
Just often we don't realize it in normal life. This is because quantum systems are usually too tiny and are often destroyed in their key properties or their quantum-ness by even the subtlest changes in the environment. Fortunately, quantum control allows one to turn this around and use the quantum systems to realize extremely sensitive quantum sensing devices. However, certain realizations or properties of quantum systems are very immune to the environment or universal and are hence ideally suited to perform quantum metrology of fundamental constants or time itself. Another quantum paradox: On the one hand, quantum systems can be used to sense the tiniest variations of the environment while, on the other hand, they are so little influenced by the environment that quantum systems can serve as standards.
Quantum sensing or quantum metrology?
Quantum sensing usually refers to the application of quantum systems or quantum methods to detect weak signals and monitor them. Typical examples are magnetic, electric and electromagnetic fields, isotope ratios, acceleration, rotation, and gravitation. Quantum metrology is usually used in a wider sense, comprising additionally quantum-enhanced measurements of physical quantities or fundamental constants. Again, this can either be accomplished using specific quantum systems and applying quantum methods, such as quantum logic or entanglement. Typical quantum metrology use cases are measurements of time and frequency or of fundamental constants such as the fine structure constant, Newton's gravitational constant, the Rydberg constant or ratios of the Planck constant over atomic masses (e.g. h/mCs).
Lasers empower quantum sensing and metrology
Lasers are the fundamental tool in quantum sensing and metrology. They are used to prepare, precisely control and manipulate quantum systems like atoms, ions, color centers, quantum dots, and many more. Lasers are applied to read out measurement results with unprecedented precision and are essential for cooling and trapping of quantum particles. They are indispensable for optical quantum clocks (like atomic clocks or ion clocks) and highly sensitive quantum detectors. Tunable diode lasers, fiber amplifiers, optical frequency combs, and wavelength meters, ideally combined in laser rack systems as complete solutions, are the most relevant products for this application area.
Probably the most well-known quantum metrology application is the measurement of time and frequency with optical quantum clocks. Harnessing the full power of quantum physics and ultra-stable lasers, optical quantum clocks are redefining how we measure time, with implications for navigation, communications, scientific discovery and the very definition of the second. Achieving relative precisions and accuracies on the order of 1 part in 1,000,000,000,000,000,000 (1018) in scientific laboratories of academic research facilities or national metrology institutes, first optical quantum clocks are now commercially available.
Atomic quantum systems can be used to improve measurements of electric fields from DC to THz using the effect that the energy of highly excited electronic states in atoms, so-called Rydberg states, is strongly shifted by electric fields. High resolution laser spectroscopy with typically one to three tunable narrow linewidth lasers allows one to measure these shifts and be used to quantify the applied electric fields. Besides the high resolution, atomic Rydberg sensing offers additional advantages such as compact measuring devices, low distortion of the electric field by the sensor, and very large tuning range of the rf frequency band that shall be measured.
Quantum magnetometers (QM) are one of the closest-to-market applied quantum technologies and have already started to be commercially exploited. Industrial QM versions use color centers in diamond (typ. NV centers) or atoms (typ. Cs, Rb, or K) in gas cells. Scientific systems explore other atoms as well. Applications of QM comprise among others medical imaging, navigation, and measurements of electric currents in technological systems or components.
In quantum metrology networks, time distribution delivers precise time tags and synchronization to align measurements across distant nodes. Frequency distribution, by contrast, transfers an ultra-stable, phase-coherent reference - typically derived from an optical clock - over stabilized fiber links using ultra-stable lasers and active fiber noise cancellation. At each node, an optical frequency comb coherently distributes this stability to multiple wavelengths required by local experiments. Together, these capabilities enable clock networks, relativistic geodesy, quantum communication testbeds, and tests of fundamental physics at unprecedented precision.