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Inhibition of ground-state superradiance and light-matter decoupling in circuit QED

Tuomas Jaako, Ze-Liang Xiang, Juan José García‐Ripoll, Peter Rabl

Published Feb 18, 2016• 5 citations
OpenAlex

Abstract

We study effective light-matter interactions in a circuit QED system consisting of a single $LC$ resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which prevent the otherwise expected superradiant phase transition in the ground state of this system. Moreover, these qubit-qubit interactions are responsible for an opposite mechanism, which at very strong couplings completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings shed new light on the controversy over the existence of superradiant phase transitions in cavity and circuit QED systems, and show that the physics of ultrastrong light-matter interactions in two- or multi-qubit settings differ drastically from the more familiar one qubit case.

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We study effective light-matter interactions in a circuit QED system consisting of a single $LC$ resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that, in addition to the usual collective qubit-photon coupling, the resulting Hamiltonian contains direct qubit-qubit interactions, which have a drastic effect on the ground- and excited-state properties of such circuits in the ultrastrong-coupling regime. In contrast to the superradiant phase transition expected from the standard Dicke model, we find an opposite mechanism, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve previous controversies over the existence of superradiant phases in circuit QED, but they more generally show that the physics of two- or multiatom cavity QED settings can differ significantly from what is commonly assumed.

arXiv:1602.05756•Physical Review A•Sep 27, 2016•155 citations

Ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit circuit QED system

Wenjuan Yang, Xiang‐Bin Wang

We study ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit system. In the one-qubit case, three second-order transitions occur and the Goldstone mode emerges under the condition of ultrastrong-coupling strength. Moreover, a first-order phase transition occurs between two different superradiant phases. In the two-qubit case, a two-qubit Hamiltonian with qubit-qubit interactions is analyzed fully quantum mechanically. We show that the quantum phase transition is inhibited even in the ultrastrong-coupling regime in this model. In addition, in the three-qubit model, the superradiant quantum phase transition is retrieved in the ultrastrong-coupling regime. Furthermore, the $N$-qubit model with $U(1)$ symmetry is studied and we find that the superradiant phase transition is inhibited or restored with the qubit-number parity.

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Effective two-photon interaction in two-resonator circuit QED system

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Abstract In this work we present a scheme to generate an effective two-photon interaction for a qubit coupled to two resonators in a superconducting circuit QED system where the two light modes are either from the same cavity or from two different ones, and the coupling between the two cavity modes is taken into account. It is shown that such a two-photon interaction results from a symmetry-broken potential of the superconducting circuit. We also propose applying this effective Hamiltonian, as an example, to generate the entangled states of the light fields in two resonators.

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Ultrastrong-coupling circuit QED in the radio-frequency regime

Tuomas Jaako, Juan José García‐Ripoll, Peter Rabl

We study a circuit QED setup where multiple superconducting qubits are ultrastrongly coupled to a single radio-frequency resonator. In this extreme parameter regime of cavity QED the dynamics of the electromagnetic mode is very slow compared to all other relevant timescales and can be described as an effective particle moving in an adiabatic energy landscape defined by the qubits. The focus of this paper is placed on settings with two or multiple qubits, where different types of symmetry-breaking transitions in the ground- and excited-state potentials can occur. Specifically, we show how the change in the level structure and the wave-packet dynamics associated with these transition points can be probed via conventional excitation spectra and Ramsey measurements performed at GHz frequencies. More generally, this analysis demonstrates that state-of-the-art circuit QED systems can be used to access a whole range of particlelike quantum-mechanical phenomena beyond the usual paradigm of coupled qubits and oscillators.

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Light-Matter Interaction - from Strong to Ultrastrong Coupling in Superconducting Quantum Circuits

