news & views QUANTUM SPINTRONICS
Single spins in silicon carbide
Individual spins, associated with vacancies in the silicon carbide lattice, have been observed and coherently manipulated. This may offer new directions for integrated spintronic devices.
Andrea Morello
U
ntil about 20 years ago, physics students were taught that one never observes a single quantummechanical system. Instead, experiments should always be conducted on large ensembles of identical particles. This point of view has been superseded by landmark experiments in which individual quantum systems were measured and controlled with exquisite precision. The first physical systems of choice were individual photons, atoms held in vacuum, or single Josephson superconducting devices1. While these exciting developments on single-particle systems were reshaping our understanding of quantum mechanics and gaining Nobel prizes2,3, researchers aiming at applying this knowledge in practical applications such as secure communications and quantum computing were ramping up their interest in diamond and silicon. This was because it had been found that certain defects in these materials exhibit unique quantummechanical behaviour 4,5. Electrons trapped at these defects possess an intrinsic angular momentum, commonly referred to as spin, which can be prepared in an arbitrary quantum state, and preserved for long time periods, called the spin coherence time. The electron and nuclear spins associated with nitrogen–vacancy centres in diamond6, and phosphorus impurities in silicon7, are by far the most coherent quantum objects to have been individually observed and manipulated in the solid state. These two materials demonstrate complementary properties for possibly realizing new spintronic devices: diamond offers optically accessible quantum states and room-temperature operation, whereas silicon provides an unparalleled platform for nanofabrication and electrical interfacing. Silicon carbide, a compound of these two materials, could in principle combine the best of both worlds, allowing for a single optically addressable spin, with long coherence times even at room temperature, embedded in a highperformance electronic material. Writing in Nature Materials, two collaborations demonstrate single-spin control in silicon carbide with optically detected magnetic resonance measurements.
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Figure 1 | Two-dimensional representation of the atomic structure of silicon carbide, showing divacancy lattice defects. Dotted blue and red circles refer to single silicon and carbon vacancies, respectively. Four variants of these vacancies are possible, characterized by their h and k lattice sites8.
Christle et al.8 report the coherent control of the electronic spin of individual neutral divacancies in silicon carbide, composed simultaneously of one silicon and one carbon atom vacancy in the silicon carbide lattice. Owing to inequivalent lattice sites in the silicon carbide structure, four distinct arrangements of these divacancies are possible, as shown in Fig. 1. Their experiment was conducted at cryogenic temperatures (20 K). Meanwhile, Widmann and co-authors9 show single-spin detection and coherent manipulation at room temperature, using a single silicon atom vacancy (Fig. 2). They also enhanced the photon collection efficiency by milling a solid-immersion lens on the surface of the silicon carbide sample. Both experiments were based on detecting the spin-dependent photoluminescence of the defect, while manipulating its spin with pulses of oscillating magnetic field at microwave frequencies. The evidence that the signal
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
comes from a single spin is provided by observing that no two photons are emitted at the same time. Widmann et al. placed a lower bound to the spin coherence time of 160 μs, and an upper bound of 1 ms, limited by the spin relaxation time. The experiment of Christle et al. at cryogenic temperatures revealed a coherence time of 1.2 ms. Owing to limitations in their photon collection efficiency, they measured single-spin echo decay up to 100 μs and used a many-spin ensemble to measure the coherence decay up to 1 ms. Therefore, the differences between the two results are most probably related to the temperature of operation and the photon collection method, rather than intrinsic to the different defects under study. Several clear messages emerge from these papers. First, although the spindependent photoluminescence signal of defects in silicon carbide is shown not to be very intense (at least in comparison with nitrogen–vacancy centres in diamond), it is sufficient to allow single-spin detection, particularly when collected through a solid-immersion lens. Next, both studies demonstrate that single-spin optically detected magnetic resonance can be performed on a variety of defects in silicon carbide. Given the hundreds of different polytypes and associated defects present in silicon carbide10, it is likely that many other optically active single-spin centres will be found and addressed in the near future. Each of these defects can be expected to possess different spin states and Hamiltonians, potentially allowing for greater flexibility in engineering and addressing multi-spin registers. Finally, the measured spin coherence times of the defects in silicon carbide are remarkably long, both at ambient conditions and at cryogenic temperatures. The spin coherence times measured in these experiments are actually limited by the randomly fluctuating magnetic field produced by the nuclear spins of 13C and 29Si atoms in the vicinity of the vacancies. Therefore, and as already shown in diamond6 and silicon7, a further increase in spin coherence time might be achievable by removing the 13C or 29Si spins, an option 1
news & views that has recently become available also in silicon carbide11. Looking ahead, the accomplishment of single-spin control in silicon carbide paves the way for integrating highly coherent spins with a variety of optical and electronic structures. Silicon carbide is becoming an increasingly mature material platform for the fabrication of high-quality optical resonators12 and photonic crystals13, as well as high-power and high-frequency electronic devices. In the long term, one can envisage building spin-based quantum information processors by engineering arrays of spins coupled through cavity photons and locally addressed by electric or magnetic fields. As suggested by the room-temperature experiments conducted by Widmann and colleagues, this quantum processor could potentially operate in ambient conditions. In addition, the luminescence of the defects in silicon carbide is in the near-infrared, which is advantageous for integration with optical telecommunication systems. There are still formidable difficulties to overcome before such defect-based quantum processors can become a reality. First among these are the deterministic creation of defects at precise locations, and
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read-out single spins in silicon carbide is a promising step towards integrating the most attractive features of carbon and silicon into a modern, flexible and high-performance material platform. ❐
0
Si C
Andrea Morello is at the Centre for Quantum Computation and Communication Technology, School of Electrical Engineering and Telecommunications, UNSW Australia, Sydney New South Wales 2052, Australia. e-mail: [email protected] References 1. 2. 3. 4.
