Abstract
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent manner, while largely preserving a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which aim to solve the quantum measurement problem through modified dynamical laws. Whether such theories describe nature, however, is not known. Here we describe and implement an experimental protocol capable of constraining theories of this class, that is more noise tolerant and conceptually transparent than the original Leggett–Garg test. We implement the protocol in a superconducting flux qubit, and rule out (by ∼84 s.d.) those theories which would deny coherent superpositions of 170 nA currents over a ∼10 ns timescale. Further, we address the ‘clumsiness loophole' by determining classical disturbance with control experiments. Our results constitute strong evidence for the superposition of states of nontrivial macroscopic distinctness.
Highlights
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent manner, while largely preserving a fully quantum mechanical description of the microscopic world
When ‘shuffling’ operations S1 and S2 intervene respectively between t1 and t2, and between t2 and t3, Leggett-Garg inequality (LGI) can be violated by a quantum mechanical system
We call this equality the ‘non-disturbance condition’ (NDC). This condition has been derived by others and has been termed a quantum witness14, ‘non disturbing measurement’[15,16] or ‘no signalling in time’ condition[17]. It follows from the same assumptions as LGI (Supplementary Note 1 and Supplementary Table 1) and demands a zero effect of the choice of measurement at t2 on the statistics of a measurement at the later time t3
Summary
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent manner, while largely preserving a fully quantum mechanical description of the microscopic world. The systems LG had in mind are micrometre scale loops of superconducting material interrupted with one or more nonlinear elements known as Josephson junctions Such circuits define two possible states of magnetic flux threading the loop, and modern variants[2] are among the most macroscopic candidates for a quantum bit (or qubit), the basic constituent of various proffered quantum-enhanced technologies such as the quantum computer. When a measurement is made, the qubit is found in one of the two possible states |gi or |ei with a probability that oscillates in time Observation of such so-called ‘Rabi oscillations’ is consistent with a textbook quantum mechanical prediction (which generally ascribes a nonzero complex amplitude to each of the states), but not necessarily inconsistent with a classical ‘value-definite’ description (which prescribes that the system is in exactly one state at any given moment)[3]. If the system is sufficiently large (super-critically macroscopic), on the other hand, macrorealism predicts that no such violation is possible
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