Pei-Rong Han, Fan Wu, Xin-Jie Huang, Huai-Zhi Wu, Chang-Ling Zou, Wei Yi, Mengzhen Zhang, Hekang Li, Kai Xu, Dongning Zheng, Heng Fan, Jianming Wen, Zhen-Biao Yang, Shi-Biao Zheng
Oct 10, 2022·quant-ph·PDF Non-Hermitian (NH) extension of quantum-mechanical Hamiltonians represents one of the most significant advancements in physics. During the past two decades, numerous captivating NH phenomena have been revealed and demonstrated, but all of which can appear in both quantum and classical systems. This leads to the fundamental question: what NH signature presents a radical departure from classical physics? The solution of this problem is indispensable for exploring genuine NH quantum mechanics, but remains experimentally untouched so far. Here, we resolve this basic issue by unveiling distinct exceptional entanglement phenomena, exemplified by an entanglement transition, occurring at the exceptional point of NH interacting quantum systems. We illustrate and demonstrate such purely quantum-mechanical NH effects with a naturally dissipative light-matter system, engineered in a circuit quantum electrodynamics architecture. Our results lay the foundation for studies of genuinely quantum-mechanical NH physics, signified by exceptional-point-enabled entanglement behaviors.
Xian-Peng Zhang, Li-Tuo Shen, Zhang-Qi Yin, Huai-Zhi Wu, Zhen-Biao Yang
Jul 12, 2014·quant-ph·PDF We demonstrate quantum bath engineering for preparation of any orbital state with controllable phase factor of a superconducting flux qubit assisted by a microwave coplanar waveguide resonator. We investigate the polarization efficiency of the arbitrary direction rotating on the Bloch sphere, and obtain an effective Rabi frequency by using the convergence condition of Markovian master equation. The processes of polarization can be implemented effectively in a dissipative environment created by resonator photon loss when the spectrum of the microwave resonator matches with the specially tailored Rabi and resonant frequencies of the drive. Our calculations indicate that state-preparation fidelities in excess of 99\% and the required time on the order of magnitude of microsecond are in principle possible for experimentally reasonable sample parameters. Furthermore, our proposal could be applied to other systems with spin-based qubits.
Xin-Yu Chen, Li-Tuo Shen, Zhen-Biao Yang, Huai-Zhi Wu, Mei-Feng Chen
We propose a scheme for the dissipative preparation of W-type entangled steady-states of three atoms trapped in an optical cavity. The scheme is based on the competition between the decay processes into and out of the target state. By suitable choice of system parameters, we resolve the whole evolution process and employ the effective operator formalism to engineer four independent decay processes, so that the target state becomes the stationary state of the quantum system. The scheme requires neither the preparation of definite initial states nor the precise control of system parameters and preparation time.
Pei-Rong Han, Fan Wu, Xin-Jie Huang, Huai-Zhi Wu, Chang-Ling Zou, Wei Yi, Mengzhen Zhang, Hekang Li, Kai Xu, Dongning Zheng, Heng Fan, Jianming Wen, Zhen-Biao Yang, Shi-Biao Zheng
Jan 27, 2025·quant-ph·PDF Dissipation usually plays a negative role in quantum metrological technologies, which aim to improve measurement precision by leveraging quantum effects that are vulnerable to environment-induced decoherence. Recently, it has been demonstrated that dissipation can actually be used as a favorable resource for enhancing the susceptibility of signal detection. However, demonstrations of such enhancement for detecting physical quantities in open quantum systems are still lacking. Here we propose and demonstrate a protocol for realizing such non-Hermitian quantum sensors for probing the coupling between a qubit and a resonator subjecting to energy dissipations. The excitation-number conversion associated with the no-jump evolution trajectory enables removal of the noisy outcomes with quantum jumps, implementing the exceptional point (EP), where the Rabi splitting exhibits a divergent behavior in response to a tiny variation of the effective coupling. The sensitivity enhancement near the EP is confirmed by both theoretical calculation and experimental measurement.
