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A strained couple
Coupling between optoelectronic states in a quantum dot and vibrations in a nanowire could lead to new techniques for laser cooling and control of mechanical motion.
Philipp Treutlein
A
century after the discovery of quantum mechanics, we have become used to the idea that microscopic systems such as atoms or semiconductor quantum dots obey the laws of the quantum world. Whether quantum-mechanical phenomena can be observed on a truly macroscopic scale is still subject of theoretical discussions and inspiration for challenging experiments. In the past few years, researchers have managed to observe quantum effects in the vibrations of engineered mechanical structures, some of which are barely visible to the naked eye1. Besides being of fundamental interest, this has implications for precision measurements with gravitational wave detectors or atomic force microscopes, where mechanical elements play a key role. A current challenge is to couple a well-controlled microscopic quantum system to an engineered mechanical structure, as this would offer new ways to prepare, manipulate and read out quantum states of mechanical vibration2. Now writing in Nature Nanotechnology, Jean-Philippe Poizat, Olivier Arcizet and colleagues from institutions in Grenoble, France, report an important step towards such mechanical hybrid quantum systems3. In their experiment, they couple the vibrations of a semiconductor nanowire to the optical transition of an embedded quantum dot (Fig. 1). The nanowire, about 18 μm long but only around a micrometre thin, stands upright on a semiconductor chip and can perform transverse flexural vibrations. Embedded in the nanowire is a semiconductor quantum dot, a nanoscale structure that is often called an artificial atom, because it has quantized electron energy levels. The transition between two energy levels can be driven by a laser beam, and the observed fluorescence yields information about the level spacing. When the nanowire vibrates, the resulting deformation of its shape causes a timevarying strain field in the semiconductor crystal lattice. This results in a periodic modulation of the transition energy of the quantum dot. By monitoring the frequency of the quantum dot fluorescence, Poizat and colleagues could detect mechanical vibrations of the nanowire with an amplitude
Laser ωm
Quantum dot fluorescence Nanowire
|e ωm |g
Quantum dot Semiconductor chip
Figure 1 | Coupling a semiconductor quantum dot to a nanomechanical oscillator by material strain. The quantum dot (a two-level system with an optical transition between levels |e〉 and |g〉) is embedded in a nanowire (the mechanical oscillator). Vibrations of the nanowire at frequency ωm create a time-varying strain field that modulates the energy splitting of the quantum dot. A laser beam is coupled to the nanowire to probe the quantum dot transition (green arrow), and the quantum dot fluorescence (red arrow) allows the nanowire vibrations to be read out.
of a few hundred picometres. Although the mechanical vibrations are still classical at this level, the observed strain-mediated coupling is an important ingredient for future experiments where the quantum dot could be used for laser cooling of the nanowire. The strain-mediated coupling was proposed about 10 years ago4, but it had not been experimentally implemented up to now. Several other hybrid mechanical systems are being investigated at present 2,5 (Fig. 2). The spin of a nitrogen–vacancy centre in diamond was recently used to sense the vibrations of a mechanical cantilever with a magnetic tip6,7. In another experiment, laser-cooled atoms were used to sympathetically cool the vibrations of a membrane oscillator 8. With a superconducting qubit, it was even possible to coherently control a piezoelectric
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
oscillator on the single-phonon level9. These systems have the following in common: that the coupling between the microscopic quantum system and the oscillator needs to be engineered, by fabricating suitable electrical connections or nanomagnets on the oscillator, or by using tailored laser fields. In contrast, Poizat and co-workers use a two-level system embedded in the oscillator and a coupling mechanism that is naturally present. This results in a monolithic and compact system with appealing properties for applications. The nanowire geometry is ingenious. Not only does it allow efficient coupling between the quantum dot and the mechanical vibrations, it also serves as a waveguide to enhance the light interaction with the quantum dot. In fact, the whole quantum-dot-in-nanowire system with off-axis quantum dots, ideal for mechanical coupling, can be fully self-assembled without the need for any top-down lithography 10. An important parameter in hybrid quantum systems is the coupling strength, corresponding to the rate at which the quantum dot and the mechanical oscillator can exchange a single quantum of energy. Poizat and co-authors carefully measure this parameter and find a value comparable to the mechanical oscillation frequency, about 0.5 MHz. This implies that a single quantum of energy can be exchanged on the timescale of a single vibrational period, a regime that has been termed ‘ultrastrong coupling’. However, the coupling strength also has to be compared with the rate of decoherence processes, which tend to wash out quantum-mechanical phenomena. In the current experiment, the decoherence rate of the quantum dot is much higher than the coupling strength. Still, there are a number of interesting applications that the authors can target with their system with realistic improvements. In one application, the quantum dot could be used to manipulate the nanowire vibrations. Because of the coupling, a change in the quantum dot state corresponds to a change in the force acting on the nanowire. Periodically driving the quantum dot with a laser could be used to excite mechanical motion of the nanowire11. By driving the dot 99
news & views a
b Cantilever with magnetic tip
Nitrogen–vacancy centre
Coplanar waveguide
Diamond
60 μm
Philipp Treutlein is in the Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland. e-mail: [email protected]
Laser excitation
c
Membrane
References
Atoms Laser beam ωm
Optical lattice
ωat
Figure 2 | Examples of hybrid systems in which a microscopic quantum system is coupled to a mechanical oscillator. a, Piezoelectric oscillator (indicated by dashed rectangle) coupled to a superconducting qubit9. b, Single electron spin of a nitrogen–vacancy centre in diamond coupled to a mechanical cantilever with a magnetic tip7. c, Micromechanical membrane (vibrational frequency ωm) coupled to ultracold atoms in an optical lattice (vibrational frequency ωat)8. Panel a reproduced from ref. 9, © 2010 NPG.
