Abstract
Nonequilibrium thermal transport in circuit quantum electrodynamics emerges as one interdisciplinary field, due to the tremendous advance of quantum technology. Here, we study steady-state heat flow in a two-mode qubit-resonator model under the influence of both the qubit-resonator and resonator-resonator interactions. The heat current is suppressed and enhanced by tuning up resonator-resonator interaction strength with given weak and strong qubit-resonator couplings respectively, which is cooperative contributed by the eigen-mode of coupled resonators and qubit-photon scattering. Negative differential thermal conductance and significant thermal rectification are exhibited at weak qubit-resonator coupling, which are dominated by cycle transition processes. Moreover, the heat flow through the resonator decoupled from the qubit can be dramatically enhanced via the resonator-resonator interaction, which is attributed by the generation of eigen-mode channels of resonators.
Introduction
The microscopic interaction between bosonic fields (e.g., light and vibration) and quantum matter plays a cornerstone role in the study of quantum optics [1–4], quantum phononics [5, 6], and quantum technology [7–9]. The significant advance of superconducting and quantum-dot circuit quantum electrodynamics (cQED) fertilizes nonequilibrium heat transport in light-matter interacting systems, by integrating cQED components (e.g., resonator and qubit) with thermal and electric reservoirs [10–16]. Bounded by the second law of thermodynamics, the heat flow can be modulated via the nonequilibrium temperature bias [10], geometric heat pump [17, 18], and quantum correlations [19].
The seminal quantum Rabi model (QRM) is one generic system to describe quantum light-matter interaction, i.e., a single-mode photon field transversely coupled with a two-level qubit. QRM nowdays has been extensively realized in various quantum systems, ranging from quantum electrodynamics [2, 20], trapped ions [21, 22], and semiconductors with spin-orbital coupling [23]. Theoretically, though the eigensolution is rather difficult to obtain, D. Braak [24] and Chen et al. [25] separately proposed mapping ways a decade ago, trying to achieve the exact solution of QRM. While considering the interface between the subsystems in cQED and mesoscopic thermal reservoirs, the powerful platform to detect steady-state heat transport has been established [10, 12] to deepen the understanding of quantum thermodynamics, condensed-matter physics, and functional thermal devices. The analogous setup nowadays can also be found in optomechanics [26]. As a consequence, theoretical studies of quantum thermal transport in two-bath QRM have been preliminarily carried out [27–29].
Meanwhile, the longitudinal qubit-resonator model, as an alternative representative system, was also experimentally realized based on cQED devices [30–32], which is able to reach quite strong qubit-resonator coupling. Such longitudinal interaction leads to tremendous intriguing applications, e.g., generation of nonclassical photon states [33, 34], scalable quantum computation [35], and efficient quantum readout [36, 37]. Moreover, as a basic member in the multi-mode qubit-resonator family [30, 38, 39], the two-mode qubit-resonator model has been designed in cQED with longitudinal coupling [40–42], which is able to exhibit quantum switch effect [43]. While for nonequilibrium thermal transport in the longitudinal qubit-resonator system, an inspiring cooling by heating effect was reported, which causes the photon antibunchinig [44]. Besides, several typical nonlinear thermal effects were also proposed [45–47]. Hence, it should be intriguing to explore the transport pictures and microscopic mechanisms in the dissipative two-mode qubit-resonator model.
In this work, we apply the quantum dressed master equation (DME) combined with coherent photon states to study the steady-state heat current of the dissipative two-mode qubit-resonator model. The main points of this study are listed as follows: 1) The steady-state heat flow is suppressed under the influence of the resonator-resonator interaction with given weak qubit-resonator coupling, whereas it is enhanced by increasing the resonator-resonator interaction strength at strong qubit-resonator coupling, which is unavailable for the single-mode qubit-resonator model [29, 45]. The underlying mechanisms are illustrated based on the eigen-modes of resonators (4) and coherent state overlap coefficient (15). 2) The nonlinear thermal effects, e.g., negative differential thermal conductance and thermal rectification are unraveled in weak qubit-resonator coupling regime. They are attributed by two kinds of cycle transition processes (20a-20 b), which fertilizes microscopic transport pictures in the single-mode qubit-resonator model. 3) The resonator-resonator interaction induced indirect heat transport is also investigated. It is interesting to find that at the condition of identical resonators, even weak resonator-resonator interaction can significantly enhance heat current via the resonator isolated from the qubit, due to the eigen-modes of resonators.
