Quantum dots are promising candidates to replace the trapped atoms originally posited as the backbone of quantum computing, but it must be possible to manipulate and measure their quantum states. Now researchers at Technische Universität München in Garching, Germany, have found a method to do so with an InGaAs dot in the active region of a GaAs photodiode.They grew the structure by molecular beam epitaxy, using aluminum shadow masks with apertures of 100 to 500 nm to isolate a single dot. A mode-locked Ti:sapphire laser excited the dot through the apertures with 1-ps pulses, inducing Rabi oscillations between two exitonic energy levels that the researchers detected as deterministic photocurrent. For optical excitation with π pulses, each laser pulse triggers the flow of a single electron through the circuit, which is the key to single-electron optoelectronics and quantum gate readout. The work was reported in the Aug. 8 issue of Nature.