These two pictures hold an incredible mystery for reality.
They demonstrate clearly that quantum particles (we are all made from atoms, and therefore quantum particles), have a very different reality to our classical intuition.
The picture below shows the arrangement of a relatively simple experiment: a source of light from (S), impacts a beam splitter (BS1). A half of the light is transmitted along the path (T) and a half of the light is reflected along (R). Both paths of light hit a mirror (M) and then make their way to detectors (D1) and (D2). The detectors each record that half of the light comes from path (T) and half from path (R), exactly as expected. This is a simple experiment that has been repeated many times. It has also been undertaken using a statistically significant number of single photons of light, where again the detectors record on average, one half of the photons at each detector.
A beam splitter comprises two triangular glass prisms, glued together with an epoxy, with the epoxy set at a thickness to ensure that light of a particular frequency transmits a half and reflects a half. It is a standard piece of optical equipment.
The following picture shows the same arrangement as above, but with a further beam splitter (BS2) located at the intersection of the transmitted and reflected light paths. Intuition would suggest that as for the first beam splitter (BS1), one half of the photons along the transmitted path (T) would be transmitted and one half reflected, and one half of the photons along the reflected path (R) would be transmitted and one half reflected, resulting in each detector again recording 50% of the light reaching each of them. Indeed, if the reflected path is blocked from reaching the second beam splitter, then the light from the transmitted path (T) is indeed split into transmitted and reflected halves that lead to the detectors recording 50% of the light reaching each of them. Exactly as expected.
But, the picture below shows that if the reflected path (R) is allowed to interact with the transmitted path (T), a very unexpected thing happens – ALL of the light ends up at (D1) and NONE of the light ends up at (D2). This is true, even if single photons are passed through the beam experiment, one at a time.
What is going on?
The pictures above are from quantum Tutorials by Frank Rioux, Emeritus Professor of Chemistry
In terms of quantum mechanics, which describes the behaviour of quantum particles such as photons, there is no mystery, and the expected result is obtained. This is because, in reality, the single light photon interacting with the first beam splitter (BS1) results in a new quantum state that is a superposition of both transmitted and reflected photon states; the superposition expresses the 50% probability of the photon being transmitted or reflected, and like Shrodinger’s cat, the photon exists in a quantum state of being both reflected and transmitted, until “detected”. When this quantum superposition interacts with the second beam splitter (BS2), the transmitted and reflected parts of the superposition interfere, just as in the experiment discussed in the first Blog July 2024 Blog, such that constructive interference occurs at (D1) and destructive interference occurs at (D2): all the photons are detected at (D1) when the second beam splitter (BS2) is in position, demonstrating that the classical interpretation that a photon exists and is real and passes through the experiment in either (T) or (R) is incorrect, and conflicts with this simple experiment which has been repeated many time. The quantum interpretation is that the photon passes through the experiment as a superposition of both (T) and (R), with a probability of being reflected of 50% and a probability of being transmitted of 50%. Mathematically, the photon is actually represented as \( {\frac {1 } {\sqrt(2)} } (T + iR ) \), very different to a classical representation which would be either T or R.
So in reality, the quantum photon particle actually goes through all paths to the detector in a state that represents the probability of being a particular state when “detected”, rather than as a specific state – reality, really is strange!
Recently, I was in a shop with four others, in which one of us asked the others, how much do we think this dress costs – four very different values were assigned to this garment. It was as if, at this point, until one of us took the dress to the front of the shop to ask the price thereby “detecting” the real value of this dress, the value for this dress for the four of us was a quantum superposition of all these guessed values; the analogy to the experiment above breaking down at the point we attempted to pay for it, as the shop had a value in mind much greater than any of the guessed values!
Next time we will look at Bell’s Inequality, which conclusively shows that quantum particles have no defined values for their parameters until “detected” – future Blogs will look at the measurement problem of quantum mechanics (i.e. what does “detected” actually mean), and how is it that since we and pretty much everything we interact with are comprised of quantum particles, that the world we live in appears so different to this strange quantum world of probabilities and quantum reality.
Mind-blowing stuff. Look forward to future episodes!