The paper is organized as follows: In Section 2 we introduce the two-mode qubit-resonator model, obtain the corresponding eigenvalues and coherent photon eigenstates, and derive the DME. In Section 3 we study steady-state heat current under the influence of both qubit-resonator interaction and resonator-resonator coupling. Typical nonlinear thermal effects and resonator-resonator induced indirect heat transport is discussed. Finally, we give a summary in Section 4.
Model and method
Two-mode qubit-resonator model
The Hamiltonian of the longitudinal two-mode qubit-resonator system, composed of one two-level qubit longitudinally coupled with two optical resonators in Figure 1A, reads
FIGURE 1
Where creates (annihilates) one photon in the i-th resonator with the frequency ωi, t describes inter-resonator hopping strength, denotes the Pauli operator of the qubit, composed of two states |↑⟩ and |↓⟩, e.g., and , ɛ is the splitting energy of the qubit, and λi is the longitudinally coupling strength between the i-th resonator and the qubit. In this paper, we set ℏ = 1 and select the identical frequency case of resonators ωi = ωa by default. The generalization to the distinct case is straightforward. The resonator-resonator linear interaction could be approximately formed in several ways, e.g., collective spin-photon interaction in quantum Dicke model at normal phase under the Holstein-Primakoff transformation [48, 49] and the coupling between two coplanar waveguides in cQED [40, 43]. While for the mechanical oscillators, the resonator-resonator interaction, i.e., , should generally include the reorganization terms ( and ), which may become negligible compared to ωa in the weak interaction limit.
Next, we try to obtain the eigenvalues and eigenstates of the Hamiltonian (Section 1). Specifically, if we include the coordinate and momentum operators of the resonators as and , the Hamiltonian can be reexpressed as . To diagonalize , we further introduce the canonical coordinate operators and , with the angleHence, the Hamiltonian (Section 1) is reexpressed aswhere is the new bosonic operator, with the momentum operator , which creates (annihilates) one photon with the eigen-mode energyunder the bound as shown in Figure 1B, and the modified qubit-resonator coupling strengths are given by.With θ1 = θ, θ2 = θ + π. The constant coefficient in Eq. 3 is ignored for safety for dissipative dynamics and steady state. Hence, the eigenstates are expressed aswith σ = {↑, ↓}, , , and the vacuum state . And the corresponding eigenvalues are shown aswith ɛ↑ = ɛ, ɛ↓ = −ɛ, and n = [n1, n2] (ni = 0, 1, 2, ⋯ ). In absence of t, the eigen-energy Ei (4) is reduced to ωi, and the modified coupling strengths become . Then, the eigenstates are characterized as , and the corresponding eigenvalues are shown as .
Quantum dressed master equation
In practice, the quantum system inevitably interacts with surrounding environments. Hence, the qubit and optical resonators may individually interact with bosonic thermal baths. Accordingly, the total Hamiltonian, including the quantum system, thermal reservoirs, and their interactions, is expressed asSpecifically, the r-th thermal reservoir connecting is described as , where creates (annihilates) one boson in the r-th thermal reservoir with the frequency ωk,r. The interaction between the i-th resonator and the reservoir is given bywhere denotes the coupling strength. Here, we consider each optical resonator is separately coupled with the r-th thermal reservoir. Hence, the interaction between the i-th resonator and the reservoir can be characterized as the spectral function . And the r-th reservoir induced interference is ignored [50], i.e., for i ≠ j. While the q-th thermal reservoir coupled with the qubit is described as , where means the bosonic creation (annihilation) operator with the frequency ωk,q. The qubit-reservoir interaction is given bywith gk,q being the coupling strength, which is characterized as the spectral functions γq(ω) = 2π|gk,q|2δ(ω − ωk). We select the Ohmic case of the spectral functions in this work, i.e., γu(ω) = αuω exp( − |ω|/ωc), with αu the dissipation strength and ωc the cutoff frequency.