Rudolf Groß

Light-Matter Interaction – from Strong to Ultrastrong Coupling in Superconducting Quantum Circuits Rudolf Gross TU Munchen / Walther-Meisner-Institut fur Tieftemperaturforschung Superconducting quantum circuits imple-mented by nanostructured superconduc-tors allow for the investigation of funda-mental quantum phenomena on a macro-scopic scale and the implementation of solid state quantum information systems. A big advantage of superconducting quantum two-level systems (qubits) over natural atoms is their design flexibility and wide tunability by means of external control parameters such as magnetic fields. We address the coupling of superconducting flux qubits [1,2] to on-chip microwave resonators, giv-ing rise to the prospering field of superconducting circuit quantum electrodynamics (c-QED), which allows us to study the fundamental interaction between artificial solid state atoms and single microwave photons as the basis for communicating quantum information. Recently, we succeeded to realize for the first time c-QED systems operating in the ultra-strong coupling re-gime, where the atom-cavity coupling rate reaches a considerable fraction of the atom transi-tion frequency [3]. In this regime new objects are formed consisting of matter and light. We present spectroscopy data on these new objects providing insight into novel phenomena that could not be studied in atom cavity-QED so far. We also address the interplay of multi-photon processes and symmetries in a qubit-resonator system by spectroscopically analyzing a super-conducting qubit-resonator system under oneand two-photon driving [4]. [1] T. Niemczyk et al., Supercond. Sci. Techn. 22, 034009 (2009). [2] F. Deppe et al., PRB 76, 214503 (2007); K. Kakuyanagi, et al., PRL 98, 047004 (2007). [3] T. Niemczyk et al., Nature Physics 6, 772-776 (2010). [4] F. Deppe et al., Nature Physics 4, 686 (2008); T. Niemczyk et al., arXiv:1107.0810v1 (2011). Gastgeber: Prof. Martin Dressel, Universitat Stuttgart, Telefon: 0711 685-64946

•Jan 1, 2011•0 citations

Non-Gaussian superradiant transition via three-body ultrastrong coupling

Fabrizio Minganti, Louis Garbe, Alexandre Le Boité +1 more

We introduce a class of quantum optical Hamiltonians characterized by three-body couplings and propose a circuit-QED scheme based on state-of-the-art technology that implements the considered model. Unlike two-body light-matter interactions, this three-body coupling Hamiltonian is exclusively composed of terms which do not conserve the particle number. We explore the three-body ultrastrong-coupling regime, showing the emergence of a superradiant phase transition which is of first order, is characterized by the breaking of ${\mathbb{Z}}_{2}\ifmmode\times\else\texttimes\fi{}{\mathbb{Z}}_{2}$ symmetry, and has a strongly non-Gaussian nature. Indeed, in contrast to what is observed in any two-body-coupling model, in proximity of the transition the ground state exhibits a divergent coskewness, i.e., quantum correlations that cannot be captured within semiclassical and Gaussian approximations. Furthermore, we demonstrate the robustness of our findings by including dissipative processes in the model, showing that the steady state of the system inherits from the ground states the most prominent features of the transition.

