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A macroscopic superposition of atoms and molecules

John Calsamiglia, Matt Mackie, Kalle‐Antti Suominen

Published Mar 21, 2001• 0 citations
OpenAlex

Abstract

In a now famous gedanken experiment, highlighted the absurdity of applying quantum theory to macroscopic objects by coupling the fate of a cat to the decay of a radioactive atom, thereby forcing the animal into a superposition of alive and dead. This nefarious situation marks the fundamental difference of principle between microscopic quantum mechanics and macroscopic realism. The technology of superconducting quantum interference devices (SQUIDs) opened the door to the first proposed macroscopic superposition, and the latest SQUID experiments have taken a giant step towards demonstrating a Schroedinger cat. Other candidates for creating macroscopic superpositions include nanoscale magnets, trapped ions, photons in a high-Q cavity, second-harmonic generated and amplitude-dispersed photons, C60 molecules, and two-component Bose-Einstein condensates (BECs). Even more provocative, we propose coherent photoassociation of a BEC as a means of creating a seemingly impossible macroscopic superposition of two different objects: a cat of atoms and molecules. With photoassociation currently on the verge of being coherent, this state should be realizable in the next generation of experiments.

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In a now famous gedanken experiment, highlighted the absurdity of applying quantum theory to macroscopic objects by coupling the fate of a cat to the decay of a radioactive atom, thereby forcing the animal into a superposition of alive and dead. This nefarious situation marks the fundamental difference of principle between microscopic quantum mechanics and macroscopic realism. The technology of superconducting quantum interference devices (SQUIDs) opened the door to the first proposed macroscopic superposition, and the latest SQUID experiments have taken a giant step towards demonstrating a Schroedinger cat. Other candidates for creating macroscopic superpositions include nanoscale magnets, trapped ions, photons in a high-Q cavity, second-harmonic generated and amplitude-dispersed photons, C60 molecules, and two-component Bose-Einstein condensates (BECs). Even more provocative, we propose coherent photoassociation of a BEC as a means of creating a seemingly impossible macroscopic superposition of two different objects: a cat of atoms and molecules. With photoassociation currently on the verge of being coherent, this state should be realizable in the next generation of experiments.
In 1935 Erwin Schrödinger proposed a thought experiment to show the absurd consequences of applying quantum mechanics to everyday objects. Schrödinger imagined placing a cat in a box, along with a vial of deadly poison that is connected to a radioactive atom. If the atom decays, it releases the poison and kills the cat. When the box is closed we do not know if the atom has decayed or not, which means that it can be in a "superposition" of two states (decayed and non-decayed) at the same time. Therefore, protested Schrödinger, the cat can be both dead and alive at the same time – which is clearly absurd. There was obviously, he thought, some boundary between the microscopic world of atoms and quantum mechanics, and the macroscopic world of cats and classical mechanics.
In 1935, Erwin Schrödinger put forward his famous gedanken experiment in which a microscopic degree of freedom - the emission state of a radioactive atom - and a macroscopic, classical state - the life or death of a cat - are entangled [1]. The resulting state is dubbed a Schrodinger-cat state. According to quantum theory, a measurement of the atom in a superposition basis projects the cat into a macroscopic superposition of being alive and dead. Here, we use coherent laser fields with different optical phases as a macroscopic model system and generate entangled Schrodinger-cat states in a deterministic way [3]. We reflect the coherent pulses from an optical cavity that contains a single strongly-coupled atom. This entangles the spin of the atom with the optical phase of the light field [2]. By manipulating and detecting the atomic state, we create a plethora of flying optical coherent-state superpositions. We reconstruct the corresponding Wigner functions (see Fig. 1) to demonstrate the tunability and non-classicality of the produced light state.
Schrödinger's famous Gedankenexperiment has inspired multiple generations of physicists to think about apparent paradoxes that arise when the logic of quantum physics is applied to macroscopic objects. The development of quantum technologies enabled us to produce physical analogues of Schrödinger's cats, such as superpositions of macroscopically distinct states as well as entangled states of microscopic and macroscopic entities. Here we take one step further and prepare an optical state which, in Schrödinger's language, is equivalent to a superposition of two cats, one of which is dead and the other alive, but it is not known in which state each individual cat is. Specifically, the alive and dead states are, respectively, the displaced single photon and displaced vacuum (coherent state), with the magnitude of displacement being on a scale of 10810^8108 photons. These two states have significantly different photon statistics and are therefore macroscopically distinguishable.
Abstract In the Schrödinger cat gedanken experiment a “cat” is in a quantum superposition of two macroscopically distinct states. There is the apparent interpretation that the “cat” is not in one state or the other, “alive” or “dead”. Here this interpretation is proved objectively. I propose the following definition of macroscopic reality: first, that the “cat” is either dead or alive, the measurement revealing which; second, that measurements on other “cats” some distance away cannot induce the macroscopic change, “dead” to “alive” and vice versa, to the “cat”. The predictions of quantum mechanics are shown to be incompatible with this premise.
The most striking feature of quantum mechanics is the existence of superposition states, where an object appears to be in different situations at the same time. The existence of such states has been previously tested with small objects, such as atoms, ions, electrons and photons (Zoller et al 2005 Eur. Phys. J. D 36 203–28), and even with molecules (Arndt et al 1999 Nature 401 680–2). More recently, it has been shown that it is possible to create superpositions of collections of photons (Deléglise et al 2008 Nature 455 510–14), atoms (Hammerer et al 2008 arXiv:0807.3358) or Cooper pairs (Friedman et al 2000 Nature 406 43–6). Very recent progress in optomechanical systems may soon allow us to create superpositions of even larger objects, such as micro-sized mirrors or cantilevers (Marshall et al 2003 Phys. Rev. Lett. 91 130401; Kippenberg and Vahala 2008 Science 321 1172–6; Marquardt and Girvin 2009 Physics 2 40; Favero and Karrai 2009 Nature Photon. 3 201–5), and thus to test quantum mechanical phenomena at larger scales. Here we propose a method to cool down and create quantum superpositions of the motion of sub-wavelength, arbitrarily shaped dielectric objects trapped inside a high-finesse cavity at a very low pressure. Our method is ideally suited for the smallest living organisms, such as viruses, which survive under low-vacuum pressures (Rothschild and Mancinelli 2001 Nature 406 1092–101) and optically behave as dielectric objects (Ashkin and Dziedzic 1987 Science 235 1517–20). This opens up the possibility of testing the quantum nature of living organisms by creating quantum superposition states in very much the same spirit as the original Schrödinger's cat 'gedanken' paradigm (Schrödinger 1935 Naturwissenschaften 23 807–12, 823–8, 844–9). We anticipate that our paper will be a starting point for experimentally addressing fundamental questions, such as the role of life and consciousness in quantum mechanics.
In 1935, Erwin Schrödinger suggested his famous gedanken experiment of the cat that is simultaneously "dead" and "alive" inside its box until the box is opened. But as Tesche explains in her Perspective, such a macroscopic manifestation of quantum mechanics has remained elusive until recently. The experiments by van der Wal et al. are an important step toward demonstrating that quantum mechanics can describe macroscopic phenomena. The approach may be exploited in quantum computing and quantum cryptography.