People

Anthony Chefles
Research Fellow

E-mail: anthony.chefles@hp.com
Phone: +44 (0)117 312 9983
FAX: +44 (0)117 312 9870

Location:
Quantum Information Processing Group
Hewlett-Packard Laboratories, Bristol

Before arriving at HP, Tony held postdoctoral and lecturing positions at the Universities of Strathclyde and Hertfordshire, NUI Maynooth and University College Dublin. His work focuses on the theory of quantum states and channels (processes) with applications to quantum information science.  His main research interests are in relation to the information-theoretic properties of quantum states and the quantum channels which are used to process them.  Among the issues that arise, and which are the focus of much of his research activity, are:

Distinguishability: Sending classical information using quantum states requires us to distinguish among the possible states in order to extract the message.  To distinguish among quantum channels, we must likewise distinguish among the states that the possible channels can create.   One problem that arises with quantum states, which has no classical analogue, is that arbitrary quantum states cannot, in principle, be perfectly distinguished.  Tony has made numerous pioneering contributions to this field, relating in particular to fundamental physical limitations upon how well quantum state discrimination tasks can be accomplished.  His current research is related the applicability of quantum channel discrimination in quantum computation, in particular, quantum searching. 

Entanglement and classical communication capacities: Quantum channels can create a novel quantum mechanical effect, and useful information processing resource, known as entanglement.  They can also be used to transmit ordinary classical information.   Part of his research relates to how well these (related) tasks can be accomplished in general, and also on obtaining more detailed information about specific channels that are of importance in quantum communications and computation, such as channels that are used to compute functions. 

Quantum protocols and games:  A topic under intensive investigation is how quantum mechanics can be used carry out tasks with certain information-theoretic requirements relating to, e.g. security.  It is well-known that quantum mechanics permits the secure distribution of cryptographic keys.  However, it has numerous other applications.  One such application relates to voting, where we require the voters’ identities to remain anonymous while retaining transparency and fairness in the voting and counting procedures.  Protocols for achieving such tasks can be regarded as games.  A quantum game-theoretic framework can also be provided for assessing how noisy a given quantum channel is, which is of paramount importance in quantum communications.   Tony's current research in this area related to both quantum voting protocols and the global quantification of noise in quantum channels, where we aim to determine when one channel is preferable to another for all input quantum states.

List of publications

Thesis

1. A. Chefles, ‘Nonlocality and nonlinearity in quantum optics’ PhD Thesis, University of Strathclyde (1997).
   
   Published
    

2. A. Chefles and S. M. Barnett, ‘Complementarity and Cirel’son’s inequality’ J. Phys. A  237 (1996).
    

3. A. Chefles and S. M. Barnett, ‘Quantum theory of two-mode nonlinear directional couplers’ J. Mod. Opt.  709 (1996).
    

4. A. Chefles, ‘Nearest-neighbour level spacing distribution for the nonperiodic discrete nonlinear Schrödinger equation’ J. Phys. A  4515 (1996).
    

5. A. Chefles and S. M. Barnett, ‘Diagonalisation of the Bell-CHSH operator’ Phys. Lett. A  4 (1997).
    

6. A. Chefles and S. M. Barnett, ‘Collective observables and enhanced violation of Bell’s inequality’ Phys. Rev. A  1721 (1997).
    

7. A. Chefles and S. M. Barnett, ‘Entanglement and unambiguous discrimination between non-orthogonal states’ Phys. Lett. A  177 (1997).
    

8. A. Chefles, ‘Unambiguous discrimination between linearly-independent quantum states’, Phys. Lett. A  339 (1998).
    

9. A. Chefles and S. M. Barnett, ‘Optimum unambiguous discrimination between linearly-independent symmetric states’, Phys. Lett. A  223 (1998).
    

10. A. Chefles and S. M. Barnett, ‘Strategies for discriminating between non-orthogonal quantum states’ J. Mod. Opt.  1295 (1998)
     

11. A. Chefles and S. M. Barnett, ‘Quantum state separation, unambiguous discrimination and exact cloning’, J. Phys. A  10097 (1998).
     

12. S. M. Barnett, A. Chefles, D. T. Pegg and L. S. Phillips, ‘Generalised measurements, retrodiction and state manipulation’ In Proceedings of the 4th International Conference of Quantum Communication, Measurement and Computing, Evanston, Illinois, 1998 (Plenum).
     

13. A. Chefles and S. M. Barnett, ‘Strategies and networks for state-dependent quantum cloning’, Phys. Rev. A  136 (1999).
     