Jia-Hao Lü, Wen Ning, Fan Wu, Ri-Hua Zheng, Ken Chen, Xin Zhu, Zhen-Biao Yang, Huai-Zhi Wu, Shi-Biao Zheng
Critical systems near quantum phase transitions were predicted to be useful for improvement of metrological precision, thanks to their ultra-sensitive response to a tiny variation of the control Hamiltonian. Despite the promising perspective, realization of criticality-enhanced quantum metrology is an experimentally challenging task, mainly owing to the extremely long time needed to encode the signal to some physical quantity of a critical system. We here circumvent this problem by making use of the critical behaviors in the Jaynes-Cummings model, comprising a single qubit and a photonic resonator, to which the signal field is coupled. The information about the field amplitude is encoded in the qubit's excitation number in the dark state, which displays a divergent changing rate at the critical point. The most remarkable feature of this critical sensor is that the performance is insensitive to the leakage to bright eigenstates, caused by decoherence and non-adiabatic effects. We demonstrate such a metrological protocol in a superconducting circuit, where an Xmon qubit, interacting with a resonator, is used as a probe for estimating the amplitude of a microwave field coupled to the resonator. The measured quantum Fisher information exhibits a critical quantum enhancement, confirming the potential of this system for quantum metrology.
Ze-Lin Zhang, Ming-Feng Chen, Huai-Zhi Wu, Zhen-Biao Yang
On the background of the Born-Oppenheimer (adiabatic) approximation, we investigate the geometrical and topological structure in the theory of quantum gravity by using the path integral method and halfclassical approximation. As we know, Berry curvature can be extracted from the linear response of a driven two-level system to nonadiabatic manipulations of its Hamiltonian. In parameter space of the Hamiltonian, magnetic monopoles can be artificially simulated. Ripples occur in Hilbert space when the monopole travels from inside to outside the surface of energy manifold spanned by system parameters. From this point of view, we set up the connection between the ripples characterized by the fidelity of quantum states in Hilbert space and gravitational-like waves in minisuperspace. This might open a window for the study of geometrical and topological properties of quantum gravity with the help of quantum physical systems.
Li-Tuo Shen, Huai-Zhi Wu, Zhen-Biao Yang
We propose an interesting scheme for distributed orbital state quantum cloning with atomic ensembles based on the quantum Zeno dynamics. These atomic ensembles which consist of identical three-level atoms are trapped in distant cavities connected by a single-mode integrated optical star coupler. These qubits can be manipulated through appropriate modulation of the coupling constants between atomic ensemble and classical field, and the cavity decay can be largely suppressed as the number of atoms in the ensemble qubits increases. The fidelity of each cloned qubit can be obtained with analytic result. The present scheme provides a new way to construct the quantum communication network.
Xian-Peng Zhang, Li-Tuo Shen, Zhang-Qi Yin, Luyan Sun, Huai-Zhi Wu, Zhen-Biao Yang
Apr 28, 2016·quant-ph·PDF A quantum network is a promising quantum many-body system because of its tailored geometry and controllable interaction. Here, we propose an external control scheme for the qubit-photon interaction and multiqubit reset in a dissipative quantum network, which comprises superconducting circuit chains with microwave drives and filter-filter couplings. The traditional multiqubit reset of the quantum network requires physically disconnected qubits to prevent their entanglement. However, we use an original effect of dissipation, i.e., consuming the entanglement generated by qubits' interaction, to achieve an external control of the multiqubit reset in an always-connected superconducting circuit. The reset time is independent of the number of qubits in the quantum network. Our proposal can tolerate considerable fluctuations in the system parameters and can be applicable to higher-dimensional quantum networks.