Then, we assume the interactions between the quantum system and thermal reservoirs are weak. Under the Born-Makov approximation, we may properly perturb them to obtain the quantum master equation within the eigenstate basis of . Since we focus on the steady-state behavior of the qubit-resonator hybrid system, we properly obtain the DME as [51–53].
Where is the reduced density operator of the multi-mode qubit-resonator system and the dissipator is given byThe transition rate contributed by the i-th resonator and r-th reservoir is described as
By considering relations , with γu( − ω)nu( − ω) = γu(ω)[1 + nu(ω)], , and δn,n′ = 1 for n = n′ and δn,n′ = 0 for n ≠ n′. The nonzero rate assisted by the q-th reservoir is given by
Where Θ(x) = 1 for x > 0 and Θ(x) = 0 for x ≤ 0, the coherent state overlap coefficient is specified as [54, 55].and the Bose-Einstein distribution function is given by , with kB the Boltzmann constant and Tu the temperature of the u-th thermal reservoir. Both and become zero as , for the phonon with ω = 0 can not support the heat transport.
The transition rate 13) involved with the r-th thermal reservoir and the i-th resonator is individual contributed by two eigen-mode channels, (i.e., and ). For each channel, the local detailed balance relation is still valid, i.e., . While the rate 14) into the q-th reservoir via the qubit is cooperatively determined by qubit-photon scattering processes, which are characterized as the coherent state overlap coefficient (15).
Based on DME (11), the steady-state heat current mediated by the subsystem component (resonators and qubit) into the corresponding thermal reservoir is expressed as
Where the dynamical equation of the density matrix element is given by and the steady state is obtained by dρn,σ/dt = 0. During the numerical calculation, we need to truncate the photon number, i.e., ni ≤ 40, to practically obtain the steady state and the heat flow.
Results and discussions
Steady-state heat current
We first study the influence of resonator-resonator coupling t on the behavior of steady-state heat current into the q-th thermal reservoir in Figure 2A. Before the analysis, we approximately characterize the weak couplings as t/ωa≲0.01 and λ/ωa≲0.01, which may fulfill the relations t/ωa≪1 and λ/ωa≪1. While for strong couplings, we quantify as t/ωa≳0.2 and λ/ωa≳1, where multi-photon processes may be included. In the weak qubit-resonator coupling regime with a given λ/ωa, Jq is monotonically suppressed by tuning the resonator-resonator interaction strength. On the contrary, at strong qubit-resonator coupling the heat current is dramatically enhanced by increasing t. Furthermore, Jq generally exhibits nonmonotonic behavior with the increase of the qubit-resonator interaction strength. We also give a comprehensive picture of the effects of both qubit-resonator and resonator-resonator couplings on the heat current in Figure 2B. It is shown that at strong λ and large t the optimal regime of Jq is broadened compared to the one with t = 0. Hence, we conclude that the resonator-resonator interaction indeed significantly contributes to the steady-state heat current.
FIGURE 2
Next, we try to analyze the heat current from the analytical view. Based on the case of identical resonators we set in Figure 2, the modified qubit-resonator coupling strengths (5a) and (5 b) are specified as Λ1 = 0 and . Hence, the bosonic eigen-mode is effectively decoupled from the qubit, as shown in Figure 2C, which shows no contribution to the steady-state heat current. The heat transport system can be reduced to the single-mode qubit-resonator model [45].
