Abstract
We analyze the performance of optomechanical cooling of a mechanical resonator in the presence of a degenerate optical parametric amplifier within the optomechanical cavity, which squeezes the cavity light. We demonstrate that this allows to significantly enhance the cooling efficiency via the coherent suppression of Stokes scattering. The enhanced cooling occurs also far from the resolved sideband regime, and we show that this cooling scheme can be more efficient than schemes realized by injecting a squeezed field into the optomechanical cavity.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In cavity optomechanics [1], squeezed light has been studied as a powerful tool for improved detection sensitivity [2–5], for the preparation of quantum states of mechanical resonators [6,7], to achieve larger optomechanical coupling strength [8,9], and for enhanced optomechanical cooling [10–13]. In general one can consider two different scenarios. On the one hand squeezed light produced by an optical parametric oscillator (OPO) external to the optomechanical cavity can be injected into the system, and on the other the OPO can be placed within the optomechanical cavity to directly squeeze the cavity light fluctuations. In this paper we are interested in optomechanical cooling. In particular, we focus on the scheme with internal OPO, we study under which conditions it is able to outperform standard sideband cooling techniques, and we analyze in details how it compares with the results achievable in the case of injected squeezing.
The cooling dynamics in the case of injected squeezing has been analyzed [11,12] and experimentally demonstrated [12]. It has been shown that squeezed light can be tuned to coherently suppress the scattering Stokes process heating the resonator, hence allowing to surpass the quantum backaction limit of optomechanical sideband cooling [14] (see also Refs. [15–17] for an alternative, feedback-based method for beating the quantum backaction limit). Here we show that, as far as the cavity linewidth is not very large, the optimal cooling achieved with a degenerate OPO placed within the optomechanical cavity [10] is due to a similar destructive interference which contributes to the coherent suppression of the dominant Stokes scattering processes. In fact in this regime, the cooling efficiency of the two schemes is the same, and the two models can be mapped one into the other by a unitary transformation (see also Ref. [13]), even though the optimal performance is achieved at different sets of parameters. In particular the cooling with intracavity squeezing is optimized for larger squeezing.
As observed in Ref. [11,13], the cooling dynamics with squeezed light (both injected or with internal OPO) are very efficient also in the unresolved sideband regime, that is when the cavity linewidth is much larger than the mechanical frequency. In this case, however, the two schemes are no more equivalent, the scheme with the internal OPO is more efficient, and its optimal performance is achieved when the OPO is driven close to the optical instability threshold. Finally, we also study the direct mechanical effect of the OPO pump and show that in general it tends to reduce the cooling efficiency.
The article is organized as follows. In section 2 we introduce the system model. In section 3 we analyze the cooling dynamics in the limit in which the direct interaction between the OPO pump fields and the mechanical resonator can be neglected. In particular we study the regime in which the models with injected and intracavity squeezing are equivalent and we identify the parameters for which the cooling using the internal OPO is superior. Then, in section 4 we study the mechanical effect of the OPO pump field. Finally section 5 is for the conclusions.
