A hyper-fast quantum computer is the digital equivalent of a nuclear bomb: whoever possesses one will be able to shred any encryption in existence, effectively owning the digital world. The question is: Who will build it first, the United States or China?

The latest of David Ignatius's timely, sharp-eyed espionage novels follows CIA agent Harris Chang into a quantum research lab compromised by a suspected Chinese informant. The breach provokes a mole hunt that is obsessive, destructive, and--above all--uncertain: Do the leaks expose real secrets, or are they false trails meant to...