Li-Tuo Shen, Zhen-Biao Yang, Mei Lu, Rong-Xin Chen, Huai-Zhi Wu
Jun 10, 2013·quant-ph·PDF We study the ground states of the single- and two-qubit asymmetric Rabi models, in which the qubit-oscillator coupling strengths for the counterrotating-wave and corotating-wave interactions are unequal. We take the transformation method to obtain the approximately analytical ground states for both models and numerically verify its validity for a wide range of parameters under the near-resonance condition. We find that the ground-state energy in either the single- or two-qubit asymmetric Rabi model has an approximately quadratic dependence on the coupling strengths stemming from different contributions of the counterrotating-wave and corotating-wave interactions. For both models, we show that the ground-state energy is mainly contributed by the counterrotating-wave interaction. Interestingly, for the two-qubit asymmetric Rabi model, we find that, with the increase of the coupling strength in the counterrotating-wave or corotating-wave interaction, the two-qubit entanglement first reaches its maximum then drops to zero. Furthermore, the maximum of the two-qubit entanglement in the two-qubit asymmetric Rabi model can be much larger than that in the two-qubit symmetric Rabi model.
Chang-Sheng Hu, Xi-Rong Huang, Li-Tuo Shen, Zhen-Biao Yang, Huai-Zhi Wu
Jun 29, 2016·quant-ph·PDF We theoretically investigate the stability of a two cascaded cavity optomechanical system with optical parametric amplifiers (OPAs) inside the two coupled cavities, and study the steady-state entanglement between two distant mechanical resonators. We show that the parameter regime where the system is unstable without OPAs, such as relatively high laser intensity and blue detuning, can be exploited to build the steady-state mechanical entanglement by modulating the parametric gain. The application of OPAs is helpful to preserve the mechanical entanglement suffered from the dissipation at some finite temperature. The scheme provides an alternative way for improving and engineering the quantum entanglement of two distant mechanical oscillators.
Ze-Lin Zhang, Ming-Feng Chen, Huai-Zhi Wu, Zhen-Biao Yang
Jun 30, 2016·quant-ph·PDF With the help of the Berry curvature and the first Chern number $($$\textit{C}_1$$)$, we both analytically and numerically investigate and thus simulate artificial magnetic monopoles formed in parameter space of the Hamiltonian of a driven superconducting qubit. The topological structure of a spin-1/2 system $($qubit$)$ can be captured by the distribution of Berry curvature, which describes the geometry of eigenstates of the Hamiltonian. Degeneracy points in parameter space act as sources $($$\textit{C}_1$ = $1$$)$ or sinks $($$\textit{C}_1$ = $-1$$)$ of the magnetic field. We note that the strength of the magnetic field $($described by Berry curvature$)$ has an apparent impact on the quantum states during the process of topological transition. It exhibits an unusual property that the transition of the quantum states is asymmetric when the degenerate point passes from outside to inside and again outside the manifold spanned by system parameters. Our results also pave the way to explore intriguing properties of magnetic monopoles in other spin-1/2 systems.
Li-Tuo Shen, Rong-Xin Chen, Huai-Zhi Wu, Zhen-Biao Yang
We study the system involving mutual interaction between three qubits and an oscillator within the ultrastrong coupling regime. We apply adiabatic approximation approach to explore two extreme regimes: (i) the oscillator's frequency is far larger than each qubit's frequency and (ii) the qubit's frequency is far larger than the oscillator's frequency, and analyze the energy-level spectrum and the ground-state property of the qubit-oscillator system under the conditions of various system parameters. For the energy-level spectrum, we concentrate on studying the degeneracy in low energy levels. For the ground state, we focus on its nonclassical properties that are necessary for preparing the nonclassical states. We show that the minimum qubit-oscillator coupling strength needed for generating the nonclassical states of the Schrödinger-cat type in the oscillator is just one half of that in the Rabi model. We find that the qubit-qubit entanglement in the ground state vanishes if the qubit-oscillator coupling strength is strong enough, for which the entropy of three qubits keeps larger than one. We also observe the phase-transition-like behavior in the regime where the qubit's frequency is far larger than the oscillator's frequency.