At weak qubit-resonator coupling (λ/ωa≪1), the coherent state overlap coefficient 15) with Λ2/E2≪1 is simplified as . Then, the transition rate assisted by the q-th reservoir is approximated as.And the zeroth order of steady state population is obtained as.With βu = 1/kBTu. Moreover, if we reexpress the populations in the vector form |ρs⟩, the steady-state solution based on Eq. 11 becomes . Then, we approximately expand and . The steady-state solution is given by and . Accordingly, the heat current based on the expression (16) is can be obtained, which is contributed by two cyclic componentswhere.Which are described in Figure 2D,E. Physically, the loop of I1(E2) is established as , whereas the cycle transitions of I2(E2) is specified as . Trough these two heat transport processes, the energy quanta E2 is delivered from the r-th reservoir to the q-th one. As t is tuned up, both the factor and the current components (I1 + I2) are gradually suppressed, which leads to the monontonic suppression of Jq. Moreover, by comparing the current in the single-resonator limit N = 1, i.e., Jq(N = 1) = (4λ2/ωa)I1(ωa), the current without resonator-resonator coupling becomes . We note that though transport pictures of both the dissipative two-mode qubit-resonator model and the single-mode case are characterized as cycle transition processes, they are microscopically distinct. Specifically, 1) the modified qubit-resonator interaction (Λi) and eigen-mode energy (Ei) induced by the resonator-resonator coupling dramatically affect the expression of heat current Jq; (ii) the existence of the transition rate results in the generation of the cycle flux component I2.
While at strong qubit-resonator coupling, e.g., λ/ωa≳1, qubit-photon scattering effects dominate the transition processes, which are characterized as the coefficient (15). Such analogous effects generally suppress the heat transport, as shown in Figure 2A and previous works, e.g., nonequilibrium spin-boson model [56–59] and quantum Rabi model [29]. In particular, the crucial factor to quantify such scattering is Λ2/E2. Thus, by increasing t it is found that the factor shows monotonic reduction. As a consequence, the scattering processes are weakened accordingly, which finally enhances the heat current. Therefore, we explain the underlying mechanisms of the influence of the resonator-resonator interaction on the heat current in weak and strong qubit-resonator coupling limits.
Negative differential thermal conductance and thermal rectification
Inspired by the cycle transition features of the steady state heat current with weak qubit-resonator interaction, we first study the negative differential thermal conductance (NDTC) in the two-mode qubit-resonator model. NDTC is widely considered as one generic nonlinear feature in nonequilibrium heat transport [60–64], which is characterized as the heat current, e.g., Jq, being reduced with the increase of the reservoir temperature bias ΔT = Tr − Tq. In phononics, NDTC was originally introduced by B. Li and his colleges to analyze heat flow in classical phononic lattices [63]. As a consequence, tremendous open quantum systems have been unraveled to exhibit NDTC. In particular, D. Segal [65] and Cao et al. [66] individually applied the noninteracting blip approximation and canonically transformed Redfield equation to explore the effect of NDTC with strong system-bath interactions. Ren et al. [67] included the spin-boson-fermion systems to propose spin-NDTC based on the Landauer formula expression of the heat current. Moreover, Giazotto et al. [68] designed the superconducting devices to measure NDTC by modulating the superconducting gap with the temperature.
Here, we show NDTC in the two-mode qubit-resonator model with identical resonators in Figure 3A. In absence of the resonator-resonator interaction, the heat current is reduced to , which is fully determined via single-type cycle transition process in Figure 2D. And the microscopic picture is identical with single qubit-resonator model [45]. Specifically, it exhibits nonmonotonic behavior by tuning up the temperature bias, i.e., initial enhancement and later suppression. Finally the heat current is completely eliminated at large ΔT (e.g., Tr = 2ωa and Tq = 0), which originates from the empty occupation of bosons in the q-th reservoir, i.e., nq(ɛ) = 0 and nq(ɛ + ωa) = 0. Then, we tune on the resonator-resonator interaction. It is found that the signature of NDTC persists in the whole temperature bias regime. However, NDTC is determined by two kinds of cycle transition processes as shown in Figure 2D,E, corresponding to I1 and I2. In particular, I2 dramatically contributes to NDTC even at rather weak resonator-resonator coupling, e.g., the blue-solid-triangle line at t = 0.01ωa in Figure 3B. Moreover, the signature of NDTC is gradually suppressed with the increase of t, which stems from robustness of cycle transition process of I2, partially characterized as the resultant part at large bias limit, i.e., I2 ≈ γq(E2 − ɛ)nr(E2) from Eq. 19.
