  Efficient and flexible schemes for a swift, field-free control of the phase
in quantum devices have far-reaching impact on energy-saving operation of
quantum computing, data storage, and sensoring nanodevices. We report a novel
approach for an ultrafast generation of a field-free vector potential that is
tunable in duration, sign, and magnitude, allowing to impart non-invasive,
spatio-temporally controlled changes to the quantum nature of nanosystems. The
method relies on triggering a steady-state toroidal moment in a donut-shaped
nanostructure that serves as a vector-potential generator and quantum phase
modulator. Irradiated by moderately intense, few cycle THz pulses with
appropriately shaped polarization states, the nano donut is brought to a
steady-state where a nearby object does not experience electric nor magnetic
fields but feels the photo-generated vector potential. Designing the time
structure of the driving THz pulses allows for launching picosecond trains of
vector potentials which is the key for a contact-free optimal control of
quantum coherent states. During the toroidal moment rise up time radiation is
emitted which can be tuned in a very broad frequency band. We carry out
full-fledged quantum dynamic simulations, and theoretical analysis to
illuminate the underlying principles and to endorse the feasibility,
robustness, and versatility of the scheme. This research could trigger a new
class of ultrafast quantum devices operated and switched in an
energy-efficient, contact and field-free manner, enabling new techniques for
use in quantum information, magnetic nanostructures and superconducting tunnel
junctions as well as in toroidally ordered systems and multiferroics.
