news & views QUANTUM DOTS
One atom at a time
Moving individual atoms on a surface with the tip of a scanning tunnelling microscope now enables the production of artificial atoms and molecules with precisely engineered molecular orbital energy-level diagrams.
Hanno H. Weitering
Q
uantum dots exhibit properties that are highly promising for materials applications, biological sensing and imaging, and quantum computing. A quantum dot can be viewed as an artificial atom with cleverly engineered electronic energy levels and precisely controlled level occupancies. Most semiconductor quantum dot systems are collections of nanoparticles containing hundreds or thousands of atoms each. In each dot the electronic levels are sharply defined as a result of the quantummechanical confinement of the electron motion. However, quantum dots slightly differ from one another, in size, shape or precise atomic composition, leading to significant variations in the energy levels. The smearing of the energy-level diagram can be detrimental to applications that a
Top view
require utmost precision in energy-level spacing. For example, pairs of single photons emitted by two identical quantum dots could be used in quantum optics and quantum information experiments1. The challenge is to produce identical dots that can emit indistinguishable, quantum-mechanically entangled photons. Now, writing in Nature Nanotechnology 2, Stefan Fölsch and co-workers from the Paul-Drude-Institut für Festkörperelektronik in Berlin, the NTT Basic Research Laboratories in Atsugi, and the Naval Research Laboratory in Washington DC, show how to completely eliminate stochastic variations in the size, shape and orientation of minuscule quantum dots by assembling the dots one atom at a time. Indium atoms are dragged to precise lattice locations on b
Top view
an InAs substrate surface with the tip of a scanning tunnelling microscope (STM). First, the authors demonstrate extraordinary reproducibility and precision in the level location and wavefunction mapping of individual quantum dots containing up to 25 atoms. Second, they arrange these artificial atoms into quantum dot molecules with tunable coupling strength and exact predetermined point-group symmetry, realizing orbital degeneracies that are surprisingly robust against perturbation by neighbouring defects. Many cutting-edge nanophysics experiments could benefit from such fidelity, but arguably the most tantalizing ones would be a demonstration of quantum coherence and entanglement in a collection of coupled quantum dots with precisely engineered, symmetry-protected c
Side view
Side view
Figure 1 | Illustration of the different quantum dot realizations. a, Two coupled quantum dots are formed by applying a negative voltage to gate electrodes deposited on a semiconductor surface or heterostructure5. The negative gate bias depletes the regions below the gates of free charge carriers, leaving a puddle of trapped electrons (red) in the middle. Voltages on the point contacts control the transmission of electrons into or out of the quantum dots with single-electron precision5. b, A sketch of the three-dimensional arrangement of pyramidal quantum dots fabricated through heteroepitaxial growth. The selforganization of atoms into pyramidal clusters is the result of a lattice-strain-relieve mechanism at the interface between two dissimilar semiconductor materials (indicated in red and blue)6,7. The strain field of a dot in the first layer then causes the second dot to grow right on top, resulting in the vertical self-alignment. c, Illustration of the atom-by-atom assembly of quantum dot molecules by means of vertical adatom manipulation with a scanning tunnelling microscope tip, achieving the ultimate precision for quantum-level engineering2. NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
1
news & views level structures1,3,4. The latter is critical for application of quantum dot systems as building blocks for quantum computers. Quantum dots can be fabricated in many different ways5–7 (Fig. 1). They need not be physical entities. All that is needed is a three-dimensional confinement potential, which, for instance, could be defined electrostatically by applying negative voltages to an array of gate electrodes on a semiconductor surface (Fig. 1a). Alternatively, the confinement potential can be defined by the physical boundaries of a nanoparticle, as shown in Fig. 1b,c. A key requirement in all of these cases is that the size of the dot should be comparable to the de Broglie wavelength of the electrons so that electrons occupy discrete quantum levels. As with real atoms, the level occupancy of the dot follows an Aufbau principle, similar to that of the elements in the periodic table. The analogy with real atoms goes even further. By varying the electron occupancy one electron at a time, it is even possible to establish trends in chemical reactivity along rows in a periodic table of