Lasers are a direct application of stimulated emission of radiation (Quantum theory). They are used in fiber optic communication, barcode scanners, laser surgery, and manufacturing.
Einstein’s concept of Stimulated Emission (introduced in 1917) sat as a pure theory for 40 years until the first laser was built. A laser is the ultimate quantum device: it forces photons to march in lockstep, creating light that is coherent (same phase) and monochromatic (same wavelength). Applications Of Modern Physics
The most famous daily application of relativity is GPS technology. To determine your exact location, your smartphone communicates with a network of satellites orbiting Earth. Because these satellites move at high speeds relative to the ground, Special Relativity dictates that their onboard atomic clocks tick slower than clocks on Earth by about 7 microseconds per day. Furthermore, because the satellites are high above Earth's mass, General Relativity states that they experience weaker gravity, causing their clocks to tick faster by about 45 microseconds per day. Lasers are a direct application of stimulated emission