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Register a new userThe Delayed-Choice Quantum Eraser Leaves No Choice
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Tabish Qureshi
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A realizable delayed-choice quantum eraser, using a modified Mach-Zehnder (MZ) interferometer and polarization entangled photons, is theoretically analyzed here. The signal photon goes through a modified MZ interferometer, and the polarization of the idler photon provides path information for the signal photon. The setup is very similar to the delayed-choice quantum eraser experimentally studied by the Vienna group. In the class of quantum erasers with discrete output states, it is easy to see that the delayed mode leaves no choice for the experimenter. The which-way information is always erased, and every detected signal photon fixes the polarization state of the idler, and thus gives information on precisely how the signal photon traversed the two paths. The analysis shows that the Vienna delayed-choice quantum eraser is the first experimental demonstration of the fact that the delayed mode leaves no choice for the experimenter, and the which-way information is always erased. Additionally it is shown that this argument holds even in a conventional two-slit quantum eraser. Every photon registered anywhere on the screen, fixes the state of the two-state which-way detector in a unique mutually unbiased basis. In the delayed-choice quantum eraser experiments, the role of mutually unbiased basis sets for the which-way detector, has been overlooked till now.
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In a delayed-choice quantum eraser, interference fringes are obtained by erasing which-way information after the interfering particle has already been irreversibly detected. Following an introductory review of delayed-choice experiments and quantum erasure, we describe the experimental realization of an optical delayed-choice quantum eraser, suitable for advanced undergraduates, based on polarization-entangled pairs of single photons. In our experiment, the delay of the erasure is implemented using two different setups. The first setup employs an arrangement of mirrors to increase the optical path length of the photons carrying which-way information. In the second setup, we use fiber-optic cables to elongate the path of these photons after their passage through the polarization analyzer but prior to their arrival at the detector. We compare our results to data obtained in the absence of a delay and find excellent agreement. This shows that the timing of the erasure is irrelevant, as also predicted by quantum mechanics. The experiment can serve as a valuable pedagogical tool for conveying the fundamentals of quantum mechanics.
Complementarity, that is the ability of a quantum object to behave either as a particle or as a wave, is one of the most intriguing features of quantum mechanics. An exemplary Gedanken experiment, emphasizing such a measurement-dependent nature, was suggested by Wheeler using single photons. The subtleness of the idea lies in the fact that the output beam-splitter of a Mach-Zehnder interferometer is put in or removed after a photon has already entered the interferometer, thus performing a delayed test of the wave-particle complementary behavior. Recently, it was proposed that using a quantum analogue of the output beam-splitter would permit carrying out this type of test after the detection of the photon and observing wave-particle superposition. In this paper we describe an experimental demonstration of these predictions using another extraordinary property of quantum systems, entanglement. We use a pair of polarization entangled photons composed of one photon whose nature (wave or particle) is tested, and of a corroborative photon that allows determining which one, or both, of these two aspects is being tested. This corroborative photon infers the presence or absence of the beam-splitter and until it is measured, the beam-splitter is in a superposition of these two states, making it a quantum beam-splitter. When the quantum beam-splitter is in the state present or absent, the interferometer reveals the wave or particle nature of the test photon, respectively. Furthermore, by manipulating the corroborative photon, we can continuously morph, via entanglement, the test photon from wave to particle behavior even after it was detected. This result underlines the fact that a simple vision of light as a classical wave or a particle is inadequate.
Many paradoxes of quantum mechanics come from the fact that a quantum system can possess different features at the same time, such as in wave-particle duality or quantum superposition. In recent delayed-choice experiments, a quantum mechanical system can be observed to manifest one feature such as the wave or particle nature, depending on the final measurement setup, which is chosen after the system itself has already entered the measuring device; hence its behaviour is not predetermined. Here, we adapt this paradigmatic scheme to multi-dimensional quantum walks. In our experiment, the way in which a photon interferes with itself in a strongly non-trivial pattern depends on its polarisation, that is determined after the photon has already been detected. Multi-dimensional quantum walks are a very powerful tool for simulating the behaviour of complex quantum systems, due to their versatility. This is the first experiment realising a multi-dimensional quantum walk with a single-photon source and we present also the first experimental simulation of the Grover walk, a model that can be used to implement the Grover quantum search algorithm.
Wave-particle duality epitomizes the counterintuitive character of quantum physics. A striking illustration is the quantum delay-choice experiment, which is based on Wheelers classic delayed-choice gedanken experiment, but with the addition of a quantum-controlled device enabling wave-to-particle transitions. Here we realize a quantum delayed-choice experiment in which we control the wave and the particle states of photons and in particular the phase between them, thus directly establishing the created quantum nature of the wave-particle. We generate three-photon entangled states and inject one photon into a Mach--Zehnder interferometer embedded in a 187-m-long two-photon Hong-Ou-Mandel interferometer. The third photon is sent 141m away from the interferometers and remotely prepares a two-photon quantum gate according to independent active choices under Einstein locality conditions. We have realized transitions between wave and particle states in both classical and quantum scenarios, and therefore tests of the complementarity principle that go fundamentally beyond earlier implementations.
Quantum nonlocality which is conventionally invoked for describing a composite entangled system is shown here to be a possible important characteristic of a single quantum object. To this end, we analyze some interactions of a single photon released from Fabry Perot resonator with environment. The split photon state with oppositely moving parts is shown to obey quantum nonlocality despite the sharp edges truncating each part. Photon post release reflection from a plane mirror is considered. The changing shape of the form during reflection contains moving discontinuities in electric and magnetic components of the pulse. They originate from preexisting edges of the form and move together, first away from and then back to the mirror. At the end of the process, the pulse restores its original shape, with electric component reversed. Altogether, the process demonstrates conservation of moving discontinuities. The considered experimental setup may be used for some ne
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