Observation of Collective Coupling between an Engineered Ensemble of Macroscopic Artificial Atoms and a Superconducting Resonator

Abstract

The hybridization of distinct quantum systems is now seen as an effective way to engineer the properties of an entire system leading to applications in quantum metamaterials, quantum simulation, and quantum metrology. One well known example is superconducting circuits coupled to ensembles of microscopic natural atoms. In such cases, the properties of the individual atom are intrinsic, and so are unchangeable. However, current technology allows us to fabricate large ensembles of macroscopic artificial atoms such as superconducting flux qubits, where we can really tailor and control the properties of individual qubits. Here, we demonstrate coherent coupling between a microwave resonator and several thousand superconducting flux qubits, where we observe a large dispersive frequency shift in the spectrum of 250 MHz induced by collective behavior. These results represent the largest number of coupled superconducting qubits realized so far. Our approach shows that it is now possible to engineer the properties of the ensemble, opening up the way for the controlled exploration of the quantum many-body system.Quantum science and technology have reached a very interesting stage in their development where we are now beginning to engineer the properties that we require of our quantum systems [1,2]. Hybridization is a core technique in achieving this. An additional (or ancilla) system can be used to greatly change not only the properties of the overall system, but also its environment [3][4][5].Specifically, a hybrid system composed of many qubits and a common field such as cavity quantum electrodynamics [6,7] may provide an excellent way of realizing such quantum engineering, leading to an interesting investigation of many-body phenomena including quantum simulations [8,9], superradiant phase transitions [10][11][12][13][14][15], spin squeezing [16][17][18], and quantum metamaterials [19][20][21][22][23][24][25]. In this regard, one of the ways to realize such a system is to employ superconducting circuits coupled to electron spin ensembles where basic quantum control such as memory operations have been demonstrated [26][27][28][29][30]. If we are to investigate quantum many-body phenomena, we will need control over the ensemble. In most typical superconducting circuit-ensemble hybrid experiments, the ensemble has been formed from a collection of either atoms or molecules with examples including nitrogen vacancy centers [26][27][28]31], ferromagnetic magnons [32], and bismuth donor spins in silicon [33]. In these cases, the properties of the atomic ensemble system are basically defined as the ensemble is formed, and are difficult to change. However, our ensembles could be composed of artificial atoms such as superconducting qubits.Superconducting qubits are macroscopic two-level systems with a significant degree of design freedom [34,35]. Josephson junctions provide the superconducting circuit with non-linearity, and we can tailor the qubit properties by changing the design of the circuit. Moreover, in contrast to natural a...

Superradiant phase transition in the ultrastrong-coupling regime of the two-photon Dicke model

“…Similar schemes can be conceived for other atomic or solid state systems. Particularly promising are superconducting devices, where bosonic modes have been coupled to increasingly large spin ensembles [48,49]. In any implementation, the critical issue would be the number of qubits that can be effectively coupled to a single bosonic mode.…”

Quantum annealing in spin-boson model: from a perturbative to an ultrastrong mediated coupling

“…This is the case of trapped ions, where atomic degrees of freedom interact via the phonons of the ion crystal, creating an effective spin model [10]. It is also the case of superconducting circuits, where a transmission line or a cavity can couple distant superconducting qubits [24][25][26][27][28][29]. We describe this paradigm of mediated interactions using a spin-boson Hamiltonian, where the spins are our qubits and the harmonic oscillators are our bosonic mediators.…”

Relaxation of Rabi dynamics in a superconducting multiple-qubit circuit

“…Thus, we have to return to the inhomogeneous system described by equations (7) and (8). To take into account this nonidealfabrication-induced inhomogeneity, we assume a normal distribution with a standard deviation λ for different junction areas whose mean value is normalized to be unity [47]. We use C j and E J to respectively denote the mean values of self-capacitances C j,k=1,K,N and Josephson energies E J,k=1,K,N , i.e., C C systems are prepared in the same initial condition and E C,k=1,K,N and E J,k=1,K,N of each system fulfill the corresponding normal distributions.…”