arXiv:2204.03520•Physical Review A•Jan 20, 2023•12 citations
We study effective light-matter interactions in a circuit QED system consisting of a single LCLCLC resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which prevent the otherwise expected superradiant phase transition in the ground state of this system. Moreover, these qubit-qubit interactions are responsible for an opposite mechanism, which at very strong couplings completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings shed new light on the controversy over the existence of superradiant phase transitions in cavity and circuit QED systems, and show that the physics of ultrastrong light-matter interactions in two- or multi-qubit settings differ drastically from the more familiar one qubit case.
We study effective light-matter interactions in a circuit QED system consisting of a single LCLCLC resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that, in addition to the usual collective qubit-photon coupling, the resulting Hamiltonian contains direct qubit-qubit interactions, which have a drastic effect on the ground- and excited-state properties of such circuits in the ultrastrong-coupling regime. In contrast to the superradiant phase transition expected from the standard Dicke model, we find an opposite mechanism, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve previous controversies over the existence of superradiant phases in circuit QED, but they more generally show that the physics of two- or multiatom cavity QED settings can differ significantly from what is commonly assumed.
We study ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit system. In the one-qubit case, three second-order transitions occur and the Goldstone mode emerges under the condition of ultrastrong-coupling strength. Moreover, a first-order phase transition occurs between two different superradiant phases. In the two-qubit case, a two-qubit Hamiltonian with qubit-qubit interactions is analyzed fully quantum mechanically. We show that the quantum phase transition is inhibited even in the ultrastrong-coupling regime in this model. In addition, in the three-qubit model, the superradiant quantum phase transition is retrieved in the ultrastrong-coupling regime. Furthermore, the NNN-qubit model with U(1)U(1)U(1) symmetry is studied and we find that the superradiant phase transition is inhibited or restored with the qubit-number parity.
Abstract In this work we present a scheme to generate an effective two-photon interaction for a qubit coupled to two resonators in a superconducting circuit QED system where the two light modes are either from the same cavity or from two different ones, and the coupling between the two cavity modes is taken into account. It is shown that such a two-photon interaction results from a symmetry-broken potential of the superconducting circuit. We also propose applying this effective Hamiltonian, as an example, to generate the entangled states of the light fields in two resonators.
We study a circuit QED setup where multiple superconducting qubits are ultrastrongly coupled to a single radio-frequency resonator. In this extreme parameter regime of cavity QED the dynamics of the electromagnetic mode is very slow compared to all other relevant timescales and can be described as an effective particle moving in an adiabatic energy landscape defined by the qubits. The focus of this paper is placed on settings with two or multiple qubits, where different types of symmetry-breaking transitions in the ground- and excited-state potentials can occur. Specifically, we show how the change in the level structure and the wave-packet dynamics associated with these transition points can be probed via conventional excitation spectra and Ramsey measurements performed at GHz frequencies. More generally, this analysis demonstrates that state-of-the-art circuit QED systems can be used to access a whole range of particlelike quantum-mechanical phenomena beyond the usual paradigm of coupled qubits and oscillators.
Light-Matter Interaction – from Strong to Ultrastrong Coupling in Superconducting Quantum Circuits Rudolf Gross TU Munchen / Walther-Meisner-Institut fur Tieftemperaturforschung Superconducting quantum circuits imple-mented by nanostructured superconduc-tors allow for the investigation of funda-mental quantum phenomena on a macro-scopic scale and the implementation of solid state quantum information systems. A big advantage of superconducting quantum two-level systems (qubits) over natural atoms is their design flexibility and wide tunability by means of external control parameters such as magnetic fields. We address the coupling of superconducting flux qubits [1,2] to on-chip microwave resonators, giv-ing rise to the prospering field of superconducting circuit quantum electrodynamics (c-QED), which allows us to study the fundamental interaction between artificial solid state atoms and single microwave photons as the basis for communicating quantum information. Recently, we succeeded to realize for the first time c-QED systems operating in the ultra-strong coupling re-gime, where the atom-cavity coupling rate reaches a considerable fraction of the atom transi-tion frequency [3]. In this regime new objects are formed consisting of matter and light. We present spectroscopy data on these new objects providing insight into novel phenomena that could not be studied in atom cavity-QED so far. We also address the interplay of multi-photon processes and symmetries in a qubit-resonator system by spectroscopically analyzing a super-conducting qubit-resonator system under oneand two-photon driving [4]. [1] T. Niemczyk et al., Supercond. Sci. Techn. 22, 034009 (2009). [2] F. Deppe et al., PRB 76, 214503 (2007); K. Kakuyanagi, et al., PRL 98, 047004 (2007). [3] T. Niemczyk et al., Nature Physics 6, 772-776 (2010). [4] F. Deppe et al., Nature Physics 4, 686 (2008); T. Niemczyk et al., arXiv:1107.0810v1 (2011). Gastgeber: Prof. Martin Dressel, Universitat Stuttgart, Telefon: 0711 685-64946
We introduce a class of quantum optical Hamiltonians characterized by three-body couplings and propose a circuit-QED scheme based on state-of-the-art technology that implements the considered model. Unlike two-body light-matter interactions, this three-body coupling Hamiltonian is exclusively composed of terms which do not conserve the particle number. We explore the three-body ultrastrong-coupling regime, showing the emergence of a superradiant phase transition which is of first order, is characterized by the breaking of ${\mathbb{Z}}_{2}\ifmmode\times\else\texttimes\fi{}{\mathbb{Z}}_{2}$ symmetry, and has a strongly non-Gaussian nature. Indeed, in contrast to what is observed in any two-body-coupling model, in proximity of the transition the ground state exhibits a divergent coskewness, i.e., quantum correlations that cannot be captured within semiclassical and Gaussian approximations. Furthermore, we demonstrate the robustness of our findings by including dissipative processes in the model, showing that the steady state of the system inherits from the ground states the most prominent features of the transition.