Witnesses of non-classicality for simulated hybrid quantum systems

Abstract

The task of testing whether quantum theory applies to all physical systems and all scales requires considering situations where a quantum probe interacts with another system that need not obey quantum theory in full. Important examples include the cases where a quantum mass probes the gravitational field, for which a unique quantum theory of gravity does not yet exist, or a quantum field, such as light, interacts with a macroscopic system, such as a biological molecule, which may or may not obey unitary quantum theory. In this context a class of experiments has recently been proposed, where the non-classicality of a physical system that need not obey quantum theory (the gravitational field) can be tested indirectly by detecting whether or not the system is capable of entangling two quantum probes. Here we illustrate some of the subtleties of the argument, to do with the role of locality of interactions and of non-classicality, and perform proof-of-principle experiments illustrating the logic of the proposals, using a Nuclear Magnetic Resonance quantum computational platform with four qubits. OPEN ACCESS RECEIVED

Witnessing non-classicality beyond quantum theory

“…Future lines of research are to lift even the assumption that Q A and Q B are qubits, and proceed with the general theory of superinformation media, [13,15,22], where entanglement is treated as locally inaccessible information. We conjecture that even in this case, the degree of locally inaccessible information on Q A and Q B can be expressed formally as a function of the degree of non-classicality of M, generalising the formal relation existing in quantum theory between non-classicality of the mediator and degree of entanglement, [23].…”

Transforming pure and mixed states using an NMR quantum homogeniser

“…Here, we adopt the approach described in Ref. [21] for designing GRAPE pulses that totally avoid environment decoupling and also have a similar computational complexity scaling to four-qubit systems. The idea is to leave the 1 H nuclei completely untouched while implementing the quantum gate, so that they can be treated as remaining in fixed eigenstates.…”

“…Our experiment to simulate a quantum homogeniser was performed on a four-qubit liquid-state NMR quantum processor, given by the four 13 C nuclei in a sample of fully labelled crotonic acid dissolved in deuterated acetone [19,20]. The Hamiltonian parameters obtained on a 600 MHz ( 1 H frequency) Varian Unity Inova spectrometer at 300 K are given in [21]. This fourqubit quantum processor can be approximated by a linear chain with strong nearest-neighbour couplings (between 42 and 72 Hz) and weak long-range couplings (no more than 7 Hz).…”

“…The conventional approach to this problem is to use continuous 1 H decoupling [22] throughout the pulse sequence, but we were unable to obtain good results with this method as the length of the pulses limited the radio frequency (RF) power that could be used. Instead we repeated the approach used previously in this system in which we avoid decoupling during the pulse sequence, but designed each GRAPE pulse to tolerate couplings to 1 H nuclei [21] by averaging their fidelity over the variation in the 13 C Hamiltonian which arises from such coupling terms, as described in Appendix A. Pulses were also designed to tolerate some RF inhomogeneity by averaging their performance over a range of RF amplitudes.…”

Decoherence effects in non-classicality tests of gravity

“…They rely on the generation of a gravitationally induced entanglement between two masses, which would work as a witness of the nonclassical nature of the gravitational mediator [10,11]. This new approach to witnessing non-classical features of gravity has been subject of intense study and debate [13][14][15][16][17][18][19][20][21][22][23][24], and is at the core of several recent experimental proposals [25][26][27][28][29]. Since the gravitational interaction is weak, one needs to employ large masses to achieve a measurable amount of entanglement in an experimentally reasonable time.…”