Macroscopic many-qubit interactions in superconducting flux qubits

Abstract

Superconducting flux qubits are considered to investigate macroscopic many-qubit interactions. Many-qubit states based on current states can be manipulated through the current-phase relation in each superconducting loop. For flux qubit systems comprised of N-qubit loops, a general expression of low-energy Hamiltonian is presented in terms of low-energy levels of qubits and macroscopic quantum tunnelings between the many-qubit states. Many-qubit interactions classified by Ising-type or tunnel exchange interactions can be observable experimentally. Flux qubit systems can provide various artificial-spin systems to study many-body systems that cannot be found naturally. DOI: 10.1103/PhysRevB.77.212506 PACS number͑s͒: 74.50.ϩr, 03.67.Lx, 85.25.Cp It is believed that electrons are interacting in a pair. The interaction is called two-body interaction. Understanding the many-electron effects is one of the most important researches in condensed-matter physics. Normally, rich many-electron physics has been revealed by the two-body interactions of spin pairs in solid-state materials. In a strongly correlated electronic system, however, a low-energy Hamiltonian can involve more than three spin interactions. 1-4 Such multiplespin interactions are known to play a significant role in strongly correlated systems. Some examples include the magnetism of solid 3 He, the high T c superconductivity, and so on. 2,3 Moreover, it is well known theoretically that higher dimensional systems can be mapped to the multiple-spin interaction Hamiltonian of one-dimensional chain unlikely found naturally. 5 The last decade has seen rapidly developing advanced material technologies that make it possible to investigate previously inaccessible quantum systems for quantum information and computation in solid-state systems. Especially, coherent manipulation of quantum states in tunable superconducting devices has enabled to demonstrate macroscopic qubits 6-8 and entangled states of qubits. 9-11 Experimentally, it has been shown that, in terms of pseudospins, different types of exchange interactions between two artificial spins such as an Ising interaction for charge qubits 9 and flux qubits 10,12 and an XY interaction for phase qubits 11 can be realized and controlled by the system parameters. Although different types of solid-state qubit systems have revealed such artificial-spin exchange interactions, multiple artificial-spin interactions have not been demonstrated yet. This Brief Report aims to discuss, in a general framework, how artificial-multiple-spin interactions are possible and realizable in superconducting qubit systems. Flux qubit systems are shown to have an intrinsic property which is multiple artificial-spin interactions. Moreover, controllable system parameters in the flux qubit systems enable to create various types of artificial-spin systems even though a flux qubit system is fabricated experimentally. Then, the manmade superconducting structures of flux qubits can offer experimental simulators for many-body physics in associat...

Tunable three-body coupler for superconducting flux qubits

“…In superconducting circuits, high-order effective interactions between qubits can be made strong, due to the fact that the underlying two-body interactions are strong. High-order interactions have been analyzed for several types of superconducting qubits and coupler circuits [14][15][16][17][18]. Recently, there has been increased interest in many-qubit couplers for quantum annealers based on superconducting qubits.…”