2. Model
We consider an optical parametric oscillator (OPO) consisting of a Fabry-Perot cavity with a second order non-linear medium (see Fig. 1) which down-converts photons of a cavity mode at frequency $\omega _c$ to photons at frequency $\omega _a=\omega _c/2$ which match a second resonant mode of the cavity according to the Hamiltonian
where $a$, $a^\dagger$, $c$ and $c^\dagger$ are the bosonic operators for the annihilation and creation of photons of the two cavity modes, and $\chi _0$ is the strength of the non-linear interaction. One cavity mirror is movable so that the resonant cavity frequencies result modulated by the mechanical vibrations. A single vibrational mode, at frequency $\omega _m$ and with annihilation and creation operators $b$ and $b^\dagger$ is assumed relevant, so that the optomechanical interaction Hamiltonian is given byWhen the pump fields are strong, the system equation in Eq. (3) can be linearized around the steady state average values of the optical and mechanical variables ($\left \langle {a} \right \rangle _{st}$, $\left \langle {b} \right \rangle _{st}$ and $\left \langle {c} \right \rangle _{st}$) neglecting non-linear terms in the fluctuations of the system operators $\delta \iota =\iota -\left \langle {\iota } \right \rangle _{st}$ for $\iota =a,b,c$. The parameters $\left \langle {a} \right \rangle _{st}$, $\left \langle {b} \right \rangle _{st}$ and $\left \langle {c} \right \rangle _{st}$ fulfill the relations
Without loss of generality we assume the phase of $\left \langle {a} \right \rangle _{st}$ to be zero so that $G_a$ and $\epsilon$ are real, moreover we explicitly define the phase of $\left \langle {c} \right \rangle _{st}=\left |{\left \langle {c} \right \rangle _{st}}\right |\ {\textrm {e}}^{2{\textrm {i}}\phi }$, and we use the substitution $G_c\to G_c\,{\textrm {e}}^{2{\textrm {i}}\phi }$ and $\chi \to \chi \,{\textrm {e}}^{2{\textrm {i}}\phi }$ (with the new $G_c$ and $\chi$ real), such that
2.1 Cooling
The cavity light can be used to cool the mechanical vibrations. Light acts as an effective thermal reservoir which compete with the natural thermal reservoir to determine the stationary mechanical energy. The effective light-controlled reservoir is characterized by an effective number of mechanical excitations $n_o$ (to which the resonator would thermalize in the absence of the natural thermal environment) and by a rate $\Gamma$ at which mechanical excitations are dissipated. Thereby the stationary number of mechanical excitations is given by
The parameters $n_o$ and $\Gamma$ are determined by how light is scattered by the mechanical resonator. Specifically the resonator can scatter inelastically incident photons to lower (Stokes processes) and higher (anti-Stokes processes) frequencies when at the same time mechanical energy is increased and reduced respectively. The rates for Stokes ($A_+$) and anti-Stokes ($A_-$) processes are determined by the fluctuations of the cavity field. In particular in the limit of weak optomechanical coupling they can be evaluated perturbatively at the lowest relevant order in terms of the power spectrum of the field operator which is coupled to the mechanical resonator (the power spectrum of the radiation pressure force operator), in our case evaluated at $\pm \omega _m$. Specifically,3. Injected vs intracavity squeezing
Let us first study the limit of large $\left |{{\kappa _c+{\textrm {i}}\,\Delta _c}}\right |$. In this limit the effect of the fluctuations of mode $c$ can be neglected and the system equations reduce to
When the nonlinearity is not too strong $\chi <\left |{{\Delta _a}}\right |$, the model described by Eq. (15) is equivalent to a model in which a standard optomechanical system is driven by a squeezed reservoir (see also Ref. [13]). This is evident when we study the dynamics for a squeezed cavity operator of the form (see the appendix)
The equations in the new representation, Eq. (20), are equivalent to the equations for a system driven by the output field of an external parametric amplifier (as the one investigated in Refs. [11,12]) in the broadband limit, and with no losses in the transmission and injection of the squeezed field form the external OPO into the optomechanical system. We therefore expect to observe the same dynamics described in Refs. [11,12] with the present system. In particular also in this case we expect that Stokes scattering processes can be suppressed so that the backaction limit of standard sideband cooling can be surpassed. It is, however, interesting to study under which parameter regime one observes these dynamics in the present case, and how this compares with the case of squeezing injection from an external OPO.
