Testing the limits of quantum mechanical superpositions

Abstract

Quantum physics has intrigued scientists and philosophers alike, because it challenges our notions of reality and locality-concepts that we have grown to rely on in our macroscopic world. It is an intriguing open question whether the linearity of quantum mechanics extends into the macroscopic domain. Scientific progress over the last decades inspires hope that this debate may be decided by table-top experiments. IntroductionThe last three decades have witnessed what has been termed 1 the second quantum revolution: A renaissance of research on the quantum foundations, hand in hand with growing experimental capabilities, 2 revived the idea of exploiting quantum superpositions for technological applications, from information science [3][4][5] to precision metrology. [6][7][8] Quantum mechanics has passed all precision tests with flying colors, but it still seems to be in conflict with our common sense. Since quantum theory knows no boundaries everything should fall under the sway of the superposition principle, including macroscopic objects. This is at the bottom of Schrödinger's thought experiment transforming a cat into a state which strikes us as classically impossible. And yet, 'Schrödinger kittens' of entangled photons 9 and ions 10 have been realized in the lab.So why are the objects around us never found in superpositions of states that would be excluded in a classical description? One may emphasize the smallness of Planck's constant, or point to decoherence theory, which describes how a system will effectively lose its quantum features when coupled to a quantum environment of sufficient size. 11,12 The formalism of decoherence, however, is based on the framework of unitary quantum mechanics, implying that some interpretational exercise is required not to become entangled in a multitude of parallel worlds. 13 More radically, one may ask whether quantum mechanics breaks down beyond a certain mass or complexity scale. As will be discussed below, such ideas can be motivated by the apparent incompatibility of quantum theory and general relativity. It is safe to state, in any case, that quantum superpositions of truly massive, complex objects are terra incognita. This makes them an attractive challenge for a growing number of sophisticated experiments.We start by reviewing several prototypical tests of the superposition principle, focusing on the quantum states of motion displayed by material objects. Particle position and momentum variables have a well-defined classical analogue, and they are therefore particularly suited to probe the macroscopic domain. We note that aspects of macroscopicity can also be addressed in experiments with photons, [14][15][16] Figure 1A): The single-valuedness of the wave function entails that the magnetic flux encircled by a closed-loop supercurrent must be quantized. In particular, one can define a symmetric and an antisymmetric linear combination of two supercurrents, which circulate simultaneously in opposing directions. Billions of electrons may contribute coherently to the wave...

A Comparison Between Models of Gravity Induced Decoherence

“…The experimental predictions of this nonlinear theory are very close to the standard theory in the micro world, but differ from the standard theory in the meso and the macro world. A decisive experiment which chooses between standard quantum theory and stochastic nonlinear theories has not yet been performed, although an extraordinary worldwide effort in this direction is currently in progress [48]. The main reason is that, even if for meso and macro systems the nonlinear effects may be relevant, in this regime the interaction with the environment plays also an important role, which masks the nonlinear effects.…”

State complexity and quantum computation

“…It is a current experimental trend to push the tests of quantum mechanics towards the macroscopic domain, see for example Refs. [3][4][5][6]. In all these experiments, the superposition indeed involves large number of particles or excitations, nevertheless the states produced are somewhat "simple": some involve the superposition of a single degree of freedom, the center of mass; others target the GHZ state, or the Dicke state with few excitations, as ideal macroscopic states.…”

Displacemon Electromechanics: How to Detect Quantum Interference in a Nanomechanical Resonator

“…In molecular interference experiments, the diffraction grating is typically an etched membrane; however, van der Waals interactions with the slits make this hard to extend to large particles [1]. More advanced implementations use optically defined gratings; the pitch, which sets the momentum separation of the diffracted beams, is then limited by the optical wavelength [31]. In our scheme, the pitch is limited neither by an optical wavelength nor by the size of the resonator, but by the qubit-resonator coupling strength.…”