FIGURE 3
We also study the thermal rectification effect by modulating the temperature bias, which is another representative nonlinear character in nonequilibrium thermal transport. The concept of thermal rectification is introduced as that the heat flow in one direction is larger than the counterpart in the opposite direction [63, 69]. We characterize the rectification by the factorwhere Jq(ΔT) denotes the current under the setting of temperatures Tr = ωa/kB + ΔT/2 and Tq = ωa/kB − ΔT/2. Such effect becomes perfect as |Jq(ΔT)|≫|Jq( − ΔT)| (i.e., R = 1) and vanishing as Jq(ΔT) ≈ − Jq( − ΔT) (i.e., R = 0). Figure 3C clearly exhibits the giant heat amplification factor at large temperature bias, mainly due to the asymmetric behavior of heat current Jq by tuning ΔT from negative to positive regimes, as shown in Figure 3D. Specifically, the disappearance of the current at ΔT = 2ωa and t = 0 results in the perfect thermal rectifier, whereas the existence of resultant current at finite t comparatively reduces thermal rectification factor. Therefore, we conclude that microscopic cycle transition processes are crucial to exhibit NDTC and significant thermal rectification in two-mode qubit-resonator model.
Resonator-resonator coupling enhances heat transport
To show the ability of the resonator-resonator interaction to modulate the heat transport, we tune off the interaction between the 1-st resonator and the qubit, as shown in Figure 4A. Straightforwardly, the heat current always keeps vanishing as t = 0. While by tuning on resonator-resonator coupling in case of identical resonators (ωi = ωa, i = 1, 2), the modified qubit-photon interaction strengths become and , which implies that two eigen-modes of resonators, i.e., and , will both contribute to the heat transport, as depicted in Figure 4B. It is interesting to find that even with weak resonator-resonator coupling (e.g., t/ωa = 0.01), the heat current in Figure 5A behaves quite similar with in Figure 5B. Based on the expression of transition rates at Eqs. 13 with θ = π/2, can be directly obtained. Each eigen-mode channel shows identical contribution to these two rates. This clearly demonstrates that actually by combing the expression of the heat current at Eq. 16. It also leads to as exhibited in Figure 5C.
FIGURE 4
FIGURE 5
Moreover, we analyze the effect of the resonator-resonator interaction on with distinct resonators. With weak resonator-resonator coupling [t/(ω2 − ω1)≪1], θ becomes vanishing. Hence, the heat flow via the 1-th resonator is dramatically blocked. It is expected to see is much weaker than , i.e., , which are also shown in Figures 5D–F. As a result, the two-mode qubit-resonator transport system is reduced to the standard single-mode qubit-resonator case [45]. While by increasing the resonator-resonator coupling, e.g., t = 0.4ωa, it is interesting to find that becomes comparable with , due to dramatic hybridization of two resonators (θ = π/2). Therefore, we conclude that though the 1-th resonator is decoupled from the qubit, the inclusion of the resonator-resonator interaction will open heat-exchange channels, which significantly contributes to the heat current .
Conclusion
To summarize, we study nonequilibrium thermal transport in the dissipative two-mode qubit-resonator model by applying quantum DME combined with extended coherent photon states, where optical resonators and the qubit are individually coupled with thermal reservoirs. By properly treating the resonator-resonator interaction, we obtain the eigensolution of the two-mode qubit-resonator model. Then, we analyze the influence of the qubit-resonator coupling and resonator-resonator interaction on steady-state behaviors of heat currents. The currents are suppressed with the increase of resonator-resonator interaction strength at weak qubit-resonator coupling. It mainly results from the eigen-mode energies of coupled resonators (4), based on the analytical expression of cycle heat current components (19). In contrast, the currents are monotonically enhanced in strong qubit-resonator coupling regime, which is mainly attributed to the reduction of the effective qubit-resonator coupling in Eq. 15 by tuning up resonator-resonator interaction strength. Hence, the resonator hybridization and the directional cycle transition processes cooperatively contribute to the nontrivial behaviors of steady-state heat currents.
