1. Detecting Genuine Multi-qubit Entanglement with Two Local Measurement Settings Géza Tóth (MPQ) Otfried Gühne (Innsbruck) Quantum Optics II, Cozumel, Dec 2004 quant-ph/0405165

2. Outline Genuine multi-qubit entanglement Entanglement detection with entanglement witnesses Witness based on projectors Our proposal: witness with few local measurements (for GHZ & cluster states) Connection to Bell inequalities

3. Genuine multi-qubit entanglement A mixed entangled state is biseparable if it is the mixture of biseparabe states (of possibly different partitions). Biseparable entanglement Genuine three-qubit entanglement

4. Entanglement witnesses I Entanglement witnesses are observables which have  positive expectation values for separable states  negative expectation values for some entangled states. Witnesses can be constructed which detect entangled states close to a state chosen by us. Witnesses can be constructed which detect only genuine multi-party entanglement.

5. S Separable states Genuine multi-qubit entangled states Biseparable entangled states Witness#1 States detected by Witness#1 Entanglement witnesses II

6. Entanglement witnesses III It is possible to construct witnesses for detecting entangled states close to a particular state with a projector. E.g., detects N-qubit entangled states close to an N-qubit GHZ state.

7. Entanglement witnesses IV So if then the system is genuinely multi-qubit entangled. Question: how can we measure the witness operator?

8. Decomposing the witness For an experiment, the witness must be decomposed into locally measurable terms See O. Gühne, P. Hyllus, quant-ph/0301162; M. Bourennane et. al., PRL 92 087902 (2004).

9. Main topic of the talk: How can one decrease the number of local terms The number of local terms increases rapidly (exponentially?) with the number of qubits. More importantly, we need more and more measurement settings to measure. (This is also true for Bell inequalities.) Entanglement detection becomes harder and harder for increasing number of qubits.

10. Entanglement witnesses based on the stabilizer formalism

11. Stabilizer witnesses We propose new type of witnesses. E.g., for three-qubit GHZ states All correlation terms are +1 for the GHZ state.

12. Stabilizer witnesses II Our general method for constructing witnesses for states close to Here S k stabilize

13. Stabilizing operators For an N-qubit GHZ state For an N-qubit cluster state

14. Cluster state Can easily be obtained from Ising spin chain dynamics. Often encountered in error correction. For N=3 qubits it is equivalent to a GHZ state For N=4 qubits it is equivalent to See Briegel, Raussendorf, PRL 86, 910 (2001).

15. Stabilizer witnesses III Optimal witness for N-qubit GHZ state Optimal witness for N-qubit cluster state

16. Stabilizer witnesses VI The projector witness is also the sum of stabilizing operators An alternative witness with the fewest terms: Stabilizer witnesses VI

17. zzzxxzzxxx xxxxxzzzzz Only the minimal two measurement settings are needed

18. Noise In an experiment the GHZ state is never prepared perfectly For each witness there is a noise limit. For a noise larger than this limit the GHZ state is not detected as entangled.

19. Noise tolerance: our witnesses are optimal Witness for N-qubit GHZ state  for N=3: 40%  for large N: >33% Witness for N-qubit cluster state  for N=4:33%  for large N: >25%

20. Noise tolerance: 40% (2 settings) Noise tolerance: 50% (4 settings) Bell ineq.! Noise tolerance: 57% (4 settings) Projector!! Connection to Bell inequalities

21. Summary Detection of genuine N-qubit entanglement was considered with few local measurements. The methods detect entangled states close to N-qubit GHZ and cluster states. Home page: http://www.mpq.mpg.de/ Theorygroup/CIRAC/people/toth *************** THANK YOU!!! ************* quant-ph/0405165

22. Stabilizer witnesses V Why do these witnesses detect genuine N-qubit entanglement? Because Any state detected by our witness is also detected by the projector witness. Later detects genuine N-qubit entanglement. Stabilizer witnesses V

23. Main topic of the talk: How can one decrease the number of local terms As the number of qubits increases, the number of local terms increases exponentially. Similar thing happens to Bell inequalities for the GHZ state. Q: How can we construct entanglement witnesses with few locally measurable terms?

24. What is a measurement setting? Measurement setting is the basic unit of experimental effort. At each qubit operator O k is measured. After repeating the measurements several times, two-point correlations, three-point correlations, etc., can be obtained. O1O1 O2O2 O3O3 O4O4 O5O5...

25. Stabilizer witnesses III Characteristics for our N-qubit entanglement witnesses  Usually the minimal 2 measurement settings  For large N, tolerates noise p noise <33% (GHZ) / 25% (cluster)  For small N, noise tolerance is better (N=3; 40% / N=4; 33%)

26. Entanglement witnesses I Bell inequalities Classical: no knowledge of quantum mechanics is used to construct them. Need many measurements. Entanglement witnesses QM is used for constructing them. Can one detect entanglement with fewer measurements? (Yes)

27. Entanglement witnesses II S Separable states Genuine multi-qubit entangled states Biseparable entangled states Witness#1 Witness#2 States detected by Witness#1