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Physical Review A

Inhibition of double excitation and strong quantum entanglement via engineered cavity interactions

Chengjie Zhu, W. Li, Yaping Yang, G. S. Agarwal

Published Dec 20, 2022• 1 citation
Publisher

Abstract

We propose a method for realizing the inhibition of simultaneous excitation of two identical qubits in a single-mode cavity QED system, where two qubits are driven by a coherent field and coupled to a cavity with engineered qubit-cavity interactions. By suitably chosen placements of the qubits in the cavity and by adjusting the relative decay strengths of the qubits and cavity field, we close many unwanted excitation pathways. This leads to an inhibition of the doubly excited state. In addition, we show that these two qubits are strongly entangled over a broad regime of the system parameters. We show that a strong signature of nonexcitation of the doubly excited state is the bunching property of the cavity photons, which thus provides a possible measurement of this nonexcitation. We also present dynamical features of the inhibition of simultaneous excitation of two qubits. The proposal presented in this paper can be realized not only in traditional cavity QED, but also in noncavity topological photonics involving edge modes.

DOI
10.1103/physreva.106.063713

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We propose a method for realizing the inhibition of simultaneous excitation of two identical qubits in a single-mode cavity QED system, where two qubits are driven by a coherent field and coupled to a cavity with engineered qubit-cavity interactions. By suitably chosen placements of the qubits in the cavity and by adjusting the relative decay strengths of the qubits and cavity field, we close many unwanted excitation pathways. This leads to an inhibition of the doubly excited state. In addition, we show that these two qubits are strongly entangled over a broad regime of the system parameters. We show that a strong signature of nonexcitation of the doubly excited state is the bunching property of the cavity photons, which thus provides a possible measurement of this nonexcitation. We also present dynamical features of the inhibition of simultaneous excitation of two qubits. The proposal presented in this paper can be realized not only in traditional cavity QED, but also in noncavity topological photonics involving edge modes.
The dipole blockade phenomenon is a direct consequence of strong dipole-dipole interaction, where only single atom can be excited because the doubly excited state is shifted out of resonance. The corresponding two-body entanglement with non-zero concurrence induced by the dipole blockade effect is an important resource for quantum information processing. Here, we propose a novel physical mechanism for realizing dipole blockade without the dipole-dipole interaction, where two qubits coupled to a cavity, are driven by a coherent field. By suitably chosen placements of the qubits in the cavity and by adjusting the relative decay strengths of the qubits and cavity field, we kill many unwanted excitation pathways. This leads to dipole blockade. In addition, we show that these two qubits are strongly entangled over a broad regime of the system parameters. We show that a strong signature of this dipole blockade is the bunching property of the cavity photons which thus provides a possible measurement of the dipole blockade. We present dynamical features of the dipole blockade without dipole-dipole interaction. The proposal presented in this work can be realized not only in traditional cavity QED, but also in non-cavity topological photonics involving edge modes.
We theoretically study the quantum-interference-induced photon blockade and antiblockade phenomenon in an atom-cavity QED system, where the destructive [constructive] interference between two different transition pathways prohibits [enhances] two-photon excitation. To begin with, we study a single-atom--cavity QED system where the atom or cavity is driven by a coherent pump field. We show that the cavity-driven case will lead to the quantum-interference-induced photon blockade under a specific condition, but such an interference-induced photon blockade cannot be realized in the atom-driven case. This is an important consequence of the pathways possible for the two types of drives. Then, we investigate the two-atom case, and find that an additional transition pathway appears in the atom-driven case. We show that this additional transition pathway results in the quantum-interference-induced photon blockade only when the atomic resonant frequency is different from the cavity mode frequency otherwise antiblockade occurs. Moreover, in this case, the condition for realizing the interference-induced photon blockade is independent of the system's intrinsic parameters, which can be used to generate antibunched photons in both weak and strong coupling regimes.
We present an all-fiber emitter-cavity quantum electrodynamics (QED) scheme which makes it possible to realize the strong photon blockade phenomenon in the strong coupling regime. To begin with, we study the emitter-cavity system where the two-level emitter and cavity are driven by a laser field under different driving strengths, respectively. Then, we theoretically derive that through driving the single-mode nanofiber cavity and a single two-level emitter simultaneously with two classical fields, the photon antibunching effect and the intracavity mean photon number will be tremendously enhanced. Compared with the systems driving an emitter or the cavity, respectively, the second-order correlation function at zero time decay g(2)(0){g}^{(2)}(0)g(2)(0) in our system can reach the minimum value as 0.0003, which is much smaller than a single-emitter-cavity QED system of emitter drive or cavity drive. In particular, under the condition of quantum interference model, the strong phenomenon of photon antibunching can be flexibly reached by controlling of the intensity relationship between the dual-pumping fields in the strong coupling regime. Our proposal can be achieved in the near future and has great potential applications in high-quality single-photon sources and quantum communication processing.
We study a model for a pair of qubits that interact with a single off-resonant cavity mode and, in addition, exhibit a direct interqubit coupling. Possible realizations for such a system include coupled superconducting qubits in a line resonator as well as exciton states or electron spin states of quantum dots in a cavity. The emergent dynamical phenomena are strongly dependent on the relative energy scales of the interqubit coupling strength, the coupling strength between qubits and cavity mode, and the cavity mode detuning. We show that the cavity mode dispersion enables a measurement of the state of the coupled-qubit system in the perturbative regime. We discuss the effect of the direct interqubit interaction on a cavity-mediated two-qubit gate. Further, we show that for asymmetric coupling of the two qubits to the cavity, the direct interqubit coupling can be controlled optically via the ac Stark effect.
The output incoherent spectrum and the photon blockade for the two-photon near-resonant excitation are numerically investigated by using the modified theories which are valid for an arbitrary degree of the qubit-cavity interaction. Owing to the effects of strong coupling of the system, the relaxation coefficients of the system are very sensitive to the coupling strength between the qubit and the cavity mode, which results in the redistribution of the populations and the appearance of the population inversion between the ground state and the first excited state, and it is demonstrated by the intensity of the output incoherent spectrum. The redistribution of the populations leads to one cascaded decay channel being weakened and the other possibly being enhanced significantly. Moreover, by adjusting the detuning between the qubit and the cavity mode, the phenomenon of the intercrossing in the energy levels appears. Through choosing the proper two-photon near-resonant excitation of the external classical driving field, the strong-coupling qubit-cavity system effectively causes one cascaded decay channel to realize the two-photon blockade. It is shown that the two-photon blockade can be further enhanced by one cascaded decay channel.