Figure 2 | Three-dimensional representation of the silicon carbide lattice. The single spin of the silicon vacancies (red arrows) is controlled and measured9 while an external magnetic field B0 is applied, as indicated.
the engineering of their mutual couplings. However, the now-demonstrated ability to coherently manipulate and optically
Ladd, T. D. et al. Nature 464, 45–53 (2010). Haroche, S. Rev. Mod. Phys. 85, 1083–1102 (2013). Wineland, D. J. Rev. Mod. Phys. 85, 1103–1114 (2013). Childress, L. & Hanson, R. Mater. Res. Soc. Bull. 38, 134–138 (2013). 5. Zwanenburg, F. A. et al. Rev. Mod. Phys. 85, 961–1019 (2013). 6. Maurer, P. C. et al. Science 336, 1283–1286 (2012). 7. Muhonen, J. T. et al. Nature Nanotech. http://dx.doi.org/10.1038/nnano.2014.211 (2014). 8. Christle, D. J. et al. Nature Mater. http://dx.doi.org/10.1038/nmat4144 (2014). 9. Widmann, M. et al. Nature Mater. http://dx.doi.org/10.1038/nmat4145 (2014). 10. Falk, A. L. et al. Nature Commun. 4, 1819 (2013). 11. Ivanov, I. G. et al. Mater. Sci. Forum 778, 471–474 (2014). 12. Cardenas, J. et al. Opt. Express 21, 16882–16887 (2013). 13. Calusine, G., Politi, A. & Awschalom, D. D. Appl. Phys. Lett. 105, 011123 (2014).
Published online: 1 December 2014
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
Single spins in silicon carbide
Individual spins, associated with vacancies in the silicon carbide lattice, have been observed and coherently manipulated. This may offer new directions for integrated spintronic devices.
Andrea Morello
U
ntil about 20 years ago, physics students were taught that one never observes a single quantummechanical system. Instead, experiments should always be conducted on large ensembles of identical particles. This point of view has been superseded by landmark experiments in which individual quantum systems were measured and controlled with exquisite precision. The first physical systems of choice were individual photons, atoms held in vacuum, or single Josephson superconducting devices1. While these exciting developments on single-particle systems were reshaping our understanding of quantum mechanics and gaining Nobel prizes2,3, researchers aiming at applying this knowledge in practical applications such as secure communications and quantum computing were ramping up their interest in diamond and silicon. This was because it had been found that certain defects in these materials exhibit unique quantummechanical behaviour 4,5. Electrons trapped at these defects possess an intrinsic angular momentum, commonly referred to as spin, which can be prepared in an arbitrary quantum state, and preserved for long time periods, called the spin coherence time. The electron and nuclear spins associated with nitrogen–vacancy centres in diamond6, and phosphorus impurities in silicon7, are by far the most coherent quantum objects to have been individually observed and manipulated in the solid state. These two materials demonstrate complementary properties for possibly realizing new spintronic devices: diamond offers optically accessible quantum states and room-temperature operation, whereas silicon provides an unparalleled platform for nanofabrication and electrical interfacing. Silicon carbide, a compound of these two materials, could in principle combine the best of both worlds, allowing for a single optically addressable spin, with long coherence times even at room temperature, embedded in a highperformance electronic material. Writing in Nature Materials, two collaborations demonstrate single-spin control in silicon carbide with optically detected magnetic resonance measurements.
(hk)
(kh)
(kk)
(hh)
Figure 1 | Two-dimensional representation of the atomic structure of silicon carbide, showing divacancy lattice defects. Dotted blue and red circles refer to single silicon and carbon vacancies, respectively. Four variants of these vacancies are possible, characterized by their h and k lattice sites8.