100
with a red-detuned laser, the mechanical vibrations could be laser-cooled4. On the other hand, as the force on the nanowire is different for a quantum dot in the excited state and the ground state, reading out the nanowire deflection with a laser could reveal the state of the quantum dot, without directly collecting the dot fluorescence. Last but not least, strain-mediated coupling as demonstrated by Poizat and colleagues could also be exploited in other systems, such as nitrogen–vacancy centres in diamond resonators. Experiments along these lines are ongoing at present in several laboratories. ❐
1. Aspelmeyer, M., Meystre, P. & Schwab, K. Physics Today 65, 29–35 (July, 2012). 2. Treutlein, P., Genes, C., Hammerer, K., Poggio, M. & Rabl, P. Preprint at http://arXiv.org/abs/ 1210.4151 (2012). 3. Yeo, I. et al. Nature Nanotech. 9, 106–110 (2014). 4. Wilson-Rae, I., Zoller, P. & Imamoglu, A. Phys. Rev. Lett. 92, 075507 (2004). 5. Hunger, D. et al. C. R. Physique 12, 871–887 (2011). 6. Arcizet, O. et al. Nature Phys. 7, 879–883 (2011). 7. Kolkowitz, S. et al. Science 335, 1603–1606 (2012). 8. Camerer, S. et al. Phys. Rev. Lett. 107, 223001 (2011). 9. O’Connell, A. D. et al. Nature 464, 697–703 (2010). 10. Heiss, M. et al. Nature Mater. 12, 439–444 (2013). 11. Auffèves, A. & Richard, M. Preprint at http://arXiv.org/abs/1305.4252 (2013).
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
A strained couple
Coupling between optoelectronic states in a quantum dot and vibrations in a nanowire could lead to new techniques for laser cooling and control of mechanical motion.
Philipp Treutlein
A
century after the discovery of quantum mechanics, we have become used to the idea that microscopic systems such as atoms or semiconductor quantum dots obey the laws of the quantum world. Whether quantum-mechanical phenomena can be observed on a truly macroscopic scale is still subject of theoretical discussions and inspiration for challenging experiments. In the past few years, researchers have managed to observe quantum effects in the vibrations of engineered mechanical structures, some of which are barely visible to the naked eye1. Besides being of fundamental interest, this has implications for precision measurements with gravitational wave detectors or atomic force microscopes, where mechanical elements play a key role. A current challenge is to couple a well-controlled microscopic quantum system to an engineered mechanical structure, as this would offer new ways to prepare, manipulate and read out quantum states of mechanical vibration2. Now writing in Nature Nanotechnology, Jean-Philippe Poizat, Olivier Arcizet and colleagues from institutions in Grenoble, France, report an important step towards such mechanical hybrid quantum systems3. In their experiment, they couple the vibrations of a semiconductor nanowire to the optical transition of an embedded quantum dot (Fig. 1). The nanowire, about 18 μm long but only around a micrometre thin, stands upright on a semiconductor chip and can perform transverse flexural vibrations. Embedded in the nanowire is a semiconductor quantum dot, a nanoscale structure that is often called an artificial atom, because it has quantized electron energy levels. The transition between two energy levels can be driven by a laser beam, and the observed fluorescence yields information about the level spacing. When the nanowire vibrates, the resulting deformation of its shape causes a timevarying strain field in the semiconductor crystal lattice. This results in a periodic modulation of the transition energy of the quantum dot. By monitoring the frequency of the quantum dot fluorescence, Poizat and colleagues could detect mechanical vibrations of the nanowire with an amplitude
Laser ωm
Quantum dot fluorescence Nanowire
|e ωm |g
Quantum dot Semiconductor chip
Figure 1 | Coupling a semiconductor quantum dot to a nanomechanical oscillator by material strain. The quantum dot (a two-level system with an optical transition between levels |e〉 and |g〉) is embedded in a nanowire (the mechanical oscillator). Vibrations of the nanowire at frequency ωm create a time-varying strain field that modulates the energy splitting of the quantum dot. A laser beam is coupled to the nanowire to probe the quantum dot transition (green arrow), and the quantum dot fluorescence (red arrow) allows the nanowire vibrations to be read out.
