1. Delayed-choice Experiment in Cavity QED Rameez-ul-Islam National Institute of Lasers and Optronics, Islamabad.

2. Layout of the Talk 1.Introduction to Quantum Eraser 2. Cavity QED based Quantum Eraser  Initial State  Proposed Experimental Schematics  Delayed-Choice, Asymmetric Erasing  Distunguishability and Visibility: Tuneability 3. Conclusions

3. COMPLEMENTARITYCOMPLEMENTARITY Introduction to Quantum Eraser

4. Double Slit Experiment: Core Mystery of the Quantum Theory Feynman famously said: “The double-slit experiment has in it the heart of quantum mechanics. In reality, it contains the only mystery” How a particle goes through both the slits simultaneously !!

5. Double Slit Experiment and Which-Path Information When we observe………!!

6. As Thomas Young taught us two Hundred years ago, photons interfere. But now we know that: Knowledge of path (1 or 2) is the reason why interference is lost. Its as if the photon knows it is being watched. But now we discover that: Erasing the knowledge of photon path brings interference back. Erasing Knowledge! “No wonder Einstein was confused.” Quantum Eraser: Newsweek, June 19, 1995, p. 68

7. Quantum Eraser: Mathematical Treatment

9. Kim, Yu, Kulik, Shih, and Scully, PRL 84, 1 (2000) Do the case of Quantum Eraser similar to the Bertelsmann Socks ?? IS IT JUST DATA RECOVERY OR MANIPULATION??

10. Cavity QED based Quantum Eraser Initial State: 1.Three-level ladder atom prepared in the coherent superposition of lower two-levels 2.A cavity Field prepared in the superposition of zero and one photon 3. Therefore the initial state comes to be; Next we want to tag or entangled the atom with the cavity field.

11. Proposed Experimental Schematics Tagging or entangling the atom with the cavity field. M.S. Zubairy, G.S. Agarwal, M.O. Scully, Phys. Rev. A 70 (2004) 012316.

13. Delayed-Choice, Asymmetric Erasing We note that the initial coherence of the atomic superposition state and the related accessibility of the Ramsey interference pattern is now lost due to its tagging or entanglement with the cavity field. To recover back the Ramsey fringes, we need to erase the information carried by the cavity field.

16. Distinguishability and Visibility: Englert–Greenberger–Yasin duality relation

17. Delayed-choice adjustability for the path distinguishability and fringe visibility Strange and Mystifying: Fringe visibility/distinguishability can be tuned in delayed-choice manner, even much later after the recording !! M. Imran et al, Annals of Physics 364, 160 (2016).

18. Conclusions: Time, information and the Quantum The QUANTUM ERASER highlights all the counterintuitive features of the quantum theory and may serve as a test for any interpretation of the theory whatsoever. 1. No experimental difference between real time and delayed-choice. Concept of time is contrastly different while dealing with classical and quantum scenarios. Moreover, entropy of a pure quantum state is always zero. So the arrow of time is something of a mystery here. Our universe appears to have a space-time structure in which all of time is laid-out in a "block universe", i.e., there is no actual "flow" of time, no movement of a "now" point.

19. 2. In quantum theory, one must not talk of any result, e.g. which path the quantum entity followed, in concrete terms unless an actual measurement is performed. Similarly it also hints out that state reduction should be better treated as a mental process concerning information rather than a physical process. 3. Recently, a lot of work has been carried out suggesting the ontological nature of the state vector or wavefunction. However, when such a conjectured reality of the wavefunction is matched with the DCQE results, it points out that the proposed reality of the wavefunction is not ‘something existing out in the universe having classical attributes’ and rather the state vector still carries all the characteristics of the quantum theory including retrocausality and the nonlocality. These ontological results therefore only marginally rule out the Ψ-epistemic models and shed little further light on the prominent foundational issues.

20. "If we attempt to attribute an objective meaning to the quantum state of a single system, curious paradoxes appear: quantum effects mimic not only instantaneous action-at-a- distance, but also, as seen here, influence of future actions on past events, even after these events have been irrevocably recorded." Asher Peres

21. We note here: The present depends on the future but there is no way the future can send a signal to the present !!

22. “How wonderful that we have met with a paradox. Now we have some hope of making progress.” Niels Bohr Thanks and any Questions………