artificial elements8. Fölsch et al. achieved perfect fidelity in atom placement by using an atomically clean reconstructed InAs(111) surface, prepared in ultrahigh vacuum, as a template for positioning individual indium atoms. The surface is characterized by a perfectly ordered array of indium-atom vacancies. However, tiny amounts of excess indium atoms may still be present on the surface and these adatoms can be dragged across the surface with an STM tip (Fig. 1c), a technology once heralded as a landmark achievement 9,10. By placing the atom at a predetermined vacancy site and keeping the temperature low enough to avoid surface diffusion, the authors acquired the ultimate fabrication precision of the size, shape and quantum-level structure of the artificial entities. The authors then made a very significant next step by positioning individual quantum
2
dots in various symmetrical arrangements. Such coupled quantum dot systems or molecules are currently of great interest. For instance, the two-atom ‘molecule’ depicted in Fig. 1a could be considered a physical realization of a ‘qubit’ for quantum computing where the quantum information could be stored in the location of an electron and/or in the orientation of its spin. The two dot indices could play the role of an ‘isospin’ or qubit while coherent tunnelling between the dots would lead to a superposition and entanglement of two quantum states4. Each dot could be as small as a single atom, such as a phosphorus dopant in silicon11 or a surface dangling bond12. In the experiments of Fölsch and co-workers, the dots contain several atoms. The quantum tunnelling between the dots or ‘hopping rate’ was controlled with extreme precision by varying the distance between dots with atomic accuracy and/or by placing additional adatoms in the gap region, so as to provide auxiliary pathways for electron hopping. The experimentally observed amplitude and nodal structure of the resulting molecular orbitals conform to the symmetry of the molecule. Three dots arranged in a trigonal planar arrangement on a substrate exhibit a type of symmetry for which the lowest energy level is non-degenerate and where the next two levels are twofold degenerate. The molecular orbital diagram is similar to that of the NH3 molecule. The lowest state corresponds to a quantum state where the valence charge piles up at the centre of the molecule, whereas the two degenerate levels exhibit distinct lobes and nodal planes in the charge distribution. Indeed, this is visualized in the tunnelling experiments. Most importantly, the degeneracy of the excited state seems to be remarkably robust against external perturbations, such as nearby charged impurity atoms. Such a property would be highly desirable for designing physical qubits, where the coupling between electrons in the dots and environmental
perturbations such as charged impurities (but also phonons and other disturbances) easily destroy quantum coherence. Practical implementation of these molecules as charge qubits remains a formidable challenge, especially with regard to decoherence, atomic-scale gate control, ultrafast read-out and cost-efficient scalability. On the other hand, fascinating experiments can already be done with a conventional low-temperature STM on similarly prepared systems. These include studies of Kondo physics, magnetism, nanoscale superconductivity or chemical reactivity. Most importantly, the high-fidelity synthesis route exemplified by Fölsch and co-workers elevates the concept of materials by design to a whole new level. It enables atom-by-atom synthesis of novel kinetically stabilized materials whose properties are eventually dominated by quantum correlations, coherence and entanglement. Such ‘quantum matter’ materials may one day become the new platform for technological innovation. ❐ Hanno H. Weitering is in the Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, and at Oak Ridge National Laboratory, Tennessee 37831, USA. e-mail: [email protected] References 1. Trotta, R. et al. Phys. Rev. Lett. 109, 147401 (2012). 2. Fölsch, S., Martínez-Blanco, J., Yang, J., Kanisawa, K. & Erwin, S. C. Nature Nanotech. http://dx.doi.org/10.1038/nnano.2014.129 (2014). 3. Doucot, B. & Ioffe, L. B. Rep. Prog. Phys. 75, 072001–072020 (2012). 4. Bayer, M. et al. Science 291, 451–453 (2001). 5. Waugh, F. R. et al. Phys. Rev. Lett. 75, 705–708 (1995). 6. Eaglesham, D. J. & Cerullo, M. Phys. Rev. Lett. 64, 1943–1946 (1990). 7. Leonard, D., Krishnamurthy, M., Reaves, C. M., Denbaars, S. P. & Petroff, P. M. Appl. Phys. Lett. 63, 3203–3205 (1993). 8. Tarucha, S., Austing, D. G., Honda, T., van der Hage, R. J. & Kouwenhoven, L. P. Phys. Rev. Lett. 77, 3613–3616 (1996). 9. Stroscio, J. A. & Eigler, D. M. Science 254, 1319–1326 (1991). 10. Crommie, M. F., Lutz, C. P. & Eigler, D. M. Science 262, 218–220 (1993). 11. Kane, B. E. Nature 393, 133–137 (1998). 12. Haider, M. B. et al. Phys. Rev. Lett. 102, 046805 (2009).
