# Analyzing quantum foundations and applications with single photon experiments

`2017-11-01 14:00:00``2017-11-01 15:00:00``Analyzing quantum foundations and applications with single photon experiments``Quantum optics experiments employing a single photon source, as well as a single photon detector, allow us to probe the foundations of quantum theory. In the last few years I have designed a line of experiments based on weak measurements [1] for this purpose. Two of them have already been performed: 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 noncommutative operators, practically measured on the same state. 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 to defy the statistical character of the expectation value, which up to now was always evaluated using a large ensemble of similarly prepared particles. I will discuss some consequences of these experiments, both theoretical (e.g. the meaning of the wavefunction) and practical (e.g. state and process tomography). If time allows me, I will outline some of our upcoming experiments, such as those concerned with the study of entanglement and nonlocality [6,7]. References [1] Y. Aharonov, D.Z. Albert, L. Vaidman, How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100, 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, Measuring incompatible observables by exploiting sequential weak values, 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, Determining the quantum expectation value by measuring a single Photon, forthcoming in Nat. Phys., doi:10.1038/nphys4223 (2017). [4] Y. Aharonov, L. Vaidman, Measurement of the Schrödinger wave of a single particle, Phys. Lett. A 178, 38–42 (1993). [5] Y. Aharonov, E. Cohen, A.C. Elitzur, Foundations and applications of weak quantum measurements, Phys. Rev. A 89, 052105 (2014). [6] Y. Aharonov, E. Cohen, A.C. Elitzur, Can a future choice affect a past measurement’s outcome?, Ann. Phys. 355, 258-268 (2015). [7] A. Brodutch, E. Cohen, Nonlocal Measurements via Quantum Erasure, Phys. Rev. Lett. 116, 070404 (2016).``Nano-center, 9th floor seminar room``Department of Physics``physics.dept@mail.biu.ac.il``Asia/Jerusalem``public`Quantum optics experiments employing a single photon source, as well as a single photon detector, allow us to probe the foundations of quantum theory. In the last few years I have designed a line of experiments based on weak measurements [1] for this purpose. Two of them have already been performed:

- 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 noncommutative operators, practically measured on the same state.
- 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 to defy the statistical character of the expectation value, which up to now was always evaluated using a large ensemble of similarly prepared particles.

I will discuss some consequences of these experiments, both theoretical (e.g. the meaning of the wavefunction) and practical (e.g. state and process tomography).

If time allows me, I will outline some of our upcoming experiments, such as those concerned with the study of entanglement and nonlocality [6,7].

References

[1] Y. Aharonov, D.Z. Albert, L. Vaidman, How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100, 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, Measuring incompatible observables by exploiting sequential weak values, 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, Determining the quantum expectation value by measuring a single Photon, forthcoming in Nat. Phys., doi:10.1038/nphys4223 (2017).

[4] Y. Aharonov, L. Vaidman, Measurement of the Schrödinger wave of a single particle, Phys. Lett. A 178, 38–42 (1993).

[5] Y. Aharonov, E. Cohen, A.C. Elitzur, Foundations and applications of weak quantum measurements, Phys. Rev. A 89, 052105 (2014).

[6] Y. Aharonov, E. Cohen, A.C. Elitzur, Can a future choice affect a past measurement’s outcome?, Ann. Phys. 355, 258-268 (2015). [7] A. Brodutch, E. Cohen, Nonlocal Measurements via Quantum Erasure, Phys. Rev. Lett. 116, 070404 (2016).

Last Updated Date : 17/10/2017