Formation of robust bound states of interacting microwave photons

Abstract

Systems of correlated particles appear in many fields of modern science and represent some of the most intractable computational problems in nature. The computational challenge in these systems arises when interactions become comparable to other energy scales, which makes the state of each particle depend on all other particles1. The lack of general solutions for the three-body problem and acceptable theory for strongly correlated electrons shows that our understanding of correlated systems fades when the particle number or the interaction strength increases. One of the hallmarks of interacting systems is the formation of multiparticle bound states2–9. Here we develop a high-fidelity parameterizable fSim gate and implement the periodic quantum circuit of the spin-½ XXZ model in a ring of 24 superconducting qubits. We study the propagation of these excitations and observe their bound nature for up to five photons. We devise a phase-sensitive method for constructing the few-body spectrum of the bound states and extract their pseudo-charge by introducing a synthetic flux. By introducing interactions between the ring and additional qubits, we observe an unexpected resilience of the bound states to integrability breaking. This finding goes against the idea that bound states in non-integrable systems are unstable when their energies overlap with the continuum spectrum. Our work provides experimental evidence for bound states of interacting photons and discovers their stability beyond the integrability limit.

Stable quantum-correlated many-body states through engineered dissipation

“…We explored this possibility using another paradigmatic model of quantum magnetism, the 1D XXZ spin chain ( 35 ), which is currently the subject of intense theoretical ( 36 ) and experimental ( 37 – 39 ) investigations thanks to its rich magnetic transport properties. A Floquet version of the XXZ model ( 40 , 41 ) is readily implementable using consecutive applications of fSim gates parameterized by a conditional phase φ and iSWAP angle θ (Fig. 5A).…”

Quantum information phases in space-time: measurement-induced entanglement and teleportation on a noisy quantum processor

“…S4 and is built off of the technique introduced in Ref. 48 . The pulse shape is defined by the maximum coupling strength g max , the hold time at g max , t p , and the rise time t rise .…”

“…In Ref. 48 leakage errors were minimized by implementing a trapezoidal coupler pulse with t rise set to 5 ns. Indeed this was sufficient for realizing relatively high fidelity (99%) fSim gates for a select few (θ, φ).…”

Dissipation and Dephasing of Interacting Photons in Transmon Arrays

“…The bosonic excitations described by the model are microwave photons but we use the term 'boson' for generality in this article. In modern arrays of transmons [15,24,26], we have ω /2π ∼ 5 GHz for the on-site energies, J /2π ∼ 10 − 30 MHz for the hopping frequencies, and U /2π ∼ 200 − 250 MHz for the on-site interactions.Due to the inevitable small differences in manufactured devices, the parameters of any two transmons are usually not equal. However, for the sake of simplicity, we will assume in most parts of this work that there is no disorder in the parameters of the Hamiltonian, i.e., ω = ω, J = J, and U = U .…”

“…Transmon arrays have recently taken substantial advances in size, coherence, and controllability, opening doors for exciting demonstrations of quantum information protocols [1][2][3][4][5][6][7][8][9][10][11][12][13] and many-body simulations [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Transmons are typically operated as quantum two-level systems, qubits, despite their inherent nature as anharmonic oscillators with approximately d ∼ 5 − 10 welldefined quantum states [28].…”