Light, Smoke, Gas and Gravity are everywhere. Sometimes they are beneficial, sometimes they are not wanted. Sensing them precisely is challenging though advantageous to understand where problem causality & measure climate solution impact fairly. A quantum sensor is anything that relies on a **“quantum particle”** to sense an external variable. Sensors that qualify as quantum: - **Magnetometers** to measure magnetic fields - **Accelerometers** to measure acceleration - **Gyroscopes** to measure angular velocity Quantum sensors can be - Ultracold - Atom-based - Photon-based In application, there are **advantages to each,** the advantages depend on the distinct physical construction of the sensor. Different sensors measure **different forces**. **Different compositions** make different sensors The distinguishing feature of all quantum sensors is that they are **limited by quantum noise.** These particles are quantised, and when you measure them you have fundamental noise from the **collapse of the quantum wave function**. The collapse of the wave function of a photon is not very different from the collapse of the wave function of an atom. > Note: All quantum sensors do not use ultra cold atoms, but also “normal temperature atoms”. These atoms are usually flying around at ~300 m/s in a tiny glass cell and we interact with them in the short amount of time when they fly through the laser beam. Ultra cold technologies do exist too where the atom is trapped inside the laser beam and we cool them down so that they are not moving at all. However, **warm “quantum sensing”** uses technology that is smaller, requires less lasers and less custom big vacuum chambers, so is most practical. Though commercialization is still in it’s infancy. Atoms are very different from photons in that they **interact with the world very strongly.** We use atoms to detect fields because they **interact electrically with everything around them.** They make the best magnetometers, which are detectors of magnetic and electrical fields. When a photon collides with another photon, it just passes right through. **Photons** make great gyroscopes, as there we are just measuring tilts and turns and not inherent electrical or magnetic interactions. [[Quantum Metrology and Sensing Applications]] [[Quantum Sensing Benchmarks]] [[Quantum Sensing Venture]]