14. A. Chefles, ‘Quantum state discrimination’, Contemporary Physics  401 (2000).
     

15. S. J. D. Phoenix, S. M. Barnett and A. Chefles, ‘Three-state quantum cryptography’, J. Mod. Opt  507 (2000).
     

16. A. Chefles, ‘Deterministic quantum state transformations’, Phys. Lett. A  14 (2000).
     

17. A. Chefles, C. R. Gilson and S. M. Barnett, ‘Entanglement and collective quantum operations’, Phys. Lett. A  10 (2000).
     

18. S. F. Huelga, J. A. Vaccaro, A. Chefles and M. B. Plenio, ‘Quantum remote control: teleportation of unitary operations’, Phys. Rev. A , 042303 (2001)
     

19. A. Chefles, C. R. Gilson and S. M. Barnett, ‘Entanglement, information and multiparticle quantum operations’, Phys. Rev. A , 032314 (2001).
     

20. S. M. Barnett, R. B. M. Clarke, V. M. Kendon, E. Riis, A. Chefles and M. Sasaki, ‘Experimental Quantum State Discrimination’ In Proceedings of the 3rd International Conference of Quantum Communication, Measurement and Computing, Capri, Italy, 2000 (Plenum).
     

21. R. B. M. Clarke, A. Chefles, S. M. Barnett and E. Riis, ‘Experimental demonstration of optimal unambiguous state discrimination’, Phys. Rev. A , 042305 (2001).
     

22. R. B. M. Clarke, V. M. Kendon, A. Chefles, S. M. Barnett, E. Riis and M. Sasaki, ‘Experimental realisation of optimal detection strategies for overcomplete states’, Phys. Rev. A , 012303 (2001).
     

23. M. Sasaki, A. Carlini and A. Chefles, ‘Optimal phase estimation and square-root measurement’, J. Phys. A  7017 (2001).
     

24. A. Chefles, ‘Unambiguous discrimination between linearly dependent states with multiple copies’, Phys. Rev. A , 063025 (2001).
     

25. A. Chefles, ‘Quantum operations, state transformations and probabilities’, Phys. Rev. A  052314 (2002).
     

26. A. Chefles, ‘Distinguishability measures and ensemble orderings’, Phys. Rev. A  042325 (2002).
     

27. A. Chefles and M. Sasaki, ‘Retrodiction of generalised measurement outcomes’, Phys. Rev. A.  032112 (2003).
     

28. S. M. Barnett, A. Chefles and I. Jex, ‘Comparison of two unknown quantum states’, Phys. Lett. A  189 (2003).
     

29. I. Jex, E. Andersson and A. Chefles, ‘Comparing the states of many quantum systems’, J. Mod. Opt.  505 (2004).
     

30. A. Chefles, R. Jozsa and A. Winter, ‘On the existence of physical transformations between sets of quantum states’, Int. J. Quant. Info.  11 (2004).
     

31. A. Chefles, ‘Condition for unambiguous state discrimination with local operations and classical communication’, Phys. Rev. A  050307(R) (2004).
     

32. *A. Chefles, E. Andersson and I. Jex, ‘Unambiguous comparison of the states of multiple quantum systems’, J. Phys. A: Math. Gen  7315 (2004).
     

33. *A. Chefles, ‘Quantum states: discrimination and classical information transmission. A review of experimental progress’. Invited contributed chapter to monograph on ‘Quantum State Estimation’ (ed. M. Paris and J. Rehacek) (Springer, 2004).
     

34. F. Anselmi, A. Chefles and M. B. Plenio, ‘Local copying of orthogonal entangled quantum states’, New. J. Phys.  164 (2004).
     

35. A. Chefles, ‘Entangling capacity and distinguishability of two-qubit unitary operators’. Phys. Rev. A  042332 (2005).
     

36. A. Kitagawa, M. Takeoka, M. Sasaki and A. Chefles, ‘Entanglement evaluation of non-Gaussian states generated by photon subtraction from squeezed states’. Phys. Rev. A  042310 (2006). 
    
    Submitted

37. *J. A. Vaccaro, J. Spring and A. Chefles, ‘Quantum protocols for anonymous voting and surveying’. ArXiv quant-ph/0504161. To appear in Physical Review A.
    
    In preparation

38. A. Chefles, ‘Theory of maximally nonlocal bipartite unitary operations (working title)’.
     

39. A. Chefles, ‘On the quantum generalisation of Blackwell’s theorem’.
     

40. A. Chefles, A. Kitagawa, M. Takeoka, M. Sasaki and J. Twamley, ’Unambiguous discrimination among oracle operators’.