Manual Quantum Superposition: Counterintuitive Consequences of Coherence, Entanglement, and Interference

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Philip Ball investigates the debate. The quirks of quantum physics are something you might expect to find under exotic conditions in a laboratory, but not in a meadow. This involves the superpositions of electronic quantum states, which seem able to explore many energy-transmitting pathways at once. If so, quantum mechanics is assisting the fundamental energetic process that drives all life on the surface of the Earth. It was a remarkable claim. But was it true? The question has been hotly debated over the past decade.

And some researchers insist that such coherence plays no useful role in photosynthesis at all. Both plants and photosynthetic bacteria capture solar energy in much the same way figure 1.

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Containing the pigments such as chlorophyll, it is surrounded by light-harvesting complexes that enhance the absorption of photons. There, the energy separates positive and negative ions by some significant distance, rendering them unable to neutralize one another by charge recombination. This separation of charge sets up an electrochemical potential that is ultimately tapped to drive enzyme-assisted reactions for making the energy-storage molecule adenosine triphosphate ATP — the main reservoir of chemical energy for powering biochemical metabolic reactions in cells figure 2.

But this transfer of solar energy to the reaction centre is extremely efficient: almost every photon of the captured solar energy is used by the cell. The mystery is how photosynthetic organisms achieve such efficiency in a warm, wet, disorderly environment. Until quantum coherence — the synchronous phase relation between different states of a quantum system — entered the picture, energy transport in photosynthesis was thought to involve interactions between the electronic states of chromophores.

The idea was that the energy is passed on to a neighbouring chromophore with a slightly lower energy level. But there was thought to be no coherence — no in-phase resonance — in this coupling. Instead, the energy transport was pictured as a random incoherent hopping process between chromophores, guided by an overall energy gradient, like a drunken sailor staggering downhill.

But even as far back as the s it had been suggested that the efficiency of photosynthetic energy transfer might come from some kind of synchronization of the wavelike electronic excitations — known as excitons — between chromophores. In physical chemist Graham Fleming of the University of California at Berkeley and his co-workers claimed to find evidence that this is indeed what happens.

If there really was coherence among excitons, they reasoned, it should be possible to see interference between the different pathways: so-called quantum beats, like the acoustic beats audible when sound waves of similar frequency interfere.

Fleming, with postdoc Engel and others, used a technique called photon-echo spectroscopy to study the energy-transfer process in the so-called Fenna—Matthews—Olson FMO pigment—protein complex — the part of the photosynthetic apparatus that mediates energy transfer in the thermophilic bacterium Chlorobium tepidum. At the same time Fleming and his collaborators Hohjai Lee and Yuan-Chung Cheng , also at Berkeley, studied energy transfer between two types of pigments in the photosynthetic reaction centre of a different kind of photosynthesizing bacterium.

Although the researchers saw the beats that they had anticipated in both cases suggesting that coherence was involved the studies were conducted at temperatures much colder than the physiological conditions under which such organisms operate in nature. But in Engel, now in Chicago, reported that quantum coherence in the C. They too saw beats lasting for longer than a tenth of a picosecond at room temperature.

This putative electronic coherence, they said, was an order of magnitude longer-lived than had been previously thought — and long enough for the energy to get transferred from the chromophores to the photosynthetic reaction centre. This seemed remarkable.

Quantum coherence between quantum bits qubits placed in entangled states is what physicists are seeking to exploit in quantum computers. But qubits are typically cooled to within a fraction of a degree of absolute zero, and even then, coherence survives only for brief instants, among just a handful of bits, before decoherence sets in due to disturbance from the surrounding environment. Explained by a physicist with a concern for clarity and experimental achievability, the extraordinary nature of quantum superposition will fascinate the reader not only for its apparent strangeness, but also for its comprehensibility.

Mark P. Silverman is Professor of Physics at Trinity College. Entanglement-enabled delayed choice experiment. Tang, J. Shadbolt, P.

1. Introduction

Testing foundations of quantum mechanics with photons. Quantum erasure with causally disconnected choice. Natl Acad. Horodecki, R. Quantum entanglement. Lo Franco, R.

Time dilation in quantum systems and decoherence - IOPscience

Quantum entanglement of identical particles by standard information-theoretic notions. Brunner, N. Bell nonlocality. Hensen, B. Loophole-free Bell inequality violation using electron spins separated by 1. Nature , — Giustina, M. Shalm, L. Strong loophole-free test of local realism. Handsteiner, J.

Time dilation in quantum systems and decoherence

Cosmic bell test: measurement settings from milky way stars. Vedral, V. Ladd, T. Quantum computers. Nature , 45—53 Wang, X.

Experimental ten-photon entanglement. Haroche, S. Nobel lecture: controlling photons in a box and exploring the quantum to classical boundary. Brune, M. Ourjoumtsev, A. Reconstruction of non-classical cavity field states with snapshots of their decoherence. Vlastakis, B. Wang, C. Streltsov, A. Quantum Coherence as a Resource. Entanglement detection. Download references. In this work Z. ZRJL, while N.

All the authors discussed the results and contributed to the preparation of the manuscript. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Reprints and Permissions.

Physical Review A Journal of the Optical Society of America B Entropy By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Advanced search. Skip to main content. Subjects Quantum information Quantum optics Single photons and quantum effects.

Abstract Wave-particle duality is the most fundamental description of the nature of a quantum object, which behaves like a classical particle or wave depending on the measurement apparatus. Introduction Quantum mechanics is one of the most successful theories in describing atomic-scale systems albeit its properties remain bizarre and counterintuitive from a classical perspective. Results Single-photon toolbox The theoretical sketch of the wave-particle scheme for the single photon is displayed in Fig. Full size image.

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Discussion In summary, we have introduced and realized an all-optical scheme to deterministically generate single-photon wave-particle superposition states. Methods Experimental wave-particle toolbox The implementation of the wave-particle toolbox exploits both polarization and path degrees of freedom of the photons.

Acquisition system The output photons are detected by avalanche photodiode detectors, which are connected to an id Time to Digital Converter from ID Quantique that is employed to record the output single counts and two-photon coincidences. References 1. Ethics declarations Competing interests The authors declare no competing financial interests. Additional information Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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