Looking at real experiments ﬁrst: Curricular and technical approaches for teaching elementary quantum physics
Declining interest in physics among adolescents is a universal phenomenon. One approach to making physics instruction more interesting is to cover current research topics on an elementary level. Many exciting topics are covered in science magazines. Quantum optics research is very suitable as an introductory example: Research is conducted in small groups, the experiments are tabletop-size and the mathematics are relatively simple. Originally, quantum optics have been taught in graduate courses, but a number of undergraduate courses including laboratories have been established over the last decade [1–3].
Our goal is to extend quantum optics teaching to students in secondary schools. The teacher then would not only talk about modern science, but also use recent terms and tools. In particular, photons are not treated as obscure particles of light, and quantum state preparation is discussed in detail: Single photons are prepared by parametric down conversion of photon pairs and measuring temporal coincidence of detection in spatially separated detectors. Real or interactive experiments are inseparable parts of the instruction.
We postulate that talking about quantum physics should involve quantum phenomena, which are not observable in classical physics. This is very well established in mechanics, for example, a free falling body is always treated as a rigid body unless it is so small and slow that one has to treat the problem with a matter wave package. On the other hand, the term photon is often used to explain the simplest phenomena of light and vision even in lower grades of secondary schools. We believe that this habit is a source of various misconceptions.
2.1 Hierarchy of theories
Traditionally, the quantum theory of light is regarded as part of quantum electrodynamics, which are taught as an extension of quantum mechanics, which again are an upgrade to classical mechanics for very small and slow particles. Ideas and terms of both classical and quantum mechanics come from tactile perception. The great miracle of quantum mechanics is the fact that quantum objects do not behave like we think, i.e. a particle such as the electron goes two ways in the double slit experiment, to name but one obscure characteristic.
For quantum optics as a special branch of quantum physics, another approach is obvious: Instead of going through all mechanics and electrodynamics, quantum optics are simply regarded as optics for non-classical states of light. Optics itself come from visual perception. A sketch of the hierarchy of theories is shown in ﬁgure 1.
Despite practical issues, it is well worthwhile to have a closer look at the alternative road: Nonclassical light behaves like light in any case, and particular quantum states such as single photon states just add certain quantum phenomena to the set of possible observations. There is no classical to quantum boundary as in quantum mechanics, where the term particle becomes useless and has to be replaced by the completely different concept of the wave packet. When looking at matter, one realizes that matter starts to behave like light at low momentum. This is quite astonishing, but rather a matter issue than a quantum issue. It is well possible to talk about advanced quantum physics including, for example, entanglement, without discussing this peculiar behaviour of matter at the very beginning of a curriculum.
To put it in plain and simple terms: Optics is the natural approach to quantum physics. Matter optics is a suitable description of quantum phenomena, but light mechanics is not.
2.2 Classical to quantum transition
Optical phenomena are spatial phenomena. There is no easy way to observe the oscillation of the electromagnetic ﬁeld directly. In our approach, the classical to quantum transition is characterized by the introduction of temporal relationship as a necessary (but not sufficient) condition.
In a standard single photon experiment, temporal relationship is introduced by a coincidence circuit for preparing single photon states. Alternatively, single photons can be generated on demand, where on demand again means a temporal relationship. In any quantum measurement, a quantum state interacts with a macroscopic apparatus instantly and irreversibly.
3 Praxis report
3.1 High school quantum physics
Based on the concept of introducing students to quantum physics via optical terms and experiments, a course for a grade 12 physics class has been developed and tested. Classical optics is inspired by reference .
A hallmark of optics is non-locality, as opposed to particle location in mechanics: i) Visual orientation relies on viewing distant objects from a ﬁxed direction. ii) All optical paths through a lens contribute equally to an image. Blocking part of the lens will dim the image homogenously. iii) Constricting optical paths by a stop causes diffraction. There is no way of preparing a single light beam. iv) Entangled photons exhibit non-local correlation. The non-local aspects are pointed out with experiments.
3.2 Physics Experience Programme
Quantum optical experiments are based on apparatus which are not commonly found in high schools. Therefore we offer a physics experience programme where high school students can work at four stations with lasers, optical ﬁbres, polarisation rotators and research laboratory mechanics. The students get to know all optical equipment, except those necessary for single photon quantum state preparation, namely the parametric down conversion crystal and the coincidence circuit. When looking at a quantum optics experiment afterwards, the students are less overwhelmed by the many compounds, as they know almost every component on the table by own experience.
3.3 Interactive Screen Experiments
Local schools can visit our quantum optics laboratory, but the number of locations with such a facility is quite limited so far. Therefore we provide interactive screen experiments via the internet .
3.4 The size of quantum objects
It turned out, that among the different experiments possible with single photon states, entanglement of a photon pair is the most interesting to students. In our experiment, we point out the non-local correlation of entangled pairs, when each component photon seems to be transmitted or reﬂected by a beam splitter randomly . Obviously, the entangled photon pair has a size of the order of the optical table, i.e. this quantum object is not tiny at all. The experiment contradicts classical expectations, because it is a true quantum experiment, but still it ﬁts into the framework of non-locality in optics.
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Jan-Peter Meyn is professor of physics education at the University of Erlangen-Nuremberg in Germany. His fields of interest are quantum optics, renewable energy, color and music. He can be reached at firstname.lastname@example.org
Disclaimer- The articles and opinion pieces found in this issue of the APS Forum on Education Newsletter are not peer refereed and represent solely the views of the authors and not necessarily the views of the APS.