Inspired by cycle transition components of heat current at weak qubit-resonator coupling, we also investigate two representative nonlinear thermal effects, i.e., NDTC and thermal rectification. NDTC is unraveled at large temperature bias and keeps robust even with strong resonator-resonator interaction strength. The microscopic cycle transition processes (20a) and (20b) determine the appearance of NDTC. Meanwhile, the significant thermal rectification effect is observed in a wide regime of qubit-resonator and resonator-resonator couplings, which becomes perfect in absence of the resonator-resonator interaction.
Moreover, we show the effect of the resonator-resonator interaction on indirectly heat transport, by tuning off the interaction between the 1-st resonator and the qubit. It is interesting to find that for identical resonators the heat current (flowing into r-th reservoir mediated by the 1-th resonator) dramatically becomes identical with even at weak resonator-resonator coupling. While for the case of distinct resonators, the effective angle θ gradually increases from 0 to π/2 with the increase of resonator-resonator interaction strength, which leads to the monotonic enhancement of . Therefore, the resonator-resonator interaction can be considered as one route to efficiently realize the indirect heat transport.
We hope that the analysis of quantum heat transport and thermal management in the two-mode qubit-phonon system may provide physical insight for smart energy control in photon-based hybrid quantum systems. In the future, it should be intriguing to explore the steady-state heat flows in multi-mode qubit-resonator systems, e.g., three-mode qubit-resonator model [70].
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.
Author contributions
F-YW: analytical and numerical work, manuscript writing J-CL: numerical work ZW: analytical work CW: guiding project, analysis, manuscript writing JR: guiding project, analysis, manuscript writing.
Funding
F-YW, L-WD, and CW are supported by the National Natural Science Foundation of China under Grant No. 11704093 and the Opening Project of Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology. J-CL, ZW, and JR acknowledges the support by the National Natural Science Foundation of China (No. 11935010 and No. 11775159), Natural Science Foundation of Shanghai (No. 18ZR1442800 and No. 18JC1410900), and the China Postdoctoral Science Foundation (Grant No. 2020M681376).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
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Appendix A: Derivation of Eq. 11.
Starting from Eq. 8, we assume that system-bath interactions are weak. Under the Born-Markov approximation, we individually perturb and up to the second order to obtain the quantum generalized master equation as [52].
Where the rate is given by κu(ω) = γu(ω)nu(ω), with the Bose-Einstein distribution function nu(ω) = 1/[ exp(ω/kBTu) − 1], the projecting operators based on the eigenstates are given by and , with and , the eigensolution is given by , and the energy gap becomes Enm = En − Em.
It should be noted that for the finite-time evolution, the off-diagonal elements of the density operator are generally coupled with the diagonal ones in the eigenspace. However, after a long-time evolution, the off-diagonal elements gradually become ignored in the present model. Hence, the populations are naturally decoupled from the off-diagonal elements, which reduces the pair of projecting operators and to and , with and . Accordingly, the generalized mater equation is reduced to the dressed master equation [51]
Where the dissipative is given by
Based on DME (section A2) and specifying |ψk⟩ = |ψn,σ⟩ and Ek = En,σ, the dynamical equation of the density matrix element is obtained as
Summary
Keywords
nonequilibrium thermal transport, circuit quantum electrodynamics, qubit-resonator interaction, quantum master equation, cycle transition process
Citation
Wang F-Y, Lu J-C, Wang Z, Duan L-W, Wang C and Ren J (2022) Nonequilibrium thermal transport in the two-mode qubit-resonator system. Front. Phys. 10:964858. doi: 10.3389/fphy.2022.964858
Received
09 June 2022
Accepted
07 July 2022
Published
25 August 2022
Volume
10 - 2022
Edited by
Ying Li, Zhejiang University, China
Reviewed by
Dazhi Xu, Beijing Institute of Technology, China
Hai-Rui Wei, University of Science and Technology Beijing, China
Updates
Copyright
© 2022 Wang, Lu, Wang, Duan, Wang and Ren.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Chen Wang, wangchen@zjnu.cn; Jie Ren, Xonics@tongji.edu.cn
This article was submitted to Physical Acoustics and Ultrasonics, a section of the journal Frontiers in Physics
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