Environment effects generally hamper or completely destroy the “quantumness” of any complex device. Particularly fragile against environment effects is quantum entanglement (QE) in multipartite systems. This fragility may disable quantum information processing and other forthcoming quantum technologies: interferometry, metrology and lithography. This QE fragility has been the standard resolution of the Schroedinger-cat paradox: the environment has been assumed to preclude macrosystem entanglement.

We put forward the following, alternative paradigm: QE in multipartite systems may **naturally (spontaneously)** arise (albeit over limited time) in commonly encountered thermal environments (baths). This includes the spontaneous formation of Schroedinger-cat states, also known as macroscopic quantum superposition (MQS) states.

This comes about because their quantized collective dynamics can be mapped onto that of angular momentum (spin) ⃗with large eigenvalues. The finite spectral width (non-Markovian features) of most commonly encountered baths drives the spin ensemble into an entangled state, via effectively nonlinear dynamics.

Another effort is to protect multipartite entangled quantum states from decoherence by their environment. Such protection is the key to the coveted quantum computation. The challenge is: how to optimally control multiqubit entangled states? Our ability to face this challenge relies on our universal approach to decoherence control.

**Bath-induced entanglement (BIE) in open systems**

Environment effects generally hamper or completely destroy the 'quantumness' of any complex device. Particularly fragile against environment effects is quantum entanglement (QE) in multipartite systems. This fragility may disable quantum information processing and other forthcoming quantum technologies: interferometry, metrology and lithography. Commonly, the fragility of QE rapidly mounts with the number of entangled particles and the temperature of the environment (thermal 'bath'). This QE fragility has been the standard resolution of the Schrödinger-cat paradox: the environment has been assumed to preclude macrosystem entanglement. But is it inevitable that Schrödinger cats die of decoherence (as commonly believed)? Or, conversely, can a cat be both dead and alive in a thermal bath?

We shed light on these fundamental issues within the simple model of N spin-1/2 non-interacting particles that identically couple to a thermal oscillator-bath via the z-component of their Pauli operators. A single spin in such a model undergoes bath-induced pure dephasing. Yet, strikingly, an initial product state of N z-polarized spins can spontaneously evolve via such coupling to the bath, into a Schrödinger-cat state, also known as a macroscopic quantum superposition (MQS) or GHZ state, nearly deterministically

Schematic view of a product-state spin-polarized ensemble (left) that spontaneously evolves in the bath into an entangled MQS or GHZ (Schrödinger-cat) state at a particular time, as a result of bath-induced entanglement.

Publications:

Kurizki, G; Bertet, P; Kubo, Y; Molmer, K; Petrosyan, D; Rabl, P; Schmiedmayer, J (2015).Quantum Technologies With Hybrid Systems. *Proceedings of the National Academy of Sciences of the United States of America.* 112:3866-3873

Zwick, A; Alvarez, Ga; Bensky, G; Kurizki, G (2014). Optimized Dynamical Control of State Transfer Through Noisy Spin Chains. *New Journal of Physics.* 16

Shahmoon, E; Kurizki, G (2014). Nonlinear Theory of Laser-Induced Dipolar Interactions in Arbitrary Geometry. *Physical Review A.* 89

Shahmoon, E; Mazets, I; Kurizki, G (2014). Non-Additivity in Laser-Illuminated Many-Atom Systems. *Optics Letters.* 39:3674-3677

Shahmoon, E; Kurizki, G (2013). Nonradiative Interaction and Entanglement Between Distant Atoms. *Physical Review A.* 87

Gordon, G; Mazets, Ie; Kurizki, G (2013). Quantum Particle Localization by Frequent Coherent Monitoring. *Physical Review A.* 87

Bensky, G; Petrosyan, D; Majer, J; Schmiedmayer, J; Kurizki, G (2012). Optimizing Inhomogeneous Spin Ensembles For Quantum Memory. *Physical Review A.* 86

Davidson, N; Almog, I; Sagi, Y; Gordon, G; Bensky, G; Kurizki, G (2012). Measurement of the System-Environment Coupling and Its Relation to Dynamical Decoupling. *2012 Conference on Lasers and Electro-Optics (Cleo)*

Bretschneider, Co; Alvarez, Ga; Kurizki, G; Frydman, L (2012). Controlling Spin-Spin Network Dynamics by Repeated Projective Measurements. *Physical Review Letters.* 108

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

Bar-Gill, N; Rao, Ddb; Kurizki, G (2011). Creating Nonclassical States of Bose-Einstein Condensates by Dephasing Collisions. *Physical Review Letters.* 107

Shahmoon, E; Kurizki, G; Fleischhauer, M; Petrosyan, D (2011). Strongly Interacting Photons in Hollow-Core Waveguides. *Physical Review A.* 83

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.* 4

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

Erez, N; Gordon, G; Nest, M; Kurizki, G (2008). Thermodynamic Control by Frequent Quantum Measurements. *Nature.* 452:724-727

Gordon, G; Kurizki, G; Lidar, Da (2008). Optimal Dynamical Decoherence Control of a Qubit. *Physical Review Letters.* 101