We further note that the equivalence between the two models [Eqs. (15) and (20)] is true only when $\chi <\left |{{\Delta _a}}\right |$. When $\chi >\left |{{\Delta _a}}\right |$ (note however that in any case $\chi$ have to be smaller than $\sqrt {\kappa _a^2+\Delta _a^2}$ in order to guarantee the stability of the OPO) the nonlinear self-interaction term cannot be set to zero for any transformation of the form of Eq. (16). In the resolved sideband regime optimal cooling is obtained for relatively small $\chi$, hence in a regime in which the two models are equivalent (see Sec. 3). In the unresolved sideband regime, instead, optimal cooling can be obtained for large $\chi$ and specifically when $\sqrt {\kappa _a^2+\Delta _a^2} >\chi >\left |{{\Delta _a}}\right |$ (see Sec. 4). In this case the two models are not equivalent, and in Sec. 4 we show that the model with the intracavity squeezing generated by the OPO can be more efficient, especially when the cavity linewidth is large and the system is far from the resolved sideband regime.
3.1 Stokes and anti-Stokes scattering rates
As discussed in Sec. 2.1, the cooling dynamics can be efficiently analyzed analytically in perturbation theory at the lowest relevant order in the optomechanical couplings. In this case, only the cavity mode $a$ contributes to the cooling dynamics, and the heating and cooling [$A_{a,\pm }=G_a\,S_a(\mp \omega _m)$] rates are given in terms of the power spectrum $S_a(\omega )=\int _{-\infty }^\infty \,{\textrm {d}}{t}\ {\textrm {e}}^{{\textrm {i}}\omega \,t}\left \langle {X_a(t)\ X_a(0)} \right \rangle _{st}$, with $X_a=\delta a+\delta a^\dagger$, which can be computed by solving Eqs. (15) for $G_a=0$. The explicit result is
where3.2 Suppression of Stokes scattering
We observe that, in the case of internal OPO, Stokes heating processes can be fully suppressed (i.e. $A_{a,+}=0$) when $\phi =\phi {^{({opt})}}$ and $\chi =\chi {^{({opt})}}$ with
In the following sections, we compare the cooling efficiency achieved when the OPO is inside the optomechanical cavity, with that obtained when a squeezed field is injected into the optomechanical cavity from an external OPO. In particular we analyze the performance of the two protocols also at fixed source of squeezing. Namely, in both cases we define a parameter ${{\cal R}}$ which determines the amount of squeezing produced by the corresponding OPO. In the case of internal squeezing we consider the definition in Eq. (29). Instead, in the case of external squeezing, we consider an external OPO with strength of the non-linearity $\chi _s$ and cavity decay rate $\kappa _s$ that is operated at resonance (as in the case of Ref. [11]) and we introduce
and correspondingly $n_s={4\,{{\cal R}}_s^2}/{\left ( {R_s^2-1} \right )^2}$. By means of these relations we can express the system equations in terms of ${{\cal R}}_s$ and we can compare the results of the two models for ${{\cal R}}={{\cal R}}_s$. In particular, the conditions for the suppression of Stokes scattering, in the case of external squeezing, can be expressed as ${{\cal R}}_s={{\cal R}}_s^{({opt})}$ with3.3 Cooling efficiency
We observe that, when Stokes processes are suppressed, the anti-Stokes scattering rate reduces to
Let us, now analyze the cooling performance for the two models at equal value of the optomechanical coupling strength $G_a=G_a^{({s})}$. Specifically we are interested in the regime of Stokes scattering suppression defined by Eqs. (27) and (28). Under these conditions, Eq. (21) and the relation $G_a=G_a^{({s})}$ imply
3.4 Ground state cooling beyond the resolved sideband regime
Let us now look more closely to the regime of large $\kappa _a$ (third row of Fig. 3). Typically in this regime standard sideband cooling is not efficient. However we observe that the resonator is cooled to the ground state with high fidelity even if the corresponding sideband cooling result would predict a number of excitations larger than $10$. This is observed for both models; however in the case of internal squeezing the optimal parameter choice occurs close to the system instability as observed in Fig. 3 (j), where stability occurs only in a relatively narrow range of the squeezing phase $\phi$. We also observe that in this case ground state cooling is achieved for an optomechanical coupling larger than the mechanical frequency. This result hence occurs beyond the regime of validity of the analytical result discussed in 1. Hence, the study that we have discussed in the previous section, which focuses onto the regime of Stokes scattering suppression, is too limited in this case. Furthermore while in Fig. 3 we have constrained the system parameters