Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

# Quantum capacitance mediated carbon nanotube optomechanics

## Abstract

Cavity optomechanics allows the characterization of a vibration mode, its cooling and quantum manipulation using electromagnetic fields. Regarding nanomechanical as well as electronic properties, single wall carbon nanotubes are a prototypical experimental system. At cryogenic temperatures, as high quality factor vibrational resonators, they display strong interaction between motion and single-electron tunneling. Here, we demonstrate large optomechanical coupling of a suspended carbon nanotube quantum dot and a microwave cavity, amplified by several orders of magnitude via the nonlinearity of Coulomb blockade. From an optomechanically induced transparency (OMIT) experiment, we obtain a single photon coupling of up to g0 = 2π 95 Hz. This indicates that normal mode splitting and full optomechanical control of the carbon nanotube vibration in the quantum limit is reachable in the near future. Mechanical manipulation and characterization via the microwave field can be complemented by the manifold physics of quantum-confined single electron devices.

## Introduction

The technically challenging integration of suspended single-wall cabon nanotubes into complex qantum devices has recently made significant advances1,2,3,4,5,6, as has also the integration of nanotube quantum dots into coplanar microwave cavities7,8,9. Both regarding their nanomechanical10,11 as well as their electronic properties12,13, carbon nanotubes are a prototypical experimental system. However, small vibrational deflection and length have made their optomechanical coupling to microwave fields14 so far impossible.

In this work, we demonstrate large optomechanical coupling of a suspended carbon nanotube quantum dot and a microwave cavity. The nanotube is deposited onto source and drain electrodes close to the coplanar waveguide cavity; a finger-like extension of the cavity center conductor, passing below the suspended nanotube, serves as capacitively coupling gate. We find that the optomechanical coupling of the transversal nanotube vibration and the cavity mode is amplified by several orders of magnitude via the inherent nonlinearity of Coulomb blockade. With this, full optomechanical control of the carbon nanotube vibration in the quantum limit15 is reachable in the near future. A unique experimental system becomes accessible, where the nanomechanically active part directly incorporates a quantum-confined electron system16.

## Results

### Device precharacterization

Our device, depicted in Fig. 1a, combines a half-wavelength coplanar microwave cavity with a suspended carbon nanotube quantum dot. Near the coupling capacitor, the center conductor of the niobium-based cavity is connected to a thin gate electrode, buried between source and drain contacts of the carbon nanotube, see the sketch of Fig. 1b. At the cavity center, i.e., the location of the voltage node of its fundamental mode, a bias connection allows additional application of a dc voltage Vg to the gate. The device is mounted at the base temperature stage (T 10 mK) of a dilution refrigerator; for details see Supplementary Note 4 and Supplementary Fig. 4.

At cryogenic temperatures, electronic transport through the carbon nanotube is dominated by Coulomb blockade, with the typical behavior of a small band gap nanotube12. Near the electronic band gap, sharp Coulomb oscillations of conductance can be resolved; measurements are shown in Fig. 1c and Supplementary Fig. 3. A well-known method to detect the transversal vibration resonance of a suspended nanotube quantum dot is to apply a rf signal and measure the time-averaged dc current17,18,19. On resonance, the oscillating geometric capacitance, effectively broadening the Coulomb oscillations, leads to an easily recognizable change in current. This was used to identify the transversal vibration resonances of the device; Fig. 1d plots the resonance frequencies over a wide gate voltage range. Two coupled vibration modes are observed (see also Supplementary Note 5), one of which clearly displays electrostatic softening20,21. At low gate voltages, Vg ≤ 1.2 V, where subsequent experiments are carried out, the resonance which we will utilize in the following is at ωm 2π 502.5 MHz, with typical quality factors around or exceeding Qm ~ 104 observed in time-averaged dc current detection17.

The combined suspended nanotube—cavity device forms a dispersively coupled optomechanical system14. The cavity has a resonance frequency of ωc = 2π 5.74 GHz with a decay rate of κc = 2π 11.6 MHz, dominated by internal losses. Nevertheless, due to the large mechanical resonance frequency ωm of the carbon nanotube, the coupled system is far in the resolved sideband regime ωmκc, the most promising parameter region for a large number of optomechanical protocols including ground state cooling and quantum control.

