a nanofabricated logic device , operable at multiple logic levels comprises asymmetrically coupled quantum point contacts provided with respective sources and drains and a coupling region between gate electrodes . the quantum mechanical carrier wavefunction in the region of qpc1 , 2 is spatially asmmetric with alternate quantised energy levels lying either in qpc1 , or qpc2 , so that by changing the energy level , the conductance of the device can be switched between multiple stable conductance levels . the device can be used to provide a multilevel output switched in response to terahertz pulses provided by an array of optical detectors .