within the regime in which the two models are equivalent $\chi <\left |{{\Delta _a}}\right |$ (i.e. ${{\cal R}}<\left |{{\Delta _a}}\right |/\sqrt {\kappa _a^2+\Delta _a^2}$), here we actually find that for large $\kappa _a$ and in the case of internal OPO, the optimal cooling is obtained for values of $\chi$ larger than $\left |{\Delta }\right |$ (i.e. for ${{\cal R}}>\left |{\Delta }\right |/\sqrt {\kappa _a^2+\Delta _a^2}$). This is reported in Figs. 5 and 6, where we explore this regime in more detail. In plots (a) and (b) of both Fig. 5 and 6 we report the steady state phonon number as a function of the squeezing phase $\phi$ ($\phi _s$) and of the value of squeezing ${{\cal R}}$ (${{\cal R}}_s$) for both models and large values of $\kappa _a$, namely $\kappa _a=10\omega _m$ in Fig. 5 and $\kappa _a=100\omega _m$ in Fig. 6. Cuts along specific values of the phases $\phi$ and $\phi _s$ corresponding to the minimum $N_{st}$ are reported in the plots (c) of the two figures. These plots are evaluated minimizing the value of $N_{st}$, for each point in the $\phi -{{\cal R}}$ space, as a function of both $\Delta _a$ and $G_a$. The specific values of $\Delta _a$ and $G_a$ corresponding to the plots (c) are reported in plots (d) and (e) of Figs. 5 and 6. In the case of internal OPO, the minimum of $N_{st}$ is observed at large squeezing ${{\cal R}}\sim 1$. Specifically, the larger is the cavity linewidth, the larger is the value of ${{\cal R}}$. In the case of external squeezing, instead, the minimum of $N_{st}$ is achieved at lower values ${{\cal R}}$ and the system results more stable [the dip in plot (c) has a larger width]. We note that, while in both cases the resonator is cooled to the ground state with high fidelity, lower mechanical energy is observed in the case of internal squeezing. In particular, while in the case of injected squeezing the cooling efficiency is reduced as $\kappa _a$ increases, the value of the minimum of $N_{st}$ with internal squeezing is essentially unaffected by the specific value of the cavity linewidth.
4. Mechanical effects of the OPO pump field
So far, we have studied how the cooling of the mechanical resonator is affected by the squeezing of the mode $a$, which is supported by the OPO pump field. However, in the case of internal squeezing also the OPO pump field (the mode $c$) interacts directly, via the radiation pressure interaction, with the mechanical resonator. Therefore, in this case, it is important to analyze its direct effect on the cooling dynamics. In general we expect that the results of the previous section are valid as far as $\left |{{\kappa _c+{\textrm {i}}\Delta _c}}\right |$ is sufficiently large, such that the mechanical interaction is not resonant and negligible. Here we analyze what are the dominant mechanical effects when this condition is not exactly fulfilled. We can extend the approximated analytical result presented in the previous section by including also the mechanical effects of the pump at the lowest relevant order in the optomechanical coupling $G_c$ and in the light-modes coupling $\epsilon$. Specifically, we can decompose the scattering rates, defined in Eq. (13), as the sum of three contributions
where the first is due to the mode $a$ and is equal to the one reported in Eq. (24), the second is due to the OPO pump mode $c$, and the last is due to the cross correlations. They are defined by the relations $S_c(\omega )=\int _{-\infty }^\infty \,{\textrm {d}}{t}\ {\textrm {e}}^{{\textrm {i}}\omega \,t}\left \langle {X_c(t)\ X_c(0)} \right \rangle _{st}$, with $X_c=\delta c+\delta c^\dagger$, and $S_{a,c}(\omega )=\int _{-\infty }^\infty \,{\textrm {d}}{t}\ {\textrm {e}}^{{\textrm {i}}\omega \,t} \left \langle {X_c(t)\ X_a(0)+X_a(t)\ X_c(0)} \right \rangle _{st}$, where the averages are evaluated for a cavity with no mechanical resonator using Eq. (10) and expanding the result at first order in $\epsilon$. The explicit expressions for the spectra areWe have tested this prediction by comparing the results obtained form the model in Eq. (15) with those obtained by solving numerically Eq. (10). Specifically, we consider parameters consistent with realistic lithium-niobate microdisk cavities which can exhibit large non-linearity and constitute the OPO [19–22]. The resonator can be instead either a vibrational mode of the microdisk itself [18], or of a nearby evanescently coupled mechanical resonator [23]. In these systems the single photon non-linearities ($\chi _0$) are of the order of kHz while the single photon optomechanical interaction strength ($g_a$) for a MHz mechanical mode (for example the breathing mode), is below one Hz [2].
