Last summer, a stream of photons arced northwest across Qinghai Lake—China's biggest—traveling about 100 kilometers in the blink of an eye before impinging on a detector on the opposite shore. The photons were entangled, meaning they were produced in pairs with identical quantum states; measuring the entangled pairs provided a key that could be used to unlock a coded message, decipherable to only those who know the photon pair's quantum state. Any attempt to read the key alters it, making it theft-proof. The technology is expected to become the gold standard of cryptography. Qinghai was a proving ground for distributing quantum keys in the final frontier: space.