DELAYED CHOICE QUANTUM ERASER (LAB VERSION) by Robert Nemiroff Michigan Tech Physics X: About This Course • Officially "Extraordinary Concepts in Physics" • Being taught for credit at Michigan Tech o Light on math, heavy on concepts o Anyone anywhere is welcome • No textbook required o Wikipedia, web links, and lectures only o Find all the lectures with Google at: "Starship Asterisk" then "Physics X" o http://bb.nightskylive.net/asterisk/viewforum.php?f=39 DELAYED CHOICE QUANTUM ERASER This is an actual experiment conducted invoking a "delayed choice quantum eraser", one of the strangest and most thought provoking in physics. This experiment demonstrates a specific type of double slit experiment where "which slit" information can be found and even erased by actions AFTER the experiment has been completed. Y-H. Kim et al., PRL, 1999 Scully, M., Phys. Rev A, 1982 Explanation help from B. Greene's book The Fabric of the Cosmos SPONTANEOUS PARAMETRIC DOWNCONVERSION SPDC: A process that converts a single photon into a pair of entangled photons. One output photon is typically labeled the "signal" photon, while the other the "idler" photon. DELAYED CHOICE QUANTUM ERASER: EXPERIMENTAL SETUP DELAYED CHOICE QUANTUM ERASER: DESCRIPTION Overview: Light is emitted toward a double slit screen. Behind each slit lies a SPDC that converts each entering photon into two entangled photons. One set of photons, the "signal" photons, goes north (up). The other set of photons, the "idler" photons go south (down). DELAYED CHOICE QUANTUM ERASER: EXPERIMENT I Assume that the two possible photon streams heading south are kept separate and strike separate detectors. Therefore a southern experimenter would know which slit each southern photon went through. The corresponding entangled two photon streams heading north, however, are combined on a single image screen. What pattern appears on this northern image screen? 1. An interference pattern. 2. No interference pattern. 3. Directions out of crazy-ville. DELAYED CHOICE QUANTUM ERASER: EXPERIMENT I 2. No interference pattern. Since the southern set of photons can be traced back to specific slits, and since the southern photons are entangled with the northern photons, "which-slit" information for the northern photons can be inferred, and so no interference pattern will result. DELAYED CHOICE QUANTUM ERASER: EXPERIMENT II Same as Experiment I, except now some of the "which slit" information is erased for the southern photons. In this experiment, two distinct southern groups have their "which slit" information erased. What happens to these distinct groups of southern photons? 1. 2. 3. 4. Each group shows an interference pattern. Neither groups shows an interference pattern. One group shows an interference pattern, the other does not. The photons move to an easier experiment. DELAYED CHOICE QUANTUM ERASER: EXPERIMENT II 1. Each group shows an interference pattern. Since which slit information is lost, these southern photons could be considered, again, to have gone through both slits and interfered. A further oddity is that the two southern interference patterns are offset compared to the other. Specifically, when one interference pattern peaks, the other valleys. (This will turn out to be important.) DELAYED CHOICE QUANTUM ERASER: EXPERIMENT II Using timing coincidence information, the northern photons can be found that are entangled with the two sets of southern photons that showed the interference patterns. Together on the northern screen, what pattern do these northern photons show? 1. 2. 3. 4. An interference pattern. No interference pattern. Two interference patterns superposed. God does not play dice with the universe. DELAYED CHOICE QUANTUM ERASER: EXPERIMENT II 2. No interference pattern. AND 3. Two interference patterns superposed. Since they were entangled with the southern photons, they show the same patterns as the southern photons whose "which slit" information had been erased. Therefore they show two interference patterns -- but this time superposed on the same screen. Since each interference pattern was offset from the other, the peaks and valleys filled in to APPEAR like there was no interference at all. DELAYED CHOICE QUANTUM ERASER: CONCLUSIONS Therefore • • • • even though the northerner sees no interference pattern, she still does not know which of his northern photons went through which slits. the northerner thinks that the southerner can use her entangled photons to figure out "which slit" information for each northern photon. in Experiment I, the southerner would indeed infer which slit information or each northern photon. in Experiment II, the southerner would infer which slit information for only some of the northern photons. DELAYED CHOICE QUANTUM ERASER: CONCLUSIONS Furthermore • • • the southern half of the experiment could be chosen to be either Experiment I or Experiment II YEARS AFTER the photons have impacted the northerners' image screen. Whether some northern photons take one path or both paths depends on a southern experiment that can happen years in the future. the present depends on the future. DELAYED CHOICE QUANTUM ERASER: DESCRIPTION Finally: • no matter which experiment the southerner does (I or II), the northerner will see the same result: no interference pattern. • there is no way the future can send a signal to the present.