In a series of lectures, Susskind gives a \"classical\" version of entanglement.

Question

In a series of lectures, Susskind gives a "classical" version of entanglement. Al places two coins in two boxes: a dime in one and a nickel in the other. A courier comes, chooses one at random and delivers it to Nathan 10,000 km away. Nathan calls Al and says the box has arrived. Al opens the box left behind and knows instantly what is in the box Nathan received, even before he opens it. Nathan still doesn't know what is in his box unless Al tells him or he opens his box.

1. How does the quantum version differ from this example?

2. In quantum, before either particle is detected, what is known about the two particles which are entangled?

3. Compared to #2. above, what do the results of the delayed choice double slit experiment seem to say about the states of the entangled particles before they are detected? Are they already set or are we just ignorant of what state they are already in?

Explanation / Answer

1.The key difference then is at what point the photon is observed / interacted with and therefore localised - either close enough to the slits to reveal which slit it passed through, or far enough such that it remains uncertain. The former will produce the two bar pattern, the latter the interference pattern.

2.The double-slit experiment strikingly demonstrates the wave-particle duality of quantum objects. In this famous experiment, particles pass one-by-one through a pair of slits and are detected on a distant screen. A distinct wave-like pattern emerges after many discrete particle impacts as if each particle is passing through both slits and interfering with itself. While the direct event-by-event buildup of this interference pattern has been observed for massive particles such as electrons, neutrons, atoms and molecules, it has not yet been measured for massless particles like photons. Here we present a temporally- and spatially-resolved measurement of the double-slit interference pattern using single photons. We send single photons through a birefringent double-slit apparatus and use a linear array of single-photon detectors to observe the developing interference pattern. The analysis of the buildup allows us to compare quantum mechanics and the corpuscular model, which aims to explain the mystery of single-particle interference. Finally, we send one photon from an entangled pair through our double-slit setup and show the dependence of the resulting interference pattern on the twin photon's measured state. Our results provide new insight into the dynamics of the buildup process in the double-slit experiment, and can be used as a valuable resource in quantum information applications.

3.

Delayed choices undermines past to future causality only if one assumes that the initial state of a system must directly determine measurement results, and that wave functions represent our ignorance of an underlying state which is already determined and only revealed by measurements, but there are other interpretations.

In sum, the situation is as follows: after a measurement, in retrospective, we can come up with a nice classical story of what happened, but the story would have been different (including for the initial state) had we decided to measure something else. But before the measurement, we cannot know the relevant aspects of the initial state that would tell us which kind of "classical story" will occur. All "classical stories" are still possible, so it's not as if the measurement changes something we knew for sure about the system, that suddenly became false. Measurement only affects something we would know if we had measured the system differently, so it's a problem only if one assumes that this "something we would know" must already be determined in some way from the start of the experiment or in other words, if we insist in having classical stories of experiments.

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.