Lecturer: Eliahu Cohen, H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, U.K.
Abstract:
Quantum optics experiments employing a single photon source, as well as a single photon detector,allow us to probe the foundations of quantum theory. Furthermore, they often serve as a proof-of-principle for various quantum technologies. In the last few years I have initiated and designed a line of experiments utilizing weak measurements [1] for these purposes. Two of them have already been performed:
1. Measuring incompatible observables by exploiting sequential weak values [2] – We
measured for the first time the polarization of single photons in two incompatible bases.
By performing a sequence of two weak measurements over a large ensemble of single
photons, we were thus able to infer the information regarding two non-commutative
operators, practically measured on the same state.
2. Determining the quantum expectation value by measuring a single photon [3] – Here we
did not use an ensemble of photons, but rather employed the quantum Zeno effect for
inferring the polarization expectation value using a genuine single photon. This was the
first demonstration of protective measurement [4], similar in spirit to our proposal in [5].
The protection mechanism allows going beyond the statistical character of the expectation
value, which up to now was always evaluated using a large ensemble of similarly prepared particles. In terms of Fisher information, this method was shown to be advantageous in comparison with ordinary projective measurement.
I will discuss several promising consequences of these experiments, both theoretical (e.g.
connection with the Feynman sum over histories [6], as well as the fundamental role of the wavefunction) and practical (e.g. state and process tomography).
Finally, I will outline some of our upcoming experiments (in Turin, Ottawa, Kyoto), mainly those related to our study of entanglement and nonlocality (e.g. [7,8,9]). I will focus on forthcoming experiments I designed for testing our newly proposed tight bound on quantum correlations [9] and for ruling out a large class of nonlocal hidden variables.
References
[1] Y. Aharonov, D.Z. Albert, L. Vaidman, Phys. Rev. Lett. 60, 1351 (1988).
[2] F. Piacentini, A. Avella, M.P. Levi, M. Gramegna, G. Brida, I.P. Degiovanni, E. Cohen, R. Lussana, F.
Villa, A. Tosi, F. Zappa, M. Genovese, Phys. Rev. Lett. 117, 170402 (2016).
[3] F. Piacentini, A. Avella, E. Rebufello, R. Lussana, F. Villa, A. Tosi, M. Gramegna, G. Brida, E. Cohen,
L. Vaidman, I.P. Degiovanni, M. Genovese, forthcoming in Nat. Phys., doi:10.1038/nphys4223 (2017).
[4] Y. Aharonov, L. Vaidman, Phys. Lett. A 178, 38–42 (1993).
[5] Y. Aharonov, E. Cohen, A.C. Elitzur, Phys. Rev. A 89, 052105 (2014).
[6] D. Georgiev, E. Cohen, arXiv:1709.08479.
[7] Y. Aharonov, E. Cohen, A.C. Elitzur, Ann. Phys. 355, 258-268 (2015).
[8] A. Brodutch, E. Cohen, Phys. Rev. Lett. 116, 070404 (2016).
[9] A. Carmi, E. Cohen, Relativistic causality limits nonlocality via indeterminism, under review in Nature Communications.