Christle et al.8 report the coherent control of the electronic spin of individual neutral divacancies in silicon carbide, composed simultaneously of one silicon and one carbon atom vacancy in the silicon carbide lattice. Owing to inequivalent lattice sites in the silicon carbide structure, four distinct arrangements of these divacancies are possible, as shown in Fig. 1. Their experiment was conducted at cryogenic temperatures (20 K). Meanwhile, Widmann and co-authors9 show single-spin detection and coherent manipulation at room temperature, using a single silicon atom vacancy (Fig. 2). They also enhanced the photon collection efficiency by milling a solid-immersion lens on the surface of the silicon carbide sample. Both experiments were based on detecting the spin-dependent photoluminescence of the defect, while manipulating its spin with pulses of oscillating magnetic field at microwave frequencies. The evidence that the signal
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
comes from a single spin is provided by observing that no two photons are emitted at the same time. Widmann et al. placed a lower bound to the spin coherence time of 160 μs, and an upper bound of 1 ms, limited by the spin relaxation time. The experiment of Christle et al. at cryogenic temperatures revealed a coherence time of 1.2 ms. Owing to limitations in their photon collection efficiency, they measured single-spin echo decay up to 100 μs and used a many-spin ensemble to measure the coherence decay up to 1 ms. Therefore, the differences between the two results are most probably related to the temperature of operation and the photon collection method, rather than intrinsic to the different defects under study. Several clear messages emerge from these papers. First, although the spindependent photoluminescence signal of defects in silicon carbide is shown not to be very intense (at least in comparison with nitrogen–vacancy centres in diamond), it is sufficient to allow single-spin detection, particularly when collected through a solid-immersion lens. Next, both studies demonstrate that single-spin optically detected magnetic resonance can be performed on a variety of defects in silicon carbide. Given the hundreds of different polytypes and associated defects present in silicon carbide10, it is likely that many other optically active single-spin centres will be found and addressed in the near future. Each of these defects can be expected to possess different spin states and Hamiltonians, potentially allowing for greater flexibility in engineering and addressing multi-spin registers. Finally, the measured spin coherence times of the defects in silicon carbide are remarkably long, both at ambient conditions and at cryogenic temperatures. The spin coherence times measured in these experiments are actually limited by the randomly fluctuating magnetic field produced by the nuclear spins of 13C and 29Si atoms in the vicinity of the vacancies. Therefore, and as already shown in diamond6 and silicon7, a further increase in spin coherence time might be achievable by removing the 13C or 29Si spins, an option 1
news & views that has recently become available also in silicon carbide11. Looking ahead, the accomplishment of single-spin control in silicon carbide paves the way for integrating highly coherent spins with a variety of optical and electronic structures. Silicon carbide is becoming an increasingly mature material platform for the fabrication of high-quality optical resonators12 and photonic crystals13, as well as high-power and high-frequency electronic devices. In the long term, one can envisage building spin-based quantum information processors by engineering arrays of spins coupled through cavity photons and locally addressed by electric or magnetic fields. As suggested by the room-temperature experiments conducted by Widmann and colleagues, this quantum processor could potentially operate in ambient conditions. In addition, the luminescence of the defects in silicon carbide is in the near-infrared, which is advantageous for integration with optical telecommunication systems. There are still formidable difficulties to overcome before such defect-based quantum processors can become a reality. First among these are the deterministic creation of defects at precise locations, and
2
B
read-out single spins in silicon carbide is a promising step towards integrating the most attractive features of carbon and silicon into a modern, flexible and high-performance material platform. ❐
0
Si C
Andrea Morello is at the Centre for Quantum Computation and Communication Technology, School of Electrical Engineering and Telecommunications, UNSW Australia, Sydney New South Wales 2052, Australia. e-mail: [email protected] References 1. 2. 3. 4.
Figure 2 | Three-dimensional representation of the silicon carbide lattice. The single spin of the silicon vacancies (red arrows) is controlled and measured9 while an external magnetic field B0 is applied, as indicated.
the engineering of their mutual couplings. However, the now-demonstrated ability to coherently manipulate and optically
Ladd, T. D. et al. Nature 464, 45–53 (2010). Haroche, S. Rev. Mod. Phys. 85, 1083–1102 (2013). Wineland, D. J. Rev. Mod. Phys. 85, 1103–1114 (2013). Childress, L. & Hanson, R. Mater. Res. Soc. Bull. 38, 134–138 (2013). 5. Zwanenburg, F. A. et al. Rev. Mod. Phys. 85, 961–1019 (2013). 6. Maurer, P. C. et al. Science 336, 1283–1286 (2012). 7. Muhonen, J. T. et al. Nature Nanotech. http://dx.doi.org/10.1038/nnano.2014.211 (2014). 8. Christle, D. J. et al. Nature Mater. http://dx.doi.org/10.1038/nmat4144 (2014). 9. Widmann, M. et al. Nature Mater. http://dx.doi.org/10.1038/nmat4145 (2014). 10. Falk, A. L. et al. Nature Commun. 4, 1819 (2013). 11. Ivanov, I. G. et al. Mater. Sci. Forum 778, 471–474 (2014). 12. Cardenas, J. et al. Opt. Express 21, 16882–16887 (2013). 13. Calusine, G., Politi, A. & Awschalom, D. D. Appl. Phys. Lett. 105, 011123 (2014).
Published online: 1 December 2014
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
