Delayed Choice Quantum Eraser - The Experiment

The Experiment

The experimental setup, described in detail in the original paper, is as follows. First, a photon is generated and passes through a double slit apparatus (vertical black line in the upper left hand corner of the diagram).

The photon goes through one (or both) of the two slits, whose paths are shown as red or light blue lines, indicating which slit the photon came through (red indicates slit A, light blue indicates slit B).

So far, the experiment is like a conventional two-slit experiment. However, after the slits a beta barium borate crystal (labeled as BBO) causes spontaneous parametric down conversion (SPDC), converting the photon (from either slit) into two identical entangled photons with 1/2 the frequency of the original photon. These photons are caused to diverge and follow two paths by the Glan-Thompson Prism.

One of these photons, referred to as the "signal" photon (look at the red and light-blue lines going upwards from the Glan-Thompson prism), continues to the target detector called D₀. The positions where these "signal" photons detected by D₀ occur can later be examined to discover if collectively those positions form an interference pattern.

The other entangled photon, referred to as the "idler" photon (look at the red and light-blue lines going downwards from the Glan-Thompson prism), is deflected by a prism that sends it along divergent paths depending on whether it came from slit A or slit B.

Somewhat beyond the path split, beam splitters (green blocks) are encountered that each have a 50% chance of allowing the idler to pass through and a 50% chance of causing it to be reflected. The gray blocks in the diagram are mirrors.

Because of the way the beam splitters are arranged, the idler can be detected by detectors labeled D₁, D₂, D₃ and D₄. Note that:

If it is recorded at detector D₃, then it can only have come from slit B.

If it is recorded at detector D₄ it can only have come from slit A.

If the idler is detected at detector D₁ or D₂, it might have come from either slit (A or B).

Thus, which detector receives the idler photon either reveals information, or specifically does not reveal information, about the path of the signal photon with which it is entangled.

If the idler is detected at either D₁ or D₂, the which-path information has been "erased", so there is no way of knowing whether it (and its entangled signal photon) came from slit A or slit B.

Whereas, if the idler is detected at D₃ or D₄, it is known that it (and its entangled signal photon) came from slit B or slit A, respectively.

By using a coincidence counter, the experimenters were able to isolate the entangled signal from the overwhelming photo-noise of the laboratory - recording only events where both signal and idler photons were detected.

When the experimenters looked only at the signal photons whose entangled idlers were detected at D₁ or D₂, they found an interference pattern.

However, when they looked at the signal photons whose entangled idlers were detected at D₃ or similarly at D₄, they found no interference.

This result is similar to that of the double-slit experiment, since interference is observed when it is not known which slit the photon went through, while no interference is observed when the path is known.

However, what makes this experiment possibly astonishing is that, unlike in the classic double-slit experiment, the choice of whether to preserve or erase the which-path information of the idler need not be made until after the position of the signal photon has already been measured by D₀.

There is never any which-path information determined directly for the photons that are detected at D₀, yet detection of which-path information by D₃ or D₄ means that no interference pattern is observed in the corresponding subset of signal photons at D₀.

The results from Kim, et al. have shown that whether the idler photon is detected at a detector that preserves its which-path information (D₃ or D₄) or a detector that erases its which-path information (D₁ or D₂) determines whether interference is seen at D₀, even though the idler photon is not observed until after the signal photon arrives at D₀ due to the shorter optical path for the latter.

Some have interpreted this result to mean that the delayed choice to observe or not observe the path of the idler photon will change the outcome of an event in the past. However, an interference pattern may only be observed after the idlers have been detected (i.e., at D₁ or D₂).

Note that the total pattern of all signal photons at D₀, whose entangled idlers went to multiple different detectors, will never show interference regardless of what happens to the idler photons. One can get an idea of how this works by looking carefully at both the graph of the subset of signal photons whose idlers went to detector D₁ (fig. 3 in the paper), and the graph of the subset of signal photons whose idlers went to detector D₂ (fig. 4), and observing that the peaks of the first interference pattern line up with the troughs of the second and vice versa (noted in the paper as "a π phase shift between the two interference fringes"), so that the sum of the two will not show interference.