### Optomechanically induced transparency (OMIT)

To probe for optomechanical coupling, we perform an OMIT type experiment22, cf. Fig. 2a, b: a strong, red-detuned drive field (ωdωc − ωm) pumps the microwave cavity; the transmission of a weak, superimposed probe signal ωp near ωc is detected. A distinct, sharp OMIT absorption feature within the transmission resonance of the cavity becomes visible in the measurements of Fig. 2c–e. It occurs due to destructive interference of the probe field with optomechanically upconverted photons of the drive field, when the two-photon resonance condition ωp − ωd = ωm is fulfilled22, and shifts in frequency as expected when ωd is detuned from the precise red sideband condition, see Fig. 2d, e. Fitting the OMIT feature allows to extract the optomechanical coupling parameter $$g=\sqrt{{n}_{{\rm{c}}}}(\partial {\omega }_{{\rm{c}}}/\partial x){x}_{{\rm{zpf}}}$$, describing the cavity detuning per displacement of the mechanical harmonic oscillator14,22, see Supplementary Note 9 for details. Surprisingly, from Fig. 2c, one obtains a single-photon coupling on the order of $${g}_{0}=g/\sqrt{{n}_{{\rm{c}}}} \sim 2\pi \cdot 100\ {\rm{Hz}}$$.

Such a value of g0 strongly exceeds expectations from the device geometry23. For a mechanical oscillator dispersively coupled to a coplanar waveguide resonator, the coupling is given by

$${g}_{0}=\frac{{\omega }_{{\rm{c}}}}{2{C}_{{\rm{c}}}}\frac{\partial {C}_{{\rm{c}}}}{\partial x}{x}_{{\rm{zpf}}},$$
(1)

where Cc is the total capacitance of the cavity, x is the mechanical displacement, and xzpf the mechanical zero-point fluctuation length scale. Assuming a metallic wire over a metallic plane and inserting device parameters23, the coupling calculated from the change in geometric gate capacitance Cg(x) becomes ∂Cg/∂x ~10−12 F m−1. This leads to $${g}_{0}^{* }=2\pi \cdot 2.9\ {\rm{mHz}}$$, more than four orders of magnitude smaller than the measured g0. To explain this discrepancy, we need to focus on the properties of the carbon nanotube as a quantum dot, with a strongly varying quantum capacitance CCNT(x) as the displacement-dependent component of Cc dominating g0.

Figure 2f depicts OMIT measurements for similar parameters as in Fig. 2c–e, however, we now keep the drive frequency ωd constant and vary the gate voltage Vg across a Coulomb oscillation of conductance. The mechanical resonance frequency ωm shifts to lower frequencies in the vicinity of the charge degeneracy point. This electrostatic softening is a well-known characteristic of suspended carbon nanotube quantum dots18,24. More interestingly, the resulting gate-dependent coupling g(Vg) (along with g0(Vg)) is plotted in Fig. 2g. It is maximal at the edges of the finite conductance peak, whereas at its center and on the outer edges, the coupling vanishes; the enhancement of g0 is intrinsically related to Coulomb blockade.

### Mechanism of enhanced coupling

Figure 3 explores the nature of this enhanced coupling mechanism. We treat the nanotube as a single quantum dot; see Supplementary Note 3 for a discussion of the validity of this assumption. Further, we assume a full separation of time scales ωmωc Γ, where Γ describes the tunnel rates of the quantum dot. We can then introduce the quantum capacitance25,26

$${C}_{{\rm{CNT}}}=e\frac{{C}_{{\rm{g}}}}{{C}_{{\rm{dot}}}}\frac{\partial \langle N\rangle }{\partial {V}_{{\rm{g}}}},$$
(2)

where $$\left\langle N\right\rangle ({V}_{{\rm{g}}})$$ is the number of charge carriers (here holes) on the quantum dot averaged over the tunneling events, and Cdot is the total quantum dot capacitance; see Supplementary Note 12 for a derivation. In a quantum dot, each Coulomb oscillation corresponds to the addition of one electron or hole. The charge occupation $$\left\langle N\right\rangle ({V}_{{\rm{g}}})$$ resembles a step function, with the sharpness of the step given for zero bias voltage by lifetime and temperature broadening. This is plotted in Fig. 3a, for the limit of kBT Γ. The quantum capacitance CCNT(Vg) becomes a Lorentzian, as plotted in Fig. 3b.

Any motion δx modulates the geometric capacitance Cg(x). It thus shifts the position of the Coulomb oscillations in gate voltage, acting equivalent to an effective modulation of the gate voltage δVg. With this, the optomechanical coupling g, scaling with $$\left|\partial {C}_{{\rm{CNT}}}/\partial x\right|$$, becomes proportional to the derivative ∂CCNT/∂Vg and thus the second derivative of $$\left\langle N\right\rangle ({V}_{{\rm{g}}})$$, as is illustrated in Fig. 3c. The functional dependence has been fitted to the data points of Fig. 2g, here again shown in the background.

