Macroscopic superpositions require tremendous measurement devices

Abstract

We consider fundamental limits on the detectable size of macroscopic quantum superpositions. We argue that a full quantum mechanical treatment of system plus measurement device is required, and that a (classical) reference frame for phase or direction needs to be established to certify the quantum state. When taking the size of such a classical reference frame into account, we show that to reliably distinguish a quantum superposition state from an incoherent mixture requires a measurement device that is quadratically bigger than the superposition state. Whereas for moderate system sizes such as generated in previous experiments this is not a stringent restriction, for macroscopic superpositions of the size of a cat the required effort quickly becomes intractable, requiring measurement devices of the size of the Earth. We illustrate our results using macroscopic superposition states of photons, spins, and position. Finally, we also show how this limitation can be circumvented by dealing with superpositions in relative degrees of freedom.

arXiv comment20 pages (including appendices), 1 Figure

References in corpus (27)

• Decoherence, einselection, and the quantum origins of the classical
• Experimental long-lived entanglement of two macroscopic objects
• Reference frames, superselection rules, and quantum information
• Invariant Variation Problems
• The Quantum Chernoff Bound
• Quantum state discrimination and its applications
• Matter-wave interference with particles selected from a molecular library with masses exceeding 10000 amu
• Optimal states and almost optimal adaptive measurements for quantum interferometry
• Classical world arising out of quantum physics under the restriction of coarse-grained measurements
• Entangling quantum and classical states of light
• Macroscopic quantum states: measures, fragility and implementations
• A Kapitza-Dirac-Talbot-Lau interferometer for highly polarizable molecules
• Optimal purification of single qubits
• Efficient use of quantum resources for the transmission of a reference frame
• Schroedinger Cat: Entanglement test in a Micro-Macroscopic system
• Observation of micro-macro entanglement of light
• Optimal estimation of group transformations using entanglement
• Conservation laws, uncertainty relations, and quantum limits of measurements
• Quantum reverse-engineering and reference frame alignment without non-local correlations
• Covariant quantum measurements which maximize the likelihood
• The size of quantum superpositions as measured with "classical" detectors
• How difficult it is to prove the quantumness of macroscropic states?
• Exact decoherence to pointer states in free open quantum systems is universal
• Quantum interference of a single spin excitation with a macroscopic atomic ensemble
• Generalized Coherent States as Preferred States of Open Quantum Systems
• Light-matter micro-macro entanglement
• Are there fundamental limits for observing quantum phenomena from within quantum theory?

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