Solid state realisation of Werner quantum states via Kondo spins

1. Solid state realisation of Werner quantum states via Kondo spins Ross McKenzie Sam Young Cho Reference: S.Y. Cho and R.H.M, Phys. Rev. A 73, 012109 (2006)

2. Thanks to Discussions with • Briggs (RKKY in nanotubes) • Doherty and Y.-C. Liang (Werner states) • Dawson, Hines, and Milburn (decoherence and entanglement sharing)

3. Big goals for quantum nano-science • Create and manipulate entangled quantum states in solid state devices • Understand the quantum-classical boundary, e.g., test quantum mechanics versus macro-realism (Leggett) • Understand the competition between entanglement and decoherence

4. Entanglement vs. decoherence • Interaction of a qubit with its environment leads to decoherence and entanglement of qubit with environment. • Interactions between qubits entangles them with one another. • We will also see that the environment can entangle the qubits with one another.

5. Outline • Classical correlations vs. entanglement vs. violation of Bell inequalities (Werner states) • Experimental realisations of two impurity Kondo model • Competition between Kondo effect and RKKY interaction • Entanglement between the two Kondo spins • How to create Werner states in the solid state.

6. Quantum correlations in different regions of Hilbert space Entangled states No correlations Violate Bell inequalities Correlations but no entanglement

7. 0 0 5 7 8 < < p p p s s s : : Werner states Mixed states of two qubits In the Bell basis Reduced density matrix is probability of a singlet No entanglement Bell-CSSH inequalities satisfied

8. Two impurity spins A and B Two impurity Kondo system Conduction electrons C Model system: two Kondo spins interact with metallic environment via Heisenberg exchange interaction

9. Two impurity Kondo system Experimental realisation I N. J. Craig et al., Science 304, 565 (2004) 2DEG between spins in quantum dots induces an RKKY interaction between spins. Gates vary J

10. Two impurity Kondo system Experimental realisation II • Endohedral fullerenes inside nanotubes A. Khlobystov et al. Angewandte Chemie International Edition 43, 1386-1389 (2004)

11. Single impurity Kondo system Single impurity Kondo model Hamiltonian Conduction electrons J is the spin exchange coupling Conduction-electron spin density at impurity site R = 0 Low temperature properties determined by single energy scale . Kondo temperature Band width D and the single particle density of state at the Fermi surface

12. Single impurity Kondo system Tuneable quantum many-body states: Kondo effect in quantum dots For a review, L. Kouwenhoven and L. Glazman, Physics World 14, 33 (2001) Conduction electron spin Impurity spin Kondo temperature can be varied over many orders of magnitude

13. RKKY interaction Two impurity Kondo system Two impurity Kondo model Hamiltonian To second order J, the indirect RKKY (Ruderman Kittel-Kasuya-Yosida)interaction is Ground state determined by competition between Kondo of single spins and RKKY

14. Single impurity Kondo system Impurity spin A Spin-rotational invariant! Conduction electrons C Spin singlet Entanglement in single impurity Kondo model [T. A. Costi and R. H. McKenzie, Phys. Rev. A 68, 034301 (2003)] J S=1/2 Subsystem A Subsystem B Total system A+B Ground state Reduced density matrix for the impurity von Neumann entropy The impurity spin is maximally entangled with the conduction electrons c.f., Yosida’s variational wavefunction [K. Yosida, Phys. Rev. 147, 233 (1966)]

15. ~ ~ S S < > ¢ A B Entanglement between the two Kondo spins • Given by concurrence of the reduced density matrix for the two localised spins (Wootters) • Ground state is a total spin singlet (S=0) and thus invariant under global spin rotations • Entanglement is determined by . $ \langle \vec{S}_A . \vec{S}_B \rangle $

16. Two impurity spins A and B Two impurity Kondo system Conduction electrons C Reduced density matrix for the impurities In the Bell basis

17. Non Fermi-liquid behaviour Low temperature behaviour of two impurity Kondo model [B. A. Jones, C. M. Varma, and J. W. Wilkins, Phys. Rev. Lett. 61, 125 (1988)] Numerical renormalization group calculation shows that Left: the staggered susceptibility and the specific heat coefficients diverge. The spin-spin correlation is continuously varying and approaches at the critical value of around the divergence of susceptibility. Right:

18. Entanglement & Quantum Phase transition

19. I T 2 2 ' K : Unstable fixed point • At the fixed point [Gan, Ludwig, Affleck, and Jones] • Thus, for the critical coupling there is no entanglement between two qubits.

20. Questions for future • Can the competition between Kondo and RKKY be better understood in terms of entanglement sharing? • Why does the entanglement between Kondo spins vanish at the quantum critical point? • What effect does temperature have?

21. Conclusions • Two spin Kondo model provides a model system to study competition between entanglement of two qubits with each other and entanglement of each qubit with environment • Entanglement between the two Kondo spins vanishes at the unstable fixed point. • Varying system parameters will produce all the Werner states S.Y. Cho and RHM, Phys. Rev. A 73, 012109 (2006)

22. Non Fermi-liquid behaviour Low temperature behaviours of two impurity Kondo model [B. A. Jones, C. M. Varma, and J. W. Wilkins, Phys. Rev. Lett. 61, 125 (1988)] Numerical renormalization group calculation shows that Left: the staggered susceptibility and the specific heat coefficients diverge. The spin-spin correlation is continuously varying and approaches at the critical value of around the divergence of susceptibility. Right:

23. Unstable fixed point [B. A. Jones and C. M. Varma, Phys. Rev. B 40, 324 (1989)] Renormalization group flows

24. Two impurity spins A and B Two impurity Kondo system Impurity spin A Impurity spin B Conduction electrons C Conduction electrons C One impurity spin A Three types of entanglements (i) and (ii) and (iii) and Subsystem A Subsystem B

25. spin-spin correlation singlet state triplet state Probabilities for spin singlet/triplet states for singlet state for triplet state For P(S)=P(T)=1/2, the state for the two spins can be regarded as an equal admixture of the total spin of impurities Simp=0 and Simp=1. at ps=1/2 spin-spin correlation

26. Two impurity Kondo system Impurity spin A Impurity spin B Entanglement (ii) between the impurities (ii) and Total system A+B+C Although the total system is in a pure state, the two impurity spins are in a mixed state. Need to calculate the concurrence as a measure of entanglement [W. K. Wootters, Phys. Rev. Lett. 80, 2245 (1998)]

27. Concurrence Critical correlation Concurrence & Critical Correlation In terms of the Werner state Hence, at ps=1/2, there exists a critical value of the spin-spin correlation separating entangled state from disentangled state.

28. Comparison of criteria singlet fidelity [42] R. Horodecki, P. Horodecki, and M. Horodecki, Phys. Lett. A 200, 340 (1995) [48] S. Popescu, Phys. Rev. Lett. 72, 797 (1994)

29. Two impurity spins A and B Two impurity Kondo system Conduction electrons C Entanglement (iii) S=1/2 Subsystem A and B Subsystems C Total system A+B+C von Neumann entropy