Verified Quantum Information Scrambling

TL;DR

This paper presents a quantum circuit method to unambiguously detect quantum information scrambling, overcoming limitations of traditional OTOC measurements, demonstrated on a 7-qubit ion trap quantum computer with high fidelity.

Contribution

The authors introduce a novel quantum circuit that provides a positive test for quantum scrambling, distinguishing it from decoherence effects, and implement it on a multi-qubit ion trap system.

Findings

Teleportation fidelities around 80% demonstrate effective scrambling detection.

The method bounds the decay of OTOC measurements caused by scrambling.

Experimental implementation confirms the circuit's capability to identify quantum scrambling.

Abstract

Quantum scrambling is the dispersal of local information into many-body quantum entanglements and correlations distributed throughout the entire system. This concept underlies the dynamics of thermalization in closed quantum systems, and more recently has emerged as a powerful tool for characterizing chaos in black holes. However, the direct experimental measurement of quantum scrambling is difficult, owing to the exponential complexity of ergodic many-body entangled states. One way to characterize quantum scrambling is to measure an out-of-time-ordered correlation function (OTOC); however, since scrambling leads to their decay, OTOCs do not generally discriminate between quantum scrambling and ordinary decoherence. Here, we implement a quantum circuit that provides a positive test for the scrambling features of a given unitary process. This approach conditionally teleports a quantum state through the circuit, providing an unambiguous litmus test for scrambling while projecting potential circuit errors into an ancillary observable. We engineer quantum scrambling processes through a tunable 3-qubit unitary operation as part of a 7-qubit circuit on an ion trap quantum computer. Measured teleportation fidelities are typically $\sim80\%$, and enable us to experimentally bound the scrambling-induced decay of the corresponding OTOC measurement.