Zhong-Sheng Chen, Wei-Xin Chen, Fan Wu, Zhong-Wei Xu, Jing Ma, Yun-Kun Jiang, Huai-Zhi Wu, Shi-Biao Zheng
Sep 17, 2025·quant-ph·PDF Compared with Hermitian theory, non-Hermitian physics offers a fundamentally different mathematical framework, enabling the observation of topological phenomena that have no analogue in Hermitian systems. Among these, the exceptional point (EP) ring stands out as a quintessential topological feature unique to non-Hermitian systems. In this study, we employ single-photon interferometry to overcome the experimental challenge of precise phase control in quantum systems, thereby enabling a complete simulation of the non-Hermitian EP ring in three-dimensional parameter space without invoking any additional symmetry assumptions. By measuring the non-Hermitian dynamics in three-dimensional parameter space, we determine the system's eigenstates, which allows us to characterize the topological band structure of the system under different conditions. We describe the topological properties of the EP ring by extracting the Chern number and Berry phase for different parameter manifolds and observe the topological critical phenomena of the system. Our work paves the way for further exploration of topological non-Hermitian systems.
Li-Tuo Shen, Xin-Yu Chen, Zhen-Biao Yang, Huai-Zhi Wu, Shi-Biao Zheng
We propose a scheme for the generation of entangled states for two atoms trapped in separate cavities coupled to each other. The scheme is based on the competition between the unitary dynamics induced by the classical fields and the collective decays induced by the dissipation of two delocalized field modes. Under certain conditions, the symmetric or asymmetric entangled state is produced in the steady state. The analytical result shows that the distributed steady entanglement can be achieved with high fidelity independent of the initial state, and is robust against parameter fluctuations. We also find out that the linear scaling of $F$ has a quadratic improvement compared to distributed entangled state preparation protocols based on unitary dynamics.
Li-Tuo Shen, Zhen-Biao Yang, Huai-Zhi Wu, Xin-Yu Chen, Shi-Biao Zheng
Jul 13, 2012·quant-ph·PDF The dynamical evolution of a quantum system composed of two coupled cavities, each containing a two-level atom and a single-mode thermal field, is investigated under different conditions. The entanglement between the two atoms is controlled by the hopping strength and the detuning between the atomic transition and the cavities. We find that when the atomic transition is far off-resonant with both the eigenmodes of the coupled cavity system, the maximally entangled state for the two atoms can be generated with the initial state in which one atom is in the ground state and the other is in the excited state. When both the two atoms are initially in the excited state, the entanglement exhibits period sudden birth and death. By choosing appropriate parameter values, the initial maximal entanglement of the two atoms can be frozen. The relation between the concurrence and cooperative parameter is calculated.
Li-Tuo Shen, Xin-Yu Chen, Zhen-Biao Yang, Huai-Zhi Wu, Shi-Biao Zheng
Jun 24, 2012·quant-ph·PDF We describe a scheme with analytic result that allows to generate steady-state entanglement for two atoms over a dissipative bosonic medium. The resonant coupling between the mediating bosonic mode and cavity modes produces three collective atomic decay channels. This dissipative dynamics, together with the unitary process induced by classical microwave fields, drives the two atoms to the symmetric or asymmetric entangled steady state conditional upon the choice of the phases of the microwave fields. The effects on the steady-state entanglement of off-resonance mediating bosonic modes are analyzed. The entanglement can be obtained with high fidelity regardless of the initial state and there is a linear relation in the scaling of the fidelity with the cooperativity parameter. The fidelity is insensitive to the fluctuation of the Rabi frequencies of the classical driving fields.