of a few hundred picometres. Although the mechanical vibrations are still classical at this level, the observed strain-mediated coupling is an important ingredient for future experiments where the quantum dot could be used for laser cooling of the nanowire. The strain-mediated coupling was proposed about 10 years ago4, but it had not been experimentally implemented up to now. Several other hybrid mechanical systems are being investigated at present 2,5 (Fig. 2). The spin of a nitrogen–vacancy centre in diamond was recently used to sense the vibrations of a mechanical cantilever with a magnetic tip6,7. In another experiment, laser-cooled atoms were used to sympathetically cool the vibrations of a membrane oscillator 8. With a superconducting qubit, it was even possible to coherently control a piezoelectric
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
oscillator on the single-phonon level9. These systems have the following in common: that the coupling between the microscopic quantum system and the oscillator needs to be engineered, by fabricating suitable electrical connections or nanomagnets on the oscillator, or by using tailored laser fields. In contrast, Poizat and co-workers use a two-level system embedded in the oscillator and a coupling mechanism that is naturally present. This results in a monolithic and compact system with appealing properties for applications. The nanowire geometry is ingenious. Not only does it allow efficient coupling between the quantum dot and the mechanical vibrations, it also serves as a waveguide to enhance the light interaction with the quantum dot. In fact, the whole quantum-dot-in-nanowire system with off-axis quantum dots, ideal for mechanical coupling, can be fully self-assembled without the need for any top-down lithography 10. An important parameter in hybrid quantum systems is the coupling strength, corresponding to the rate at which the quantum dot and the mechanical oscillator can exchange a single quantum of energy. Poizat and co-authors carefully measure this parameter and find a value comparable to the mechanical oscillation frequency, about 0.5 MHz. This implies that a single quantum of energy can be exchanged on the timescale of a single vibrational period, a regime that has been termed ‘ultrastrong coupling’. However, the coupling strength also has to be compared with the rate of decoherence processes, which tend to wash out quantum-mechanical phenomena. In the current experiment, the decoherence rate of the quantum dot is much higher than the coupling strength. Still, there are a number of interesting applications that the authors can target with their system with realistic improvements. In one application, the quantum dot could be used to manipulate the nanowire vibrations. Because of the coupling, a change in the quantum dot state corresponds to a change in the force acting on the nanowire. Periodically driving the quantum dot with a laser could be used to excite mechanical motion of the nanowire11. By driving the dot 99
news & views a
b Cantilever with magnetic tip
Nitrogen–vacancy centre
Coplanar waveguide
Diamond
60 μm
Philipp Treutlein is in the Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland. e-mail: [email protected]
Laser excitation
c
Membrane
References
Atoms Laser beam ωm
Optical lattice
ωat
Figure 2 | Examples of hybrid systems in which a microscopic quantum system is coupled to a mechanical oscillator. a, Piezoelectric oscillator (indicated by dashed rectangle) coupled to a superconducting qubit9. b, Single electron spin of a nitrogen–vacancy centre in diamond coupled to a mechanical cantilever with a magnetic tip7. c, Micromechanical membrane (vibrational frequency ωm) coupled to ultracold atoms in an optical lattice (vibrational frequency ωat)8. Panel a reproduced from ref. 9, © 2010 NPG.
100
with a red-detuned laser, the mechanical vibrations could be laser-cooled4. On the other hand, as the force on the nanowire is different for a quantum dot in the excited state and the ground state, reading out the nanowire deflection with a laser could reveal the state of the quantum dot, without directly collecting the dot fluorescence. Last but not least, strain-mediated coupling as demonstrated by Poizat and colleagues could also be exploited in other systems, such as nitrogen–vacancy centres in diamond resonators. Experiments along these lines are ongoing at present in several laboratories. ❐
1. Aspelmeyer, M., Meystre, P. & Schwab, K. Physics Today 65, 29–35 (July, 2012). 2. Treutlein, P., Genes, C., Hammerer, K., Poggio, M. & Rabl, P. Preprint at http://arXiv.org/abs/ 1210.4151 (2012). 3. Yeo, I. et al. Nature Nanotech. 9, 106–110 (2014). 4. Wilson-Rae, I., Zoller, P. & Imamoglu, A. Phys. Rev. Lett. 92, 075507 (2004). 5. Hunger, D. et al. C. R. Physique 12, 871–887 (2011). 6. Arcizet, O. et al. Nature Phys. 7, 879–883 (2011). 7. Kolkowitz, S. et al. Science 335, 1603–1606 (2012). 8. Camerer, S. et al. Phys. Rev. Lett. 107, 223001 (2011). 9. O’Connell, A. D. et al. Nature 464, 697–703 (2010). 10. Heiss, M. et al. Nature Mater. 12, 439–444 (2013). 11. Auffèves, A. & Richard, M. Preprint at http://arXiv.org/abs/1305.4252 (2013).
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
