Abstract: People often say, and this is impossible to understand:“The photon learned the bomb was there without interacting with it.”
In that famous QUANTUM BOMB TESTER (QBT) experiment one can detect the presence of an object with CERTAINTY without interfering with it, and from OBSERVING A SINGLE PHOTON (appearing in a particular place).
Many view this as ultimate in quantum weirdness. However, it is actually not surprising because Patrice Ayme shows that the famous Quantum Bomb experiment is simply a variation on the theme of the two-slit experiment (which can be rigged carefully in a bomb-like setup, namely knowing something is not there from receiving a single photon).
The Quantum Bomb Tester is only mysterious if one denies ontological status to the wave. In other words the QBT is a mystery only for the fans of Copenhagen Interpretation of the Quantum (CIQ).
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But before I explain that iconoclastic viewpoint, let me explain what the bomb experiment is, and then how the superluminal pilot wave explains it naturally. From the horses’ mouths, slightly modified for clarity:
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QUANTUM MECHANICAL INTERACTION-FREE MEASUREMENTS Avshalom C. Elitzur(a) and Lev Vaidman School of Physics and Astronomy Tel-Aviv University 69 978 Tel Aviv, ISRAEL.
A novel manifestation of nonlocality of quantum mechanics is presented. It is shown that it is possible to ascertain the existence of an object in a given region of space without interacting with it. The method might have practical applications for delicate quantum experiments.
INTRODUCTION Nonlocality is an intriguing aspect of quantum mechanics. Bell’s inequality1 showed that nonlocality must exist, and Aspect 2 provided an experimental proof. We shall present here yet another manifestation of the nonlocality of quantum mechanics.
We shall describe a measurement which, when successful, is capable of ascertaining the existence of an object in a given region of space, though no particle and no light “touched” this object. This is a new type of an interactionfree quantum measurement which has no classical analog.
Let us begin with a brief review of nonlocal measurements which yield information about the existence of an object in a given region of space. If an object is charged or has an electric (magnetic) moments, then its existence in a given region can be inferred without any particle passing through that region, but rather by the measurement of the electric (magnetic) field the object creates outside the region.
Quantum mechanics allows inferring the existence of an object in a nonlocal way via Aharonov-Bohm effect even when the object creates no electromagnetic field outside a certain space region, but only an electromagnetic potential. Even if the object creates no detectable change at a distance, i.e., it interacts with the external world only locally, its location can often be found in a simple nonlocal interaction-free measurement (i.e., without interacting with the object).
For example, assume it is known that an object is located in one out of two boxes. Looking and not finding it in one box tells us that the object is located inside the other box.
A more sophisticated example of obtaining information in a nonlocal way is the measurement performed on a system prepared in the Einstein-Podolsky-Rosen state. If two objects are prepared in an eigenstate of relative position, the measurement of the position of one object yields the position of the other. In the above cases, what allowed us to infer that an object is located in a given place by performing an interaction-free measurement was the information about the object prior to the measurement. In the first example we knew that the object is located inside one of the two boxes, and in the second example we knew about the correlation between the position of one object and that of another.
The question we address in this Letter is this: Is it possible to obtain knowledge about the existence of an object in a certain place using interaction free measurements without any prior information about the object? The answer is, indeed, in the affirmative as we proceed to show.
Our method is based on a particle interferometer which is analogous to the Mach-Zehnder interferometer of classical optics. In principle, it can work with any type of particle.
A particle reaches the first beam splitter… The transmitted and reflected parts of the particle’s wave are then reflected by the mirrors in such a way that they are reunited at another, similar beam splitter. Two detectors collect the particles after they pass through the second beam splitter.
We can arrange the positions of the beam splitters and the mirrors so that, due to the destructive interference, no particles are detected by one of the detectors, say D1 (but all are detected by D2). If, without changing the positions of the mirrors and the beam splitters, we block one of the two arms of the interferometer, the particles which succeeded to pass through the interferometer are detected with equal probability by both detectors D1 and D2. Thus, detector D1 detects particles IF and ONLY IF if something stands in the way of particles in one of the routes of the interferometer.
A practical realization of such an interferometer with electrons and protons is hampered by strong electromagnetic interaction with the environment, but neutron interferometers operate in many laboratories. However, our method requires a single particle interferometer, i.e. an interferometer with one particle passing through it at a time, and there is no appropriate neutron source which produces a single particle state.
Recently experiments were performed with a source of single photon states. Thus we propose to use the Mach-Zehnder interferometer with such a source of single photons.
