Demonstration of long-range correlations via susceptibility measurements in a one-dimensional superconducting Josephson spin chain

Abstract

Spin chains have long been considered an effective medium for long-range interactions, entanglement generation, and quantum state transfer. In this work, we explore the properties of a spin chain implemented with superconducting flux circuits, designed to act as a connectivity medium between two superconducting qubits. The susceptibility of the chain is probed and shown to support long-range, cross-chain correlations. In addition, interactions between the two end qubits, mediated by the coupler chain, are demonstrated. This work has direct applicability in near term quantum annealing processors as a means of generating long-range, coherent coupling between qubits.

Precision of quantum simulation of all-to-all coupling in a local architecture

“…We present a solution to the exponential decrease in performance with the length of the repetition code: instead of a ferromagnetic repetition code, one needs to use a paramagnetic chain in its ground state as the interaction mediator, together with a single well-isolated hardware qubit serving as a logical qubit. This idea has already appeared under the name of paramagnetic trees [8,9], and here we provide a theoretical justification for this approach. We observe that such a mediator is a type of perturbative gadget [10], and analyze it via an exact version of perturbation theory: a Schrieffer-Wolff transformation [11].…”

“…( 1) is impossible on a chip is that to implement it, the qubits on which H target is defined will have to be extended objects, which will lead to the failure of the qubit approximation, as well as uncontrollable levels of noise. Instead, we take inspiration from the idea of paramagnetic trees [8,9] where the qubits are well isolated and highly coherent, and the extended objects connecting different qubits are the mediators of interactions.…”

“…We present a construction of a scalable paramagnetic tree [8,9]. Specifically, we show that a circuit on a chip with a constant density of elements shown in Fig.…”

“…Perturbative gadgets were previously used to implement a many-body interaction using only two body terms [12]. Here we use them instead to implement a long-range two-body interaction using only nearest-neighbor two-body terms [9]. The mediator can be any physical system.…”

Scalable and robust quantum computing on qubit arrays with fixed coupling

“…A promising physical realisation of our model is the superconducting flux and transmon qubits. A direct ZZ interaction between flux qubits can be realised by coupling the qubits inductively, as demonstrated in quantum annealers 11,18 . There is another interesting scheme based on the inductive longitudinal coupling of the flux qubits with a common bus resonator 15,16,19 , which can be scaled up to 2D arrays 20 .…”