The three key situations depending on the gate voltage are sketched in Fig. 3d–f: away from the conductance peak, the charge on the nanotube is constant, and only geometric capacitances change, see Fig. 3d. On the flank of the conductance resonance, a small change δx (δCg) strongly modulates CCNT, see Fig. 3e. At the center of the conductance resonance, the charge adapts to x, but the derivative ∂CCNT/∂Vg and with it g |CCNT/x| is approximately zero.

The detailed derivation and the full expressions and values for Fig. 3 can be found in the Supplementary Information, Supplementary Note 12, and Supplementary Table 1. The parameter entering the optomechanical coupling in Eq. (1), the derivative of the quantum capacitance ∂CCNT/∂x, is found to be

$$\frac{\partial {C}_{{\rm{CNT}}}}{\partial x}=\eta \frac{\partial {C}_{{\rm{g}}}}{\partial x}=e\frac{{\partial }^{2}\left\langle N\right\rangle }{\partial {V}_{{\rm{g}}}^{2}}\frac{{V}_{{\rm{g}}}}{{C}_{{\rm{dot}}}}\frac{\partial {C}_{{\rm{g}}}}{\partial x},$$
(3)

indicating that for significant optomechanical coupling a sharp Coulomb oscillation (i.e., low temperature and low intrinsic line width Γ, leading to large values of $${\partial }^{2}\,\left\langle N\right\rangle /\partial {V}_{{\rm{g}}}^{2}$$) and a large Vg are required. From device data, we obtain an amplification factor η ~ 104. The experimental gate voltage dependence g0(Vg) is qualitatively reproduced very well. To obtain the quantitative agreement of Fig. 3c, we have introduced an additional scaling prefactor as free fit parameter, resulting in $${g}_{0}^{{{\exp }}}/{g}_{0}^{{\rm{th}}}=5.77$$. Given the uncertainties of input parameters, this is a good agreement; see Supplementary Note 15 for a discussion of error sources.

## Discussion

In literature, many approaches have been pursued to enhance optomechanical coupling26,27,28,29,30,31,32,33,34,35. Resonant coupling, with ωm = ωc, has been demonstrated successfully for a carbon nanotube quantum dot26, but does not provide access to the wide set of experimental protocols developed for the usual case of dispersive coupling and the “good cavity limit” ωmκc. The mechanism presented here is most closely related to those where a superconducting charge qubit was coherently introduced between mechanical resonator and cavity27. However, the impact of single electron tunneling and shot noise on the optomechanical system shall require careful analysis.

Given the sizeable coupling in the good cavity limit κcωm, many experimental techniques for future experiments are at hand. First steps are demonstrated in Fig. 4 in a two-tone spectroscopy experiment: a mechanical drive signal ωa is applied simultaneously to a cavity pump signal at ωd = ωc − ωa; the plotted cavity output power at ωc clearly shows the optmechanical upconversion (anti-Stokes scattering) at mechanical resonance ωa = ωm. In Fig. 4a, the dc bias across the nanotube is set to zero, and the antenna drive kept at a minimum. In Fig. 4b, both antenna drive and bias voltage have been increased. A background signal independent of device parameters emerges; at the same time, the upconverted signal displays a phase shift and destructive interference with the background for parts of the gate voltage range, meriting further measurements and analysis.

Future improvements of the optomechanical coupling via drive power and device geometry and of the detection sensitivity via the output amplifier chain shall allow detection of the thermal motion of the carbon nanotube and subsequently motion amplitude calibration.

The observation of strong optomechanical coupling and the corresponding normal mode splitting requires a coupling g exceeding both mechanichal linewidth κm and cavity line width κc. Clean carbon nanotubes have reached mechanical quality factors up to36 Qm ~ 106, allowing for two orders of magnitude improvement and a line width of κm ~ 2π · 500 Hz. Regarding microwave resonators we have reached up to Qc = 105 in our setup so far, corresponding to κc = 2π · 57 kHz. This means that strong coupling should be reachable already at moderate increase of our so far rather low cavity photon number nc.

Regarding the cooperativity C = 0.0042 of our experiment (cf. Supplementary Table 1), already an improvement of the nanotube Qm by a factor 100 brings it into the same order of magnitude as the thermal mode occupation nm = 0.4, with significant further and independent room for improvement via the cavity photon number nc.

With this, a wide range of physical phenomena becomes experimentally accessible, ranging from side-band cooling of the vibration mode and potentially its quantum control37 all the way to real-time observation of its interaction with single electron tunneling phenomena38.