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HOW TO FIND AN OBJECT WITHOUT INTERACTING WITH IT? Our procedure for finding out about the existence of an object in a given place, without passing even one photon through it, is as follows: We arrange a photon interferometer as described above, i.e. no photons are detected by D1 when both routes of the interferometer are open, and position it in such a way that one of the routes of the photon passes through the region of space where we want to detect the existence of an object.
We send a single photon through the system. There are three possible outcomes of this measurement: i) no detector clicks, ii) detector D2 clicks, iii) detector D1clicks. In the first case, the photon has been absorbed (or scattered) by the object and never reached the detectors. The probability for this outcome is 1/2. In the second case (the probability for which is ¼), the measurement has not succeeded either. The photon could have reached D1 in both cases: when the object is, and when the object is not located in one of the arms of the interferometer. In this case there has been no interaction with the object so we can try again. Finally, in the third case, when the detector D1 clicks (the probability for which is 1/4), we have achieved our goal: we know that there is an object inside the interferometer without having “touched” the object. Indeed, we saw that the necessary condition for D1 to detect a photon is that one of the routes of the interferometer is obstructed; therefore the object must be there. This is an interaction-free measurement because we had only one photon and has it interacted with the object, it could never reach detector D1
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SQPR, (and all Pilot Wave Theories) SHOWS WHY The QUANTUM BOMB TESTER IS OBVIOUS:
In SQPR the Guiding Pilot Wave, (very) roughly the one De Broglie talked about in 1923, forges ahead and establishes a (Bohmian-like) linear interference field. If the bomb is in the way, the PILOT WAVE can’t get through, and thus the guiding field is altered. So, whereas if there is no bomb there is never any field reaching D1
(I have said in the past that the Quantum Bomb Tester proves Pilot Waves and SQPR; that’s not correct mathematically: the two theories are equivalent in this particular kind of experiment; however Pilot Waves theories make the situation conceptually obvious, no mysterious collapse, no many worlds; remark that if the interferometer is cosmic size, the usual mumbo jumbo that a particle is simultaneously in both branches sounds particularly silly…)
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The original Avshalom Elitzur–Lev Vaidman setup only succeeds 25% of the time; 50% of the time the bomb explodes. So the measurement is “interaction-free” only in the successful branch, not globally. That weakens any mystical reading. Moreover, one can craftily engineer the two slit along the same lines., as follows:
We could invert the 2 slit situation: knowing there is ONLY A SINGLE slit as soon as we detect a photon in the no photon inteference fringe: if one detects a photon in a location that is normally a destructive-interference minimum, one know coherence has been disturbed.
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Why do some physicists call the Quantum Bomb Tester mysterious? It’s because they believe in the Copehnagen Interpretation Quantum, CIQ (“sick”). The “mystery” language usually arises because: A detector’s click certifies the bomb’s presence in a branch of MZI. Yet in that branch, no energy was transferred. And it works in a single shot: no slow statistical built up. .
From a realist wave perspective — Bohmian, de Broglie, or the SQPR first-approximation regime — the bomb experiment is conceptually tame.(One of SQPR axioms is that the usual waves of QM are those of SQPR, in first approximation, at scales which are neither cosmic nor approaching Planck length…)
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PHILOSOPHICAL CORE OF THE QUANTUM BOMB:
The bomb experiment mainly exposes the following tension. Is the wave:epistemic (information only)? Or is it ontic (physically real structure)? If the latter, ontic, then the experiment is natural. If the wave is epistemic, a wave of probabilities, the experiment feels strange.
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The usual narrative says: The photon “would have exploded” the bomb if it went that way. But standard (CIQ) Quantum Mechanics does not assign truth values to that counterfactual (Heisenberg was proud of not assigning truth values to counterfactuals, telling an aghast Einsetin that he and his CIQ friends learned that trick from… Einstein…)
Instead: The amplitude associated with that path was removed by the absorber (the “bomb”). That removal changed interference structure. Nothing says that the photon had a definite unrealized trajectory. So the experiment does not validate counterfactual definiteness (a technical term in Bell experiments science and considerations on reality).
It validates something subtler: Non-actualized branches still shape physical outcomes. That is a statement about the ontology of superposition.
The bomb experiment exposes a tension between two pictures of reality: Classical ontology where Reality = what actually happens. And Quantum ontology where Reality = structure of allowed amplitudes (amplitudes replacing the First Law Of Mechanics from Burida, a discovery published by Louis De Broglie in 1923!)
In quantum mechanics, the unrealized is not nothing, it is dynamically active.. That is radical.
In Pilot-Wave theories (de Broglie/Bohm/ SQPR) what happens is clear: the Pilot Wave gets blocked. More can be said, as SQPR depends upon its own gradient… But another time.
Patrice Ayme
Patrice Ayme's Thoughts 