Quantum phenomena are expected to play an increasing role in technologies. Special attention must hence be paid to decoherence effects emerging, such as coupling of the system to an external environment (bath), noise in the classical fields controlling the system, or population leakage out of a relevant system subspace.
In operational tasks involving quantum states, these effects are detrimental and must be suppressed by dynamical control. The underlying dynamics must be Zeno-like: suppressed coupling to the bath.
There are however tasks which cannot be implemented by unitary evolution, in particular a change of the system’s entropy. Such tasks necessitate efficient coupling to a bath for their implementation. Examples are the use of measurements to cool (purify) a system, or harvest energy from the environment. If the dynamics is anti-Zeno like, enhancement of this coupling to the bath will occur and thereby facilitate the task, as discovered by us.
We have constructed a general framework for optimizing the way a system interacts with its environment to achieve a desired task. This optimization consists in maximizing the success of the task, such as controlling fidelity, entropy, entanglement, or energy by dynamical modification of the system-bath coupling spectrum on demand.
Control within the bath memory-time: Zeno and anti-Zeno dynamics
Our theory of quantum systems whose weak interaction with thermal baths is dynamically controlled treats all kinds of such control, be it coherent or projective (non-unitary), continuous or pulsed, as generalized forms of two generic effects or control paradigms. One is
Minimized bath effect ≡ Quantum Zeno effect (QZE)
which minimizes (under constraints on the control energy) the integral product (overlap) of two functions: G(ω) the coupling spectrum of the bath (obtained by Fourier-transforming its autocorrelation function) and a spectral 'filter' function F1(ω) determined by the control field-intensity spectrum and its time duration t. It is the 'filter' function that provides the control handle on our ability to optimally execute a desired task in the presence of a given bath. In the presence of several baths (a common situation), both G(ω) and F1(ω) functionals are represented by matrices.
QZE-based control is required in operational tasks related to quantum information its storage and transmission, where bath effects are detrimental and must be suppressed. Regardless of the chosen form of control, the controlled-system dynamics must then be Zeno-like, namely, resulting in suppressed system–bath interaction.
The alternative paradigm is
Maximized bath effect ≡ Anti- Zeno effect (AZE)
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