Experimental limit on non-linear state-dependent terms in quantum theory

TL;DR

This experiment tests for non-linear, state-dependent effects in quantum field theory by correlating macroscopic voltages with quantum measurements, finding no evidence and setting strict bounds on such non-linearities.

Contribution

The paper presents the first experimental bounds on non-linear, state-dependent terms in quantum field theory using a novel correlation approach with multiple control methods.

Findings

No evidence for electromagnetic quantum state-dependent non-linearity.

Set a bound on the non-linearity parameter |_{\u03b3}|<4.7 10^{-11} at 90% confidence.

Limits on electromagnetic interactions between different branches of the universe in Everett's interpretation.

Abstract

We report the results of an experiment that searches for causal non-linear state-dependent terms in quantum field theory. Our approach correlates a binary macroscopic classical voltage with the outcome of a projective measurement of a quantum bit, prepared in a coherent superposition state. Measurement results are recorded in a bit string, which is used to control a voltage switch. Presence of a non-zero voltage reading in cases of no applied voltage is the experimental signature of a non-linear state-dependent shift of the electromagnetic field operator. We implement blinded measurement and data analysis with three control bit strings. Control of systematic effects is realized by producing one of the control bit strings with a classical random-bit generator. The other two bit strings are generated by measurements performed on a superconduting qubit in an IBM Quantum processor, and on a $^{15}$N nuclear spin in an NV center in diamond. Our measurements find no evidence for electromagnetic quantum state-dependent non-linearity. We set a bound on the parameter that quantifies this non-linearity $|\epsilon_{\gamma}|<4.7\times 10^{-11}$, at 90% confidence level. Within the Everett many-worlds interpretation of quantum theory, our measurements place limits on the electromagnetic interaction between different branches of the universe, created by preparing the qubit in a superposition state.