In Quantum theory, light has both particle- and wave-like properties. For years, experiments that were able to show the particle-like nature were only feasible for well-established laboratories. However, recent advancements in technology have enabled the use of a down-converting crystal to study light's particle-like nature at an undergraduate level. Over the last few years, such experiments have become much more affordable. This talk describes two recent experiments that have been performed at Taylor University. In the first, we have used photon counters,half-wave plates, beam-splitting cubes, and a down-converting crystalto measure the quantity g2(0).The quantity g2(0) may be used to distinguish between the particle and wave nature of light.For our first experiment our results for g2(0) implied that light behaves as a particle. In the second experiment(currently in progress)the lab is currently set upto useentangled photons to make measurements related to Bell's inequalities. Bell's inequality allows one to distinguish quantum theory from theories involving hidden variables. Extensions of this experiment will include research on the quantum erasereffect.
In Quantum theory, light has both particle- and wave-like properties. For years, experiments that were able to show the particle-like nature were only feasible for well-established laboratories. However, recent advancements in technology have enabled the use of a down-converting crystal to study light's particle-like nature at an undergraduate level. Over the last few years, such experiments have become much more affordable. This talk describes two recent experiments that have been performed at Taylor University. In the first, we have used photon counters,half-wave plates, beam-splitting cubes, and a down-converting crystalto measure the quantity g2(0).The quantity g2(0) may be used to distinguish between the particle and wave nature of light.For our first experiment our results for g2(0) implied that light behaves as a particle. In the second experiment(currently in progress)the lab is currently set upto useentangled photons to make measurements related to Bell's inequalities. Bell's inequality allows one to distinguish quantum theory from theories involving hidden variables. Extensions of this experiment will include research on the quantum erasereffect.
Quantum physics, which describes the strange behavior of light and matter at the smallest scales, is one of the most successful descriptions of reality, yet it is notoriously inaccessible. Here we provide an approachable explanation of quantum physics using simple thought experiments. We derive all relevant quantum predictions using minimal mathematics, without introducing the advanced calculations that are typically used to describe quantum physics. We focus on the two key surprises of quantum physics, namely wave-particle duality, a term that was introduced to capture the fact that single quantum particles in some respects behave like waves and in other respects like particles, and entanglement, which applies to two or more quantum particles and brings out the inherent contradiction between quantum physics and seemingly obvious assumptions regarding the nature of reality. Following arguments originally made by John Bell and Lucien Hardy, we show that the so-called local hidden variables are inadequate at explaining the behavior of entangled quantum particles. This means that one either has to give up on hidden variables, i.e., the idea that the outcomes of measurements on quantum particles are determined before an experiment is actually carried out, or one has to relinquish the principle of locality, which requires that no causal influences should be faster than the speed of light and is a cornerstone of Einstein's theory of relativity. Finally, we describe how these remarkable predictions of quantum physics have been confirmed in experiments. We have successfully used the present approach in a course that is open to all undergraduate students at the University of Calgary, without any prerequisites in mathematics or physics.
While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g(2)(0)=0.0177±0.0026, which violates the classical inequality g(2)(0)⩾1 by 377 standard deviations.
The thesis documents the work done over the year to initiate an undergraduate Advanced Laboratory experiment which tests Bell's inequality.It provides reference theory for the experiment, including explanations of Bell inequalities, basics of nonlinear optics, type-I downconversion and entanglement, and polarization states of the entangled photons.A main result is the equipment and design proposal for the experiment, which will cost a total 19600, led in price by the 9000 of a four photodetector array and followed by the 5000 of a 405nm pump laser.Entangled photons are produced by pumping BBO in a two-crystal geometry.Although most of the light is transmitted, some undergoes type-I parametric downconversion.Degenerate pairs are in a tunable entangled state and can be used to show non-classical behaviour.Specifically, a violation of the CHSH Bell inequality can be observed.Usable coincidence rates of several thousand per second are expected.Experimental and data analysis methods are described as the basis of future laboratory documentation.Explanations of equipment alignment and adjustment and data collection are included, as well as derivations of relevant analyses of the experimental data.Lastly the coincidence circuit built for the experiment is reviewed.The circuit costs less than 40 to construct and demonstrates a coincidence window of between 18ns and 36ns.
We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein–Podolsky–Rosen paradox, we demonstrate strong polarization correlations of the entangled photons. Bell’s idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony, and Holt version of the Bell inequality finds S=2.307±0.035, in clear contradiction of hidden variable theories. The experiments described can be performed in an afternoon.
We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein-Podolsky-Rosen ``paradox,'' we demonstrate strong polarization correlations of the entanged photons. Bell's idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony and Holt version of the Bell inequality finds S=2.307pm0.035, in clear contradiciton of hidden variable theories. The experiments described can be performed in an afternoon.
Introduction Annus mirabilis 1964 1964 was a wonderful year for physics. In this annus mirabilis, summarising and simplifying a little, the quark model was proposed by Gell-Mann and Zweig and charm and colour properties were introduced, Higgs published his work on the scalar boson, the _ boson and CP violation were discovered, cosmic background radiation was identified and, finally, Bell inequalities were suggested. Most of these works were later subjects of a Nobel prize. Unluckily John Bell left us too early for this achievement, but, at least in my opinion, his work [1] is probably the most fruitful among all these huge advances, since now we can recognise that it was the root of a fundamental change in our vision of the physical world, providing, on one hand, the possibility of a test of quantum mechanics against a whole class of theories (substantially all the theories that we could define as “classical”) and, on the other hand, introducing a clear notion of quantum nonlocality that represents not only a challenge in understanding quantum mechanics, but also one of the most important resources paving the way to quantum technologies that were developed in the last decades based on the quantum correlations discussed in the Bell paper. In the following we will present the experimental progress in testing Bell inequalities. Rather amazingly even 50 years after Bell's paper, a conclusive test of his proposal is still missing [2, 3]. Nevertheless, huge progress has been achieved with respect to the first experiments.We will discuss these developments and the remaining problems for a conclusive test of Bell inequalities in detail. The Bell Inequalities, Their Hypotheses and the Experimental Loopholes In order to understand the experimental difficulties in achieving a final test of Bell inequalities, let us analyse a little how these inequalities are derived and the explicit and implicit hypotheses they contain. The proof of Bell inequalities usually starts by considering two separated subsystems of an entangled state, which are addressed to two independent measurement apparatuses measuring the expectation values of two dichotomic observables, Aa and B b . a, b are two parameters describing the settings of the measuring apparatus A and B , respectively.