Published online: 29 June 2014
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
One atom at a time
Moving individual atoms on a surface with the tip of a scanning tunnelling microscope now enables the production of artificial atoms and molecules with precisely engineered molecular orbital energy-level diagrams.
Hanno H. Weitering
Q
uantum dots exhibit properties that are highly promising for materials applications, biological sensing and imaging, and quantum computing. A quantum dot can be viewed as an artificial atom with cleverly engineered electronic energy levels and precisely controlled level occupancies. Most semiconductor quantum dot systems are collections of nanoparticles containing hundreds or thousands of atoms each. In each dot the electronic levels are sharply defined as a result of the quantummechanical confinement of the electron motion. However, quantum dots slightly differ from one another, in size, shape or precise atomic composition, leading to significant variations in the energy levels. The smearing of the energy-level diagram can be detrimental to applications that a
Top view
require utmost precision in energy-level spacing. For example, pairs of single photons emitted by two identical quantum dots could be used in quantum optics and quantum information experiments1. The challenge is to produce identical dots that can emit indistinguishable, quantum-mechanically entangled photons. Now, writing in Nature Nanotechnology 2, Stefan Fölsch and co-workers from the Paul-Drude-Institut für Festkörperelektronik in Berlin, the NTT Basic Research Laboratories in Atsugi, and the Naval Research Laboratory in Washington DC, show how to completely eliminate stochastic variations in the size, shape and orientation of minuscule quantum dots by assembling the dots one atom at a time. Indium atoms are dragged to precise lattice locations on b
Top view
an InAs substrate surface with the tip of a scanning tunnelling microscope (STM). First, the authors demonstrate extraordinary reproducibility and precision in the level location and wavefunction mapping of individual quantum dots containing up to 25 atoms. Second, they arrange these artificial atoms into quantum dot molecules with tunable coupling strength and exact predetermined point-group symmetry, realizing orbital degeneracies that are surprisingly robust against perturbation by neighbouring defects. Many cutting-edge nanophysics experiments could benefit from such fidelity, but arguably the most tantalizing ones would be a demonstration of quantum coherence and entanglement in a collection of coupled quantum dots with precisely engineered, symmetry-protected c
Side view
Side view
Figure 1 | Illustration of the different quantum dot realizations. a, Two coupled quantum dots are formed by applying a negative voltage to gate electrodes deposited on a semiconductor surface or heterostructure5. The negative gate bias depletes the regions below the gates of free charge carriers, leaving a puddle of trapped electrons (red) in the middle. Voltages on the point contacts control the transmission of electrons into or out of the quantum dots with single-electron precision5. b, A sketch of the three-dimensional arrangement of pyramidal quantum dots fabricated through heteroepitaxial growth. The selforganization of atoms into pyramidal clusters is the result of a lattice-strain-relieve mechanism at the interface between two dissimilar semiconductor materials (indicated in red and blue)6,7. The strain field of a dot in the first layer then causes the second dot to grow right on top, resulting in the vertical self-alignment. c, Illustration of the atom-by-atom assembly of quantum dot molecules by means of vertical adatom manipulation with a scanning tunnelling microscope tip, achieving the ultimate precision for quantum-level engineering2. NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
1
news & views level structures1,3,4. The latter is critical for application of quantum dot systems as building blocks for quantum computers. Quantum dots can be fabricated in many different ways5–7 (Fig. 1). They need not be physical entities. All that is needed is a three-dimensional confinement potential, which, for instance, could be defined electrostatically by applying negative voltages to an array of gate electrodes on a semiconductor surface (Fig. 1a). Alternatively, the confinement potential can be defined by the physical boundaries of a nanoparticle, as shown in Fig. 1b,c. A key requirement in all of these cases is that the size of the dot should be comparable to the de Broglie wavelength of the electrons so that electrons occupy discrete quantum levels. As with real atoms, the level occupancy of the dot follows an Aufbau principle, similar to that of the elements in the periodic table. The analogy with real atoms goes even further. By varying the electron occupancy one electron at a time, it is even possible to establish trends in chemical reactivity along rows in a periodic table of artificial