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)

Publications:

- Zwick, A.; Alvarez, G.A.; Kurizki, G. (2016). Maximizing Information on the Environment by Dynamically Controlled Qubit Probes.
*Physical Review Applied.*5:-014007 (17 pp.) - Clausen, J; Bensky, G; Kurizki, G (2012). Task-Optimized Control of Open Quantum Systems.
*Physical Review A.*85 - Bensky, G; Petrosyan, D; Majer, J; Schmiedmayer, J; Kurizki, G (2012). Optimizing Inhomogeneous Spin Ensembles For Quantum Memory.
*Physical Review A.*8 - Bensky, G; Amsuss, R; Majer, J; Petrosyan, D; Schmiedmayer, J; Kurizki, G (2011). Controlling Quantum Information Processing in Hybrid Systems on Chips.
*Quantum Information Processing.*10:1037-1060 - Gordon, G; Kurizki, G (2011). Scalability of Decoherence Control in Entangled Systems.
*Physical Review A.*83 - Escher, Bm; Bensky, G; Clausen, J; Kurizki, G (2011). Optimized Control of Quantum State Transfer from Noisy to Quiet Qubits.
*Journal of Physics b-Atomic Molecular and Optical Physics.*44 - Rao, Ddb; Bar-Gill, N; Kurizki, G (2011). Generation of Macroscopic Superpositions of Quantum States by Linear Coupling to a Bath.
*Physical Review Letters.*106 - Almog, I; Sagi, Y; Gordon, G; Bensky, G; Kurizki, G; Davidson, N (2011). Direct Measurement of the System-Environment Coupling as a Tool For Understanding Decoherence and Dynamical Decoupling.
*Journal of Physics b-Atomic Molecular and Optical Physics.*44 - Rao, Ddb; Kurizki, G (2011). From Zeno to Anti-Zeno Regime: Decoherence-Control Dependence on the Quantum Statistics of the Bath.
*Physical Review A.*83 - Clausen, J; Bensky, G; Kurizki, G (2010). Bath-Optimized Minimal-Energy Protection of Quantum Operations from Decoherence.
*Physical Review Letters.*104 - Khodorkovsky, Y; Kurizki, G; Vardi, A (2008). Bosonic Amplification of Noise-Induced Suppression of Phase Diffusion.
*Physical Review Letters.*100 - Gordon, G; Kurizki, G; Lidar, Da (2008). Optimal Dynamical Decoherence Control of a Qubit.
*Physical Review Letters.*101