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Information Processing and the Atomtronic Transistor Oscillator

In many circles the term quantum information processing is practically synonymous with the term quantum computing . In the realm of classical information processing, however, computing has historically been quite distinct from other domains of information processing, notably signal processing. It is only relatively recently, in fact, that digital electronic technology has played a significant role in signal processing applications.  The power of electronics applied to signal processing is the ability to easily implement functional primitives with circuitry and to combine them to achieve sophisticated signal processing capability.  Functional primitives, for example, are signal generation, amplification, filtering, mixing, and so on, while sophisticated capability can be found in radios, radar, lock-in amplifiers, cell phone base stations and so on.    In the quantum domain, it is interesting to consider the potential of quantum signal processing especially when the signals are themselves quantum, perhaps derived from quantum sensors.   To that end, one can take advantage of a faithful analog between electronics and atomtronics , in which electric potential and current are replaced by chemical potential and atom current.  Of all electronic elements, it is surely the now ubiquitous transistor that underlies the sweeping transformation that modern technology has imposed on society.  Thus, it is fascinating to wonder how one might achieve an atom analog to the electronic transistor except to operate in a regime in which the behavior is dominated by the quantum character and properties of the constituents.   In this work we discuss the atom transistor and its role in the conceptually simple coherent matterwave oscillator.  We present experiments involving ultracold (<100 nK) rubidium atoms associated with a triple-well atomtronic transistor.  The transistor is characterized by a narrow “gate” well surrounded by comparatively larger “source” and “drain” wells.  The wells are defined by narrow potential barriers produced with laser beams that are blue-detuned from atomic resonance.  The operational parameters are such that atomic transport across the barriers is non-classical.  We establish experimentally that the atom current from the source to the drain well is in the form of a coherent matterwave.

All driven oscillators, whether electronic, optical, mechanical or otherwise have a set of characteristics in common, namely, a saturable gain element, a frequency selective element, feedback, and a source of power (e.g. a battery).  It is worthwhile noting that any driven oscillator, in fact any non-trivial electronic circuit, is fundamentally a system in thermodynamic non-equilibrium.  Our system has no reservoir that can supply or remove heat, although the drain section of the transistor is open to the vacuum.    While hardly notable in a classical electronic circuit operating near room temperature, the thermodynamic aspects of an atomtronic  circuit operating in the ultracold regime are intimately connected to the quantum aspects.  Here the dual of the electrical  battery is served by an ensemble of ultracold atoms in the source well.  We show experimentally that the temperature of these atoms rises as the circuit also emits a coherent matterwave output.  Thus, perhaps unintuitively, coherent emission into the vacuum is associated with heat flow into the source well.   The evolution of entropy in different portions of the circuit reflect the connection between thermodynamic and information entropy associated with a signal-processing circuit.