## Data availability

The datasets generated during and/or analyzed during this study are available from the corresponding author on reasonable request.

## References

1. Wu, C. C., Liu, C. H. & Zhong, Z. One-step direct transfer of pristine single-walled carbon nanotubes for functional nanoelectronics. Nano Lett. 10, 1032–1036 (2010).

2. Pei, F., Laird, E. A., Steele, G. A. & Kouwenhoven, L. P. Valley-spin blockade and spin resonance in carbon nanotubes. Nat. Nanotechnol. 7, 630–634 (2012).

3. Ranjan, V. et al. Clean carbon nanotubes coupled to superconducting impedance-matching circuits. Nat. Commun. 6, 7165 (2015).

4. Gramich, J., Baumgartner, A., Muoth, M., Hierold, C. & Schönenberger, C. Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices. Phys. Stat. Solidi B 252, 2496–2502 (2015).

5. Waissman, J. et al. Realization of pristine and locally tunable one-dimensional electron systems in carbon nanotubes. Nat. Nanotechnol. 8, 569–574 (2013).

6. Blien, S., Steger, P., Albang, A., Paradiso, N. & Hüttel, A. K. Quartz tuning-fork based carbon nanotube transfer into quantum device geometries. Phys. Stat. Solidi B 255, 1800118 (2018).

7. Delbecq, M. R. et al. Photon-mediated interaction between distant quantum dot circuits. Nat. Commun. 4, 1400 (2013).

8. Viennot, J. J., Dartiailh, M. C., Cottet, A. & Kontos, T. Coherent coupling of a single spin to microwave cavity photons. Science 349, 408–411 (2015).

9. Cubaynes, T. et al. Highly coherent spin states in carbon nanotubes coupled to cavity photons. npj Quant. Inf. 5, 47 (2019).

10. Witkamp, B., Poot, M. & van der Zant, H. Bending-mode vibration of a suspended nanotube resonator. Nano Lett. 6, 2904-2908 (2006).

11. Sazonova, V. et al. A tunable carbon nanotube electromechanical oscillator. Nature 431, 284–287 (2004).

12. Laird, E. A. et al. Quantum transport in carbon nanotubes. Rev. Mod. Phys. 87, 703–764 (2015).

13. Margańska, M. et al. Shaping electron wave functions in a carbon nanotube with a parallel magnetic field. Phys. Rev. Lett. 122, 086802 (2019).

14. Aspelmeyer, M., Kippenberg, T. J. & Marquardt, F. Cavity optomechanics. Rev. Mod. Phys. 86, 1391–1452 (2014).

15. Poot, M. & van der Zant, H. S. J. Mechanical systems in the quantum regime. Phys. Rep. 511, 273–335 (2012).

16. Weig, E. M. et al. Single-electron-phonon interaction in a suspended quantum dot phonon cavity. Phys. Rev. Lett. 92, 046804 (2004).

17. Hüttel, A. K. et al. Carbon nanotubes as ultrahigh quality factor mechanical resonators. Nano Lett. 9, 2547–2552 (2009).

18. Steele, G. A. et al. Strong coupling between single-electron tunneling and nanomechanical motion. Science 325, 1103–1107 (2009).

19. Götz, K. J. G. et al. Nanomechanical characterization of the Kondo charge dynamics in a carbon nanotube. Phys. Rev. Lett. 120, 246802 (2018).

20. Wu, C. C. & Zhong, Z. Capacitive spring softening in single-walled carbon nanotube nanoelectromechanical resonators. Nano Lett. 11, 1448–1451 (2011).

21. Stiller, P. L., Kugler, S., Schmid, D. R., Strunk, C. & Hüttel, A. K. Negative frequency tuning of a carbon nanotube nano-electromechanical resonator under tension. Phys. Stat. Solidi B 250, 2518–2522 (2013).

22. Weis, S. et al. Optomechanically induced transparency. Science 330, 1520–1523 (2010).

23. Regal, C. A., Teufel, J. D. & Lehnert, K. W. Measuring nanomechanical motion with a microwave cavity interferometer. Nat. Phys. 4, 555–560 (2008).

24. Lassagne, B., Tarakanov, Y., Kinaret, J., Garcia-Sanchez, D. & Bachtold, A. Coupling mechanics to charge transport in carbon nanotube mechanical resonators. Science 325, 1107–1110 (2009).

25. Duty, T. et al. Observation of quantum capacitance in the Cooper-pair transistor. Phys. Rev. Lett. 95, 206807 (2005).

