Abstract
Engineering strong single-photon optomechanical couplings is crucial for optomechanical systems. Here, we propose a hybrid quantum system consisting of a nanobeam (phonons) coupled to a spin ensemble and a cavity (photons) to overcome it. Utilizing the critical property of the lower-branch polariton (LBP) formed by the ensemble-phonon interaction, the LBP-cavity coupling can be greatly enhanced by three orders magnitude of the original one, while the upper-branch polariton (UBP)-cavity coupling is fully suppressed. Our proposal breaks through the condition of the coupling strength less than the critical value in previous schemes using two harmonic oscillators. Also, strong Kerr effect can be induced in our proposal. This shows our proposed approach can be used to study quantum nonlinear and nonclassical effects in weakly coupled optomechanical systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the potential applications in studying optomechanically induced transparency [1–3], optical bistability [3,4], higher-orders sidebands [5,6], precision measurements [7–9], gravitational wave detectors [10,11], phonon lasers [12,13], cavity optomechanics [14] has received great interest in the past decades. However, realizing strong single-photon optomechanical coupling is still challenging for most of optomechanical setups [15–18] to date.
One experimentally used method is to apply a strong driving field on an optomechanical cavity [19], resulting in a strong linearized optomechanical coupling larger than the decay rate of the cavity. However, this method sacrifices its inherent nonlinearity. For this, various approaches are proposed for improving the single-photon optomechanical coupling, including mechanical oscillators arrays [20], squeezing effects [21,22], Kerr nonlinearity via the Josephson effect [23,24], as well as a single atom [25] and superconducting circuits [26–30]. These remarkable schemes indicate that enhancing optomechanical coupling is possible using hybrid systems. In addition, quantum criticality is utilized to efficiently improve system coupling [31,32]. In their proposals [31,32], the critical point (CP) can only be approached by unidirectionally increasing the photon-phonon coupling strength. However, it is out of work when the photon-phonon coupling exceeds the critical value.
Motivated by this, we replace one of the optomechanical cavities in [31] by a nitrogen vacancy ensemble (NVE) embedded in a diamond nanobeam. As well known, the NVE can be mapped to the bosonic mode in the large number limit [33–35], so that the NVE can be regarded as a single-mode cavity. We consider here the case that the NVE-phonon coupling exceeds the critical value, so that the low-excitation for the ensemble is broken and higher order terms are included. This gives rise to a nonlinear NVE-phonon coupling in the thermodynamics limit, which can hybridize the NVE and the phonon as two polaritons. Utilizing the critical property of the lower-branch polariton (LBP) formed by the coupled NVE-phonon, the LBP-cavity coupling can be greatly enhanced by three orders magnitude of the original one, while the upper-branch polariton (UBP)-cavity coupling is fully suppressed. In such an effective polariton-cavity optomechanical system, strong Kerr effect can be introduced. This shows our proposed approach can be used to study quantum nonlinear and nonclassical effects in weakly coupled optomechanical systems [31].
The paper is organized as follows. In Sec. II, the model and the Hamiltonian are introduced. Then we study the linearized mangnon-phonon interaction Hamiltonian in Sec. III. In Sec. IV, enhancing optomechanical coupling is investigated. Finally, a conclusion is given in Sec. V.
