If you buy a kilogram of salmon at the market, you would hope that it is at least one kilogram of fish that you paid for. The vendor, on the other hand, would like to give you at most one kilogram of fish for your money. The scale on the market should better be a good one… While this use of units is mostly anecdotal, there is a huge demand for precise measurements and the exact definition of units in science and industry. Technologies like GPS carefully rely on the precise measurement of the tiniest differences in time. Without this precision, even a small error could result in a location discrepancy of several meters, highlighting the importance of accurate timekeeping. The field of quantum metrology uses [[Quantum Mechanics|quantum]] effects to improve [[Measurement|measurements]] and the definition of units. Just imagine that even our current system of units like meter, second, and kilogram relies heavily on quantum effects, such as the transition frequency in Caesium atoms! This transition is used in [[Atomic Clocks|atomic clocks]] to define our measure of time with exceptional accuracy: today we define one second as exactly 9,192,631,770 cycles of radiation corresponding to the transition between two [[Energy|energy levels]] of the Caesium-133 atom. In recent years, the international system of units (SI) has transitioned to quantum-based definitions. For example, the kilogram was redefined in 2019 using [[Planck's Constant|Planck's constant]] instead of the physical platinum-iridium cylinder that served as the reference for over a century. This shift eliminates the uncertainties associated with physical artifacts, ensuring a stable and universally accessible definition of mass. Similarly, the meter is now defined using the [[Speed of Light|speed of light]], in combination with the definition of the second. Quantum metrology goes beyond traditional measurement techniques by leveraging quantum phenomena, such as [[Superposition|superposition]] and [[Entanglement|entanglement]], to achieve unprecedented precision and sensitivity. For instance, quantum entanglement allows particles to share information in a way that allows for better measurements. These improved measurements surpass what's possible without quantum and only with [[Classical Physics|classical physics]] given the same resources. On a more technological side, researchers in quantum metrology are developing increasingly refined sensors to detect smaller quantities, such as gravitational waves, magnetic fields, and temperature changes. For example, *quantum magnetometers* utilize entangled particles to detect minuscule changes in [[Magnetic Field|magnetic fields]], enabling applications in medical imaging, navigation, and even geological exploration. One prominent application is in quantum-enhanced [[Interference|interferometry]], used by observatories like LIGO to detect gravitational waves. By employing [[Squeezed State|squeezed]] light—a quantum state with reduced noise in one property at the expense of increased noise in another—researchers have significantly enhanced the sensitivity of these interferometers. This capability allows the detection of ripples in spacetime caused by cosmic events, such as the collision of black holes, which would otherwise be undetectable. >[!read]- Further Reading >- [[Quantum Technologies]] >- [[Atomic Clocks]] >- [[Entanglement]] >[!ref]- References