Frustration of Decoherence and Entanglement-sharing in the Spin-bath

1. Christopher Dawson Ross McKenzie Gerard Milburn Frustration of Decoherence and Entanglement-sharingin the Spin-bath Andrew Hines

2. The short version • Motivated by paper by Tessieri and Wilkie (J. Phys. A 36, 12305 (2003)), who considered a simple model of a qubit interacting with a spin-bath, with intra-bath interactions. They found a suppression of the decoherence resulting from the intra-bath interactions. • In a quantum system, interactions lead to entanglement. We were interested in entanglement and quantum phase transitions, so we were interested in entanglement in many-body systems. • In the multi-partite setting, there are bounds on how entanglement can be distributed amongst subsystems. • A quantum bath can be entangled. • Entanglement between system and bath decoheres the system, but entanglement between bath spins can limit system-bath entanglement. • Is the suppression of decoherence a manifestation of entanglement-sharing? (Stupid Hat Day 2004)

3. Decoherence Central qubit (systems, S), coupled to a bath of N spin-half particles (spin-bath B), each with their own dynamics. Initially at t = 0 we take the central spin S to be in a pure state, uncorrelated with the bath, for some initial state of the bath, usually thermal state, or for low temperatures, the ground state. As the system evolves under H, the central spin becomes correlated with the bath so at later times is no longer pure. The central spin is said to have decohered, and the amount of decoherence is typically quantified by the von Neumann entropy of its reduced density matrix,

4. Classical vs. Quantum Correlations Decoherence implies correlation between system and bath, but his doesn’t have to be entanglement. Classical: • could be quantified by the mutual information: Quantum: In both cases, the bath state contains information about the state of the system.

5. Mathematically: Practically: • cannot be prepared using only LOCC • violation of a Bell inequality For pairs of qubits: tangle Entanglement Artistically Bipartite entanglement measures (A-B) Entanglement of formation: All bipartite states with the same Efare equivalent up to LOCC R. Bloch

6. Many-particle Entanglement There are inequivalent classes of entangled states in the multipartite setting: For 3 qubits – 2 classes of three-party entanglement • GHZ-class: pure tripartite entanglement • W-class: bipartite entanglement There can be multiple “units” of entanglement, making a multipartite measure difficult to conceive. Beyond three qubits, the structure is not as simple W. Dür, G. Vidal, and J. I. Cirac, Phys. Rev. A 62, 062314 (2000)

7. Entanglement is monogamous Unlike classical correlations, entanglement may not be shared arbitrarily amongst many parties. Consider 3 qubits A,B,C, if A-B then C cannot become entangled with A, unless some entanglement between A-B is destroyed: Classical correlations: Consider a collection of random variables Mutual information between different variables is not bounded. The correlations between X and Y do not affect correlations between X and Z

8. Entanglement-sharing Inequalities Monogamy places bounds on the amount of entanglement that may be shared amongst parties in a multipartite setting.These are quantified in terms of entanglement-sharing inequalities 3-qubit example N-qubits, bipartite entanglement Maximum pair-wise entanglement in symmetric state Coffman, Kundu, Wootters, Phys. Rev. A 61, 052306 (2000)

9. A perfect classical correlation between A and B will forbid system A from being entangled to other systems: The reduced density operator for A must be a pure state. Though this does NOT restrict classical correlations between A and another party. Monogamy of entanglement and other correlations Koashi and Winter Phys. Rev. A 69, 022309 (2004) Koashi and Winter quantify this trade-off between correlations, using an operational definition of the classical correlations in a bipartite quantum state (one-way distillable common randomness). This describes how it is possible for entanglement to bound potential classical correlations.

10. Frustration Distributing entanglement between nearest neighbours around a ring of qubits ?? Spins with anti-ferromagnetic coupling High School Chemistry Exams

11. Frustration of Decoherence If a state of the system is evolving under a Hamiltonian such as and the bath, initialised in some entangled state, maintains appreciable entanglement over the evolution, then it follows there is a restriction on the entanglement between the `central spin' S and the bath. For pure states this equivalent to a restriction on the amount that S may decohere. For mixed states we must also bound the classical correlations which may be done using the result of Koashi and Winter. Entanglement between bath spins can frustrate correlations between the central spin and the bath.

12. Entanglement-Sharing Inequality For spin-baths of N particles, , the situation is more complicated due to the many different types of entanglement which exist in these baths, and the absence of good entanglement measures for them. To overcome this difficulty we will assume the Hamiltonians HB and HSB are symmetric about permutations of spins. Here the pair-wise entanglement between any two bath spins is the same, for all i,j, allowing us to quantify the intra-bath entanglement by a single parameter. Our aim is to show how this constrains the system-bath tangle .

13. Entanglement-Sharing Inequality Use the symmetry in the system, and the characteristics of W-class states to obtain an entanglement-sharing inequality for a single qubit, with a permutation symmetric bath of N-qubits. Dawson, Hines, McKenzie, Milburn, Phys Rev. A 71, 052321 (2005)

14. Frustration of Decoherence Zurek Model One simple model of decoherence where the inequality is immediately applicable is an exactly solvable model introduced by Zurek [8] recently used to investigate the structure of the decoherence induced by spin environments [9].

15. Frustration of Decoherence , Tessieri-Wilkie Model L. Tessieri and J. Wilkie, J. Phys. A 36, 12305 (2003).

16. Frustration of Decoherence Tessieri-Wilkie Model

17. Frustration of Decoherence Tessieri-Wilkie Model Rabi Oscillations

18. Entanglement and Chaotic Environments Results to date suggest that chaotic dynamics enhance the generation of entanglement, and possibly correlations in general. • Two possible viewpoints for decoherence • If chaos is introduced by the interaction between system and environment, then this will increase the generation of entanglement, leading to greater decoherence • Chaos within the bath degrees of freedom will generate intra-bath entanglement, which can act to frustrate decoherence.

19. Thank you