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Engineering bilinear mode coupling in circuit QED: Theory and experiment
摘要： Photonic states of high-Q superconducting microwave cavities controlled by superconducting transmon ancillas provide a platform for encoding and manipulating quantum information. A key challenge in scaling up the platform towards practical quantum computation is the requirement to communicate on demand the quantum information stored in the cavities. It has been recently demonstrated that a tunable bilinear interaction between two cavity modes can be realized by coupling the modes to a bichromatically driven superconducting transmon ancilla, which allows swapping and interfering the multiphoton states stored in the cavity modes [Gao et al., Phys. Rev. X 8, 021073 (2018)]. Here we explore both theoretically and experimentally the regime of relatively strong drives on the ancilla needed to achieve fast SWAP gates but which can also lead to undesired nonperturbative effects that lower the SWAP fidelity. We develop a theoretical formalism based on linear response theory that allows one to calculate the rate of ancilla-induced interaction, decay, and frequency shift of the cavity modes in terms of a susceptibility matrix. We go beyond the usual perturbative treatment of the drives by using Floquet theory, and find that the interference of the two drives can strongly alter the system dynamics even in the regime where the standard rotating wave approximation applies. The drive-induced ac Stark shift on the ancilla depends nontrivially on the drive and ancilla parameters which in turn modify the strength of the engineered interaction. We identify two major sources of infidelity due to ancilla decoherence. (i) Ancilla dissipation and dephasing lead to incoherent hopping among Floquet states which occurs even when the ancilla is at zero temperature; this hopping results in a sudden change of the SWAP rate, thereby decohering the SWAP operation. (ii) The cavity modes inherit finite decay from the relatively lossy ancilla through the inverse Purcell effect; the effect becomes particularly strong when the ac Stark shift pushes certain ancilla transition frequencies to the vicinity of the cavity mode frequencies. The theoretical predictions agree quantitatively with the experimental results, paving the way for using the developed theory for optimizing future experiments and architecture designs.
关键词： decoherence,transmon ancilla,superconducting cavities,ac Stark shift,quantum information,circuit QED,inverse Purcell effect,bilinear mode coupling,SWAP gates,Floquet theory
更新于2025-09-23 15:23:52
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Effect of the Diamagnetic term in the ultra-strong coupling regime
摘要： We consider a set of two level atoms interacting with a single quantized bosonic mode governed by the Dicke model. In this model it is well known that under a critical value of the light-matter coupling a spontaneous radiation process takes place. In the present work, we investigate the dynamics of the system and we study the Wigner distribution function to visualize the effect of the minimal coupling on the ground state wavefunction from the normal phase to the superradiant one. We show also that the entanglement of Bi-partite model is limited by the presence of the diamagnetic term.
关键词： diamagnetic term,Circuit QED,Cavity QED,Entanglement
更新于2025-09-23 15:22:29
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Quantum-dot circuit-QED thermoelectric diodes and transistors
摘要： Recent breakthroughs in quantum-dot circuit-quantum-electrodynamics systems are promising both from fundamental perspectives and from the point of view of quantum photonic devices. However, understanding such setups as potential thermoelectric devices has been missing. In this paper, via the Keldysh nonequilibrium Green’s function approach, we show that cavity-coupled double quantum-dots can serve as excellent quantum thermoelectric diodes and transistors. Using an enhanced perturbation approach based on the exact polaron transformation, we find dependencies of thermoelectric transport properties on the electron-photon interaction beyond the predictions from the conventional second-order perturbation theory. In particular, strong light-matter interaction leads to pronounced rectification effects for both charge and heat, as well as thermal transistor effects in the linear transport regime, which opens up a cutting-edge frontier for quantum thermoelectric devices.
关键词： rectification,light-matter interaction,circuit-QED,diode,transistor,thermoelectric,thermal transistor,quantum-dot
更新于2025-09-23 15:22:29
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Ultrastrong-coupling regime of nondipolar light-matter interactions
摘要： We present a circuit-QED scheme which makes it possible to reach the ultrastrong-coupling regime of a nondipolar interaction between a single qubit and a quantum resonator. We show that the system Hamiltonian is well approximated by a two-photon quantum Rabi model and propose a simple scattering experiment to probe its fundamental properties. In particular, we identify a driving scheme that reveals the change in selection rules characterizing the breakdown of the rotating-wave approximation and the transition from strong to ultrastrong two-photon interactions. Finally, we show that a frequency crowding in a narrow spectral region is observable in the output fluorescence spectrum as the coupling strength approaches the collapse point, paving the way to the direct observation of the onset of the spectral collapse in a solid-state device.
关键词： nondipolar light-matter interactions,spectral collapse,two-photon quantum Rabi model,ultrastrong-coupling regime,circuit-QED
更新于2025-09-23 15:21:01
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Gauge ambiguities imply Jaynes-Cummings physics remains valid in ultrastrong coupling QED
摘要： Ultrastrong-coupling between two-level systems and radiation is important for both fundamental and applied quantum electrodynamics (QED). Such regimes are identified by the breakdown of the rotating-wave approximation, which applied to the quantum Rabi model (QRM) yields the apparently less fundamental Jaynes-Cummings model (JCM). We show that when truncating the material system to two levels, each gauge gives a different description whose predictions vary significantly for ultrastrong-coupling. QRMs are obtained through specific gauge choices, but so too is a JCM without needing the rotating-wave approximation. Analysing a circuit QED setup, we find that this JCM provides more accurate predictions than the QRM for the ground state, and often for the first excited state as well. Thus, Jaynes-Cummings physics is not restricted to light-matter coupling below the ultrastrong limit. Among the many implications is that the system's ground state is not necessarily highly entangled, which is usually considered a hallmark of ultrastrong-coupling.
关键词： quantum Rabi model,circuit QED,gauge invariance,Jaynes-Cummings model,ultrastrong coupling,quantum electrodynamics
更新于2025-09-19 17:15:36
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Experimentally feasible scheme for a high-fidelity controlled-phase gate with general cat-state qubits
摘要： We propose an efficient scheme for implementing the universal controlled-phase gate with logical qubits encoded in the superpositions of 2d circularly distributed coherent states (d is a positive integer), which have the advantage of protecting quantum information against the (d ? 1)-photon loss errors. We first demonstrate explicitly how to engineer dispersive interaction between the microwave cavity modes and multilevel transmon ancilla in circuit quantum electro-dynamical (QED) system. With the cat-state qubits, we realize the two-qubit controlled phase gate, and its operation time scales as 1/d. This means that it will be less susceptible to environmental disturbances if we employ larger d to encode information. Finally, we employ the experimentally reachable parameters in circuit QED system to numerically study the influence of the photon loss, the qubit relaxation, and the Kerr nonlinearity. The average fidelity for a controlled-phase gate has reached 0.9955 for d = 8 when the system imperfections are considered. The results demonstrate the great potential of our scheme for quantum computation in circuit QED system.
关键词： circuit QED system,cat-state qubits,controlled-phase gate,quantum computation
更新于2025-09-16 10:30:52