In our comparison, reported in Fig. 7, we fix the optimal values of $G_a$ and $\chi$ for the system parameters used in the second row of Fig. 3, and analyze the cooling performance as a function of $g_c$ and $\chi _0$, so that $G_2=g_b\ \chi /\chi _0$ and $\epsilon =\chi _0\ G_a/g_a$. In general the mechanical effects of mode c are negligible for small $g_c$ as described in Fig. 7 (b), where the result for the full model in Eq. (10) (blue lines) reproduce that of model in Eq. (15) (red lines) for sufficiently small $g_c$. Moreover, on the one hand, if $\chi _0$ is too small then a strong pump is required to achieve sufficiently large $\chi$ so that also $G_c$ is large (while $\epsilon$ is small), and the mechanical effect of mode c is significant so that the model discussed in the previous section may fail. On the other hand, if $\chi _0$ is large then $G_c$ is small but $\epsilon$ is large, and this strong optical coupling can spoil the dynamics discussed in the previous section. Hence we expect that the model described by Eq. (15) is valid for intermediate values of $\chi _0$. This behaviour is described by Fig. 7 (a). In the limit of large $\chi _0$, which imply small $G_c$, and hence negligible direct mechanical effect, the effect of mode coupling $\epsilon$, that can be instead significant, can be taken into account at lowest order in $1/\left |{{\kappa _c+{\textrm {i}}\Delta _c}}\right |$ by considering renormalized cavity detuning $\Delta _a$ and linewidth $\kappa _a$. In details, in Fig. 7 we fix the values of the parameters of the reduced model (15), so that the blue curves, corresponding to the full model (10), have been evaluated by including the modification due to the mode coupling according to the relations $\Delta _a\to \Delta _a+{\Delta _c\ \epsilon ^2}/\left ( {\kappa _c^2+\Delta _c^2} \right )$ and $\kappa _a\to \kappa _a-{\kappa _c\ \epsilon ^2}\left ( {\kappa _c^2+\Delta _c^2} \right )$.
5. Conclusions
In conclusion we have studied how optomechanical sideband cooling is modified when the fluctuations of the cooling light are squeezed by a degenerate optical parametric oscillator (OPO). We have considered the situation in which the non-linear medium which provides the parametric interaction is placed inside the optomechanical system and we have shown that in specific limits this system is equivalent to a system in which the light generated by the OPO is injected into the optomechanical system. It has been demonstrated [11,12] for the latter that Stokes scattering transitions can be suppressed and the cooling efficiency enhanced. Correspondingly we show that an equal suppression of the Stokes processes can be observed also in the case of internal OPO, so that also in this case the backaction limit can be surpassed. However, although the two systems are ideally equivalent, they achieve equal results under different physical conditions, and each of them exhibit specific advantages and disadvantages. On the one hand the internal OPO scheme has the advantage of bypassing the detrimental effects of the inevitable losses in the transmission and injection of the squeezed light from the external OPO into the optomechanical cavity. On the other hand it is negatively affected by the OPO pump field which can also interact with the mechanical resonator and reduce the cooling efficiency. Furthermore, the system with the internal OPO requires higher values of squeezing and is optimal close to the instability of the system.
