No Synchronization for Qubits

In the late 1600s, Christiaan Huygens discovered that two pendulum clocks hung from a common support can eventually tick perfectly in phase. Today we know of many cases of one oscillator adjusting its periodic motion to synchronize with another. The long list of examples includes beating hearts, firing neurons [2], blinking fireflies [2], and sloshing clouds of cold atoms [3]. But despite its ubiquity, synchronization does not appear to be possible in the simplest kind of quantum object—the two-level system, or qubit. That’s the conclusion of theorists Alexandre Roulet and Christoph Bruder of the University of Basel, Switzerland, who report that a quantum system must have three or more energy levels in order to synchronize [4]. This fundamental limit could be relevant to experiments or devices that use synchronization to influence the properties of networks of quantum objects, like spins.