Li-Tuo Shen, Rong-Xin Chen, Huai-Zhi Wu, Zhen-Biao Yang, Elinor Irish, Shi-Biao Zheng
Feb 25, 2014·quant-ph·PDF We analyze the quantum phase transition-like behavior in the lowest energy state of a two-site coupled atom-cavity system, where each cavity contains one atom but the total excitation number is not limited to two. Utilizing the variance of the total excitation number to distinguish the insulator and superfluid states, and the variance of the atomic excitation number to identify the polaritonic characteristics of these states, we find that the total excitation number plays a significant role in the lowest-energy-state phase transitions. In both the small hopping regime and the small atom-field interaction regime, we identify an interesting coexisting phase involving characteristics of both photonic superfluid and atomic insulator. For small hopping, we find that the signature of the photonic superfluid state becomes more pronounced with the increase in total excitation number, and that the boundaries of the various phases shift with respect to the case of $N=2$. In the limit of small atom-field interaction, the polaritonic superfluid region becomes broader as the total excitation number increases. We demonstrate that the variance of the total excitation number in a single site has a linear dependence on the total excitation number in the large-detuning limit.
Li-Tuo Shen, Rong-Xin Chen, Huai-Zhi Wu, Zhen-Biao Yang
We study the dynamics of two qubits separately sent through two coupled resonators, each initially containing a coherent state field. We present analytical arguments and numerical calculations for the qubit-field system under different two-qubit initial states, photon hopping strengths, and detunings. In far off-resonant regime, the maximal entanglement of two qubits can be generated with the initial qubit state in which one qubit is in the excited state and the other is in the ground state, and the initially maximal two-qubit entanglement can be frozen and fully revived even for large mean photon number. When the qubits are both initially in their excited states or ground states, the qubit-qubit entanglement birth and death apparently appear in the regime where the photon hopping strength is close to qubit-field detuning, and its peaks do not decrease monotonically as the interaction time increases. It is interesting to observe that when there is photon hopping strength between two fields, the field-field entanglement can be larger than one and increases as the initial amplitude of the coherent state grows. By postselecting the fields both in their coherent states, the entanglement of two initially unentangled qubits can be largely improved. Our present setup is fundamental for the distributed quantum information processing and applicable to different physical qubit-resonator systems.
En-Ze Li, Dong-Sheng Ding, Yi-Chen Yu, Ming-Xin Dong, Lei Zeng, Wei-Hang Zhang, Ying-Hao Ye, Huai-Zhi Wu, Zhi-Han Zhu, Wei Gao, Guang-Can Guo, Bao-Sen Shi
Aug 20, 2019·quant-ph·PDF Cavity-free optical nonreciprocity components, which have an inherent strong asymmetric interaction between the forward- and backward-propagation direction of the probe field, are key to produce such as optical isolators and circulators. According to the proposal presented by Xia et al., [Phys. Rev. Lett. 121, 203602 (2018)], we experimentally build a device that uses cross-Kerr nonlinearity to achieve a cavity-free optical isolator and circulator. Its nonreciprocal behavior arises from the thermal motion of N-type configuration atoms, which induces a strong chiral cross-Kerr nonlinear response for the weak probe beam. We obtain a two-port optical isolator for up to 20 dB of isolation ratio in a specially designed Sagnac interferometer. The distinct propagation directions of the weak probe field determine its cross-phase shift and transmission, by which we demonstrate the accessibility of a four-port optical circulator.
Pei-Rong Han, Fan Wu, Xin-Jie Huang, Huai-Zhi Wu, Wei Yi, Jianming Wen, Zhen-Biao Yang, Shi-Biao Zheng
Understanding the dynamical behavior of a qubit in a reservoir is critical to applications in quantum technological protocols, ranging from quantum computation to quantum metrology. The effect of the reservoir depends on reservoir's spectral structure, as well as on the qubit-reservoir coupling strength. We here propose a measure for quantifying the non-Markovian effect of a reservoir with a Lorentzian spectrum, based on the maximum qubit-reservoir quantum entanglement that can be extracted. Numerical simulation shows this entanglement exhibits a monotonous behavior in response to the variation of the coupling strength. We confirm the validity of this measure with an experiment, where a superconducting qubit is controllably coupled to a lossy resonator, which acts as a reservoir for the qubit. The experimental results illustrate the maximal extractable entanglement is progressively increased with the strengthening of the non-Markovianity.