A further important advantage of adding squeezing within an optomechanical system is that it allows to achieve ground state cooling in the unresolved sideband regime, characterized by large cavity decay rate $\kappa _a$, which would be otherwise impossible. This result is observed for both intracavity and injected squeezing but, in this regime, optimal cooling is observed in a parameter regime in which the two models are not equivalent; in this case the scheme with internal squeezing results clearly superior, although it requires a precise fine-tuning of the squeezing strength close to the optical instability threshold.
In our analysis the strength of the squeezing is determined by the parameter ${{\cal R}}$ [see Eq. (29)] which sets the value of the variance of the squeezed quadrature, $\left \langle {X_a^2} \right \rangle =1/(1+{{\cal R}})$, with respect to the vacuum noise variance (which in our description is equal to 1). As usual, the maximum value of ${{\cal R}}=1$ corresponds to the minimum squeezing of the cavity mode equal to 1/2, i.e. 3dB below the vacuum noise level. However, in experiments one does not have direct access to the cavity mode, but to the field leaking out of the cavity and hence to the corresponding power spectrum of its quadratures measured by homodyne detection. In order to compare our numbers with experimental results it can be useful to identify the value of the minimum of the homodyne power spectrum, $S_\textrm {out}(\omega )$ (at zero frequency, $\omega =0$), of the squeezed quadrature at the output of an equivalent single sided OPO cavity with the same parameters used in our results (note that also in the case of the scheme with the OPO inside the optomechanical cavity, here, we determine the value of the output squeezing without considering the mechanical resonator). Specifically we find that the optimal cooling reported in the second row of Fig. 3 and in Fig. 4 correspond to $S^{({ext})}_\textrm {out}(\omega =0)\sim 4$dB below the vacuum noise level, for the scheme with external OPO, and to $S^{({int})}_\textrm {out}(\omega =0)\sim 8$dB for the scheme with internal OPO, which are values easily achievable in squeezing experiments [24]. More demanding is the cooling optimization in the far unresolved sideband regime. Specifically, in Fig. 5 (c), the minimum number of mechanical excitations is achieved for $S^{({ext})}_\textrm {out}(\omega =0)\sim 20$dB and $S^{({int})}_\textrm {out}(\omega =0)\sim 28$dB, and in Fig. 6 (c) for $S^{({ext})}_\textrm {out}(\omega =0)\sim 29$dB and $S^{({int})}_\textrm {out}(\omega =0)\sim 42$dB.
We finally point out that the present analysis applies also to other, apparently different, physical systems. One can, for example, consider an hybrid configuration in which a Fabry-Perot optical cavity interacts with both a mechanical resonator and an array of two or more transversely driven two-level atoms [25,26] which can realize the parametric two-photon driving [27,28]. The intracavity atoms provide the parametric nonlinearity, with the advantage of avoiding the detrimental effects of the OPO pump. Another possibility could be to include a Josephson parametric amplifier within the microwave cavity of an optomechanical system similar to that discussed in Refs. [5].
A. Mapping between the models with internal and external OPO
In this appendix we show that Eqs. (15) and (20) are related by the transformation (16). These equations can be rewritten in matrix form as equations for the vectors of operators ${\textbf {a}}=\left ( {\delta a,\delta a^\dagger } \right )^T$ and ${\textbf {a}}^{({s})}=\left ( {\delta a_s,\delta a_s^\dagger } \right )^T$, as
Funding
European Union Horizon 2020 Programme for Research and Innovation Project QuaSeRT funded by the QuantERA ERA-NET Cofund in Quantum Technologies; Horizon 2020 Framework Programme (732894 (FET Proactive HOT)).
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