elements8. Fölsch et al. achieved perfect fidelity in atom placement by using an atomically clean reconstructed InAs(111) surface, prepared in ultrahigh vacuum, as a template for positioning individual indium atoms. The surface is characterized by a perfectly ordered array of indium-atom vacancies. However, tiny amounts of excess indium atoms may still be present on the surface and these adatoms can be dragged across the surface with an STM tip (Fig. 1c), a technology once heralded as a landmark achievement 9,10. By placing the atom at a predetermined vacancy site and keeping the temperature low enough to avoid surface diffusion, the authors acquired the ultimate fabrication precision of the size, shape and quantum-level structure of the artificial entities. The authors then made a very significant next step by positioning individual quantum
2
dots in various symmetrical arrangements. Such coupled quantum dot systems or molecules are currently of great interest. For instance, the two-atom ‘molecule’ depicted in Fig. 1a could be considered a physical realization of a ‘qubit’ for quantum computing where the quantum information could be stored in the location of an electron and/or in the orientation of its spin. The two dot indices could play the role of an ‘isospin’ or qubit while coherent tunnelling between the dots would lead to a superposition and entanglement of two quantum states4. Each dot could be as small as a single atom, such as a phosphorus dopant in silicon11 or a surface dangling bond12. In the experiments of Fölsch and co-workers, the dots contain several atoms. The quantum tunnelling between the dots or ‘hopping rate’ was controlled with extreme precision by varying the distance between dots with atomic accuracy and/or by placing additional adatoms in the gap region, so as to provide auxiliary pathways for electron hopping. The experimentally observed amplitude and nodal structure of the resulting molecular orbitals conform to the symmetry of the molecule. Three dots arranged in a trigonal planar arrangement on a substrate exhibit a type of symmetry for which the lowest energy level is non-degenerate and where the next two levels are twofold degenerate. The molecular orbital diagram is similar to that of the NH3 molecule. The lowest state corresponds to a quantum state where the valence charge piles up at the centre of the molecule, whereas the two degenerate levels exhibit distinct lobes and nodal planes in the charge distribution. Indeed, this is visualized in the tunnelling experiments. Most importantly, the degeneracy of the excited state seems to be remarkably robust against external perturbations, such as nearby charged impurity atoms. Such a property would be highly desirable for designing physical qubits, where the coupling between electrons in the dots and environmental
perturbations such as charged impurities (but also phonons and other disturbances) easily destroy quantum coherence. Practical implementation of these molecules as charge qubits remains a formidable challenge, especially with regard to decoherence, atomic-scale gate control, ultrafast read-out and cost-efficient scalability. On the other hand, fascinating experiments can already be done with a conventional low-temperature STM on similarly prepared systems. These include studies of Kondo physics, magnetism, nanoscale superconductivity or chemical reactivity. Most importantly, the high-fidelity synthesis route exemplified by Fölsch and co-workers elevates the concept of materials by design to a whole new level. It enables atom-by-atom synthesis of novel kinetically stabilized materials whose properties are eventually dominated by quantum correlations, coherence and entanglement. Such ‘quantum matter’ materials may one day become the new platform for technological innovation. ❐ Hanno H. Weitering is in the Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, and at Oak Ridge National Laboratory, Tennessee 37831, USA. e-mail: [email protected] References 1. Trotta, R. et al. Phys. Rev. Lett. 109, 147401 (2012). 2. Fölsch, S., Martínez-Blanco, J., Yang, J., Kanisawa, K. & Erwin, S. C. Nature Nanotech. http://dx.doi.org/10.1038/nnano.2014.129 (2014). 3. Doucot, B. & Ioffe, L. B. Rep. Prog. Phys. 75, 072001–072020 (2012). 4. Bayer, M. et al. Science 291, 451–453 (2001). 5. Waugh, F. R. et al. Phys. Rev. Lett. 75, 705–708 (1995). 6. Eaglesham, D. J. & Cerullo, M. Phys. Rev. Lett. 64, 1943–1946 (1990). 7. Leonard, D., Krishnamurthy, M., Reaves, C. M., Denbaars, S. P. & Petroff, P. M. Appl. Phys. Lett. 63, 3203–3205 (1993). 8. Tarucha, S., Austing, D. G., Honda, T., van der Hage, R. J. & Kouwenhoven, L. P. Phys. Rev. Lett. 77, 3613–3616 (1996). 9. Stroscio, J. A. & Eigler, D. M. Science 254, 1319–1326 (1991). 10. Crommie, M. F., Lutz, C. P. & Eigler, D. M. Science 262, 218–220 (1993). 11. Kane, B. E. Nature 393, 133–137 (1998). 12. Haider, M. B. et al. Phys. Rev. Lett. 102, 046805 (2009).
Published online: 29 June 2014
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