26. Ares, N. et al. Resonant optomechanics with a vibrating carbon nanotube and a radio-frequency cavity. Phys. Rev. Lett. 117, 170801 (2016).

27. Pirkkalainen, J.-M. et al. Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator. Nature 494, 211–215 (2013).

28. Rimberg, A. J., Blencowe, M. P., Armour, A. D. & Nation, P. D. A cavity-Cooper pair transistor scheme for investigating quantum optomechanics in the ultra-strong coupling regime. New J. Phys. 16, 055008 (2014).

29. Heikkilä, T. T., Massel, F., Tuorila, J., Khan, R. & Sillanpää, M. A. Enhancing optomechanical coupling via the Josephson effect. Phys. Rev. Lett. 112, 203603 (2014).

30. Abdi, M., Pernpeintner, M., Gross, R., Huebl, H. & Hartmann, M. J. Quantum state engineering with circuit electromechanical three-body interactions. Phys. Rev. Lett. 114, 173602 (2015).

31. Lecocq, F., Teufel, J. D., Aumentado, J. & Simmonds, R. W. Resolving the vacuum fluctuations of an optomechanical system using an artificial atom. Nat. Phys. 11, 635–639 (2015).

32. Pirkkalainen, J.-M. et al. Cavity optomechanics mediated by a quantum two-level system. Nat. Commun. 6, 6981 (2015).

33. Xue, Z.-Y., Yang, L.-N. & Zhou, J. Circuit electromechanics with single photon strong coupling. Appl. Phys. Lett. 107, 023102 (2015).

34. Santos, J. T., Li, J., Ilves, J., Ockeloen-Korppi, C. F. & Sillanpää, M. Optomechanical measurement of a millimeter-sized mechanical oscillator approaching the quantum ground state. New J. Phys. 19, 103014 (2017).

35. Shevchuk, O., Steele, G. A. & Blanter, Y. M. Strong and tunable couplings in flux-mediated optomechanics. Phys. Rev. B 96, 014508 (2017).

36. Moser, J., Eichler, A., Güttinger, J., Dykman, M. I. & Bachtold, A. Nanotube mechanical resonators with quality factors of up to 5 million. Nat. Nanotechnol. 9, 1007–1011 (2014).

37. O’Connell, A. D. et al. Quantum ground state and single-phonon control of a mechanical resonator. Nature 464, 697–703 (2010).

38. Barnard, A. W., Zhang, M., Wiederhecker, G. S., Lipson, M. & McEuen, P. L. Real-time vibrations of a carbon nanotube. Nature 566, 89–93 (2019).

39. Reinhardt, S. et al. Lab::Measurement—a portable and extensible framework for controlling lab equipment and conducting measurements. Comput. Phys. Commun. 234, 216–222 (2019).

40. Kouwenhoven, L. P.  et al.  Electron Transport in Quantum Dots (Kluwer, 1997).

41. Nazarov, Y. V. & Blanter, Y. M. Quantum Transport: Introduction to Nanoscience (Cambridge University Press, Cambridge, 2009).

## Acknowledgements

The authors acknowledge funding by the Deutsche Forschungsgemeinschaft via Emmy Noether grant Hu 1808/1, SFB 631, SFB 689, SFB 1277, and GRK 1570. We would like to thank G. Rastelli, F. Marquardt, E. A. Laird, Y. M. Blanter, and D. Weiss for insightful discussions, O. Vavra for experimental help, and Ch. Strunk and D. Weiss for the use of the experimental facilities. The data have been recorded using Lab::Measurement39.

## Author information

Authors

### Contributions

A.K.H. and S.B. conceived and designed the experiment. P.S. and R.G. developed and performed the nanotube growth and transfer; N.H. and S.B. developed and fabricated the coplanar waveguide device. The low temperature measurements were performed jointly by all authors. Data evaluation and writing of the paper was done jointly by S.B., N.H., and A.K.H. The project was supervised by A.K.H.

### Corresponding author

Correspondence to Andreas K. Hüttel.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Communications thanks Joel Moser and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Rights and permissions

Reprints and Permissions

Blien, S., Steger, P., Hüttner, N. et al. Quantum capacitance mediated carbon nanotube optomechanics. Nat Commun 11, 1636 (2020). https://doi.org/10.1038/s41467-020-15433-3

• Accepted:

• Published:

• DOI: https://doi.org/10.1038/s41467-020-15433-3

• ### Visualizing nonlinear resonance in nanomechanical systems via single-electron tunneling

• Xinhe Wang
• Lin Cong
• Kaili Jiang

Nano Research (2021)