2. Model and Hamiltonian
We consider an ensemble consisting of $N$ NV center spins embedded in a crystal diamond nanobeam with frequency $\omega _m$, as shown in Fig. 1(a). The electronic ground state of the NV center spin is $S=1$ triplet states labeled by $|m_s=0,\pm 1\rangle$, as shown in Fig. 1(b). In the presence of external electric field $\textbf {E}$ and magnetic field $\textbf {B}$, the Hamiltonian for a single NV spin is [36,37] (setting $\hbar =1$)
We now consider the case of $N\rightarrow \infty$, the dynamics of the collective spin can be mapped to a bosonic mode $c$ via the Holstein-Primakoff transformation [40], i.e.,
Then, Eq. (2) reduces to3. Effective linearized Hamiltonian for NVE coupled to the nanobeam
When the effective coupling strength between the NVE and the nanobeam in Eq. (5) is strong, generally exceeding the critical coupling strength, i.e., $G\geq G_{c}=\sqrt {\omega _m\omega _q}/2$, each operator of the system can acquire a macroscopic displacement. This is due to the fact that the strong coupling results in more spins excited in the thermodynamics limit $N\rightarrow \infty$, especially for the large number of the NV spins ($\langle c^{\dagger}c\rangle \gg 1$). Therefore, we write each operator in Eq. (5) into the expectation value plus the fluctuation, i.e.,
and we have4. Quantum criticality
For the condition in Eq. (12), the equality ($\mu =1$) corresponds to $\alpha _b=\alpha _c=0$, resulting in $\Omega _q=\omega _q$, $\mathcal {G}=G$ and $\eta =0$, so Eq. (13) becomes
For the case of $\mu <1$ in Eq. (12), the system Hamiltonian is governed by Eq. (13), which can be fully diagonalized via a Bogoliubov transformation [41], as
where5. Enhancement of optomechanical coupling at the single-photon level with quantum criticality
In this part, we focus on enhancing the single-photon optomechanical coupling strength with the critical property of the system governed by Eq. (13). To realize it, we couple the nanobeam to a(n) microwave (optical) cavity with a movable mirror, forming a cavity optomechanical system [see Fig. 1(c)]. Thus, the total Hamiltonian of the hybrid system can be written as
where $H_\textrm {OM}=\omega _a a^{\dagger}a-g_0 a^{\dagger}a(b^{\dagger}+b)$ is the Hamiltonian of the cavity optomechanical system. $\omega _a$ is the frequency of the cavity with annihilation (creation) operator $a$ $(a^{\dagger})$, $g_0$ is the single-photon optomechanical coupling strength, and $H_\textrm {NMP}$ is given by Eq. (5). In the polariton presentation, the phonon operators $b$ and $b^{\dagger}$ can be expressed in terms of the polariton operators $d_\pm$ and $d_\pm ^{\dagger}$. Thus, Eq. (18) can be written asIn Fig. 3, we also plot the normalized single-photon optomechanical coupling strength as functions of the normalized $\omega _m/\omega _-$ and $(G_c-G)/\omega _m$ for different $\omega _b/\omega _m$. From Fig. 3(a), we see that the effective single-photon optomechanical coupling strength $g_-$ increases monotonously with increasing $\omega _m/\omega _-$, that is, the small $\omega _-$ leads to large $g_-$ for fixed $\omega _m$. However, $g_-$ decreases monotonously with increasing parameter $(G_c-G)/\omega _m$ describing the difference between $G_c$ and $G$ in Fig. 3(b). When $G$ approaches $G_c$, $g_-$ can be greately enhanced. This enhancement can be up to three orders of magnitude of $g_0$, and we can obtain the larger $g_-$ by increasing the ratio of $\omega _b$ and $\omega _m$ in principle [see both Figs. 3(a) and 3(b)].
The enhanced single-photon optomechanical coupling in Eq. (21) can be used to produce strong Kerr nonlinearity, via the unitary transformation $U=\exp [-(g_-/\omega _-) a^{\dagger}a({d}_-^{\dagger}-{d}_-)]$, as
where $\chi =g_-^2/\omega _-=g_0\sqrt {\omega _m/\omega _-^3}\sin \theta$. Obviously, the Kerr coefficient can be very strong due to $\omega _-\rightarrow 0$ at the CP. This Kerr has been investigated to generating Schrödinger cat and studying photon blockade effect [31].6. Conclusion
In summary, we have proposed a proposal to realize a strong single-photon optomechanical coupling in a hybrid spin-optomechanical system. Different from previous low-excitation schemes, here we consider the spins are highly excited when the ensemble-phonon coupling exceeds a critical value. This coupling hybridizes the NVE and the phonon forming two polaritons. The LBP can have critical property when the NVE-phonon coupling approaches the critical value. Owning to this critical property, the coupling between the optomechanical cavity and the UBP can be fully suppressed, while the coupling between the cavity and the LBP is greatly improved. With accessible parameters, the enhanced coupling can be about three orders of magnitude of the original value, so it can be in the strong coupling regime. This strong optomechanical coupling can be used to achieving strong Kerr nonlinearity, which can be employed to produce multi-component cat state [31]. Our proposal provides a potential way to study quantum nonlinear optics in weakly coupled optomechanical systems.
Funding
Natural Science Foundation of Hunan Province (2020JJ5466); National Natural Science Foundation of China (11804074, 11904201, 12104214).
Acknowledgments
This paper is supported by the National Natural Science Foundation of China (Grants No. 11804074, No. 11904201 and No. 12104214), and the Natural Science Foundation of Hunan Province of China (Grant No. 2020JJ5466).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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