Why not? Two reasons: First, legacy atomic clock technologies only scale so small. Second, even if you could make today’s smallest atomic clocks significantly smaller than they are already (about the size of a deck of cards), they would still have serious shortcomings in portable applications—where battery power and ambient temperatures are real issues.
Enter the chip scale atomic clock that enables a new class of atomic applications, defined by portability. These applications include geophysical sensors, backpack IED jammers, backpack military radios, unmanned aerial vehicles (UAVs) and military GPS receivers. It offers longer battery life than previous technologies and maintains high accuracy without GPS or other external time reference—all in addition to very small size and weight.