N. V. Abrosimov

Quantum information storage for over 180 s using donor spins in a 28si “semiconductor vacuum”

Extending Quantum Memory Practical applications in quantum communication and quantum computation require the building blocks—quantum bits and quantum memory—to be sufficiently robust and long-lived to allow for manipulation and storage (see the Perspective by Boehme and McCarney). Steger et al. (p. 1280) demonstrate that the nuclear spins of 31P impurities in an almost isotopically pure sample of 28Si can have a coherence time of as long as 192 seconds at a temperature of ∼1.7 K. In diamond at room temperature, Maurer et al. (p. 1283) show that a spin-based qubit system comprised of an isotopic impurity (13C) in the vicinity of a color defect (a nitrogen-vacancy center) could be manipulated to have a coherence time exceeding one second. Such lifetimes promise to make spin-based architectures feasible building blocks for quantum information science. An almost isotopically pure sample of 28Si provides a vacuumlike environment for 31P qubits. A quantum computer requires systems that are isolated from their environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spins in solids promise long coherence lifetimes, but they are difficult to initialize into known states and to detect with high sensitivity. We show how the distinctive optical properties of enriched 28Si enable the use of hyperfine-resolved optical transitions, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these resolved hyperfine transitions permit rapid nuclear hyperpolarization and electrical spin-readout. We combine these techniques to detect nuclear magnetic resonance from dilute 31P in the purest available sample of 28Si, at concentrations inaccessible to conventional measurements, measuring a solid-state coherence time of over 180 seconds.

Simultaneous Subsecond Hyperpolarization of the Nuclear and Electron Spins of Phosphorus in Silicon by Optical Pumping of Exciton Transitions

A. Yang, M. Steger, T. Sekiguchi, M. L. W. Thewalt, ∗ T. D. Ladd, K. M. Itoh, H. Riemann, N. V. Abrosimov, P. Becker, and H.-J. Pohl Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 E. L. Ginzton Laboratory, Stanford University, Stanford CA 94305, USA Keio University, Yokohama 223-8522, Japan Institute for Crystal Growth (IKZ), 12489 Berlin, Germany PTB Braunschweig, 38116 Braunschweig, Germany VITCON Projectconsult GmbH, 07745 Jena, Germany (Dated: Accepted for publication in Physical Review Letters on June 03, 2009)

Hyperfine structure and nuclear hyperpolarization observed in the bound exciton luminescence of Bi donors in natural Si.

As the deepest group-V donor in Si, Bi has by far the largest hyperfine interaction and also a large I = 9/2 nuclear spin. At zero field this splits the donor ground state into states having total spin 5 and 4, which are fully resolved in the photoluminescence spectrum of Bi donor bound excitons. Under a magnetic field, the 60 expected allowed transitions cannot be individually resolved, but the effects of the nuclear spin distribution, -9/2 < or = I(z) < or = 9/2, are clearly observed. A strong hyperpolarization of the nuclear spin towards I(z) = -9/2 is observed to result from the nonresonant optical excitation. This is very similar to the recently reported optical hyperpolarization of P donors observed by EPR at higher magnetic fields. We introduce a new model to explain this effect, and predict that it may be very fast.

Photoluminescence of deep defects involving transition metals in Si: New insights from highly enriched 28Si

Deep luminescence centers in Si associated with transition metals have been studied for decades, both as markers for these deleterious contaminants, as well as for the possibility of efficient Si-based light emission. They are among the most ubiquitous luminescence centers observed in Si, and have served as testbeds for elucidating the physics of isoelectronic bound excitons, and for testing ab-initio calculations of defect properties. The greatly improved spectral resolution resulting from the elimination of inhomogeneous isotope broadening in the recently available highly enriched 28Si enabled the extension of the established technique of isotope shifts to the measurement of isotopic fingerprints, which reveal not only the presence of a given element in a luminescence center, but also the number of atoms of that element. This has resulted in many surprises regarding the actual constituents of what were thought to be well-understood deep luminescence centers. Here we summarize the available information f...

Direct observation of the donor nuclear spin in a near-gap bound exciton transition: P31 in highly enriched S28ia)

We report on ultrahigh resolution studies of the bound exciton states associated with the shallow acceptor B and the shallow donor P in highly enriched S28i using a tuneable single frequency laser to perform photoluminescence excitation spectroscopy. The linewidths and fine structure of the transitions, which were too narrow to be resolved previously using an available photoluminescence apparatus, are now fully revealed. The P bound exciton transition shows a complicated additional structure, which the Zeeman spectroscopy demonstrates to be a result of the splitting of the donor ground state by the hyperfine interaction between the spin of the donor electron and that of the P31 nucleus. The P31 nuclear spin populations can thus be determined, and hopefully modified, by optical means. The predominant Auger recombination channel of these bound excitons is used to observe the same resolved hyperfine transitions in the photocurrent spectrum. This demonstrates that donors in specific electronic and nuclear spi...

Vacancy–oxygen complex in Si1−xGex crystals

Electronic properties of the vacancy–oxygen complex in unstrained Si1−xGex crystals (0<x⩽0.055) grown by the Czochralski method were studied by means of capacitance transient techniques. The enthalpy of electron ionization for the single acceptor level of the defect relative to the conduction band edge, ΔHn, was found to increase from 0.16 to 0.19 eV with the increase in Ge content. The change of the lattice parameter in Si1−xGex alloys is argued to be one of the main reasons of the observed ΔHn change.

Optically-detected NMR of optically-hyperpolarized 31P neutral donors in 28Si

The electron and nuclear spins of the shallow donor 31P are promising qubit candidates invoked in many proposed Si-based quantum computing schemes. We have recently shown that the near-elimination of inhomogeneous broadening in highly isotopically enriched 28Si enables an optical readout of both the donor electron and nuclear spins by resolving the donor hyperfine splitting in the near-gap donor bound exciton transitions. We have also shown that pumping these same transitions can very quickly produce large electron and nuclear hyperpolarizations at low magnetic fields, where the equilibrium electron and nuclear polarizations are very small. Here we show preliminary results of the measurement of 31P neutral donor NMR parameters using this optical nuclear hyperpolarization mechanism for preparation of the 31P nuclear spin system, followed by optical readout of the resulting nuclear spin population after manipulation with NMR pulse sequences. This allows for the observation of single-shot NMR signals with ve...

Room Temperature Direct Band-Gap Emission from an Unstrained Ge P-I-N LED on Si

We present a novel Ge on Si based LED with unstrained i-Ge active region. The device operates at room temperature and emits photons with energy of 0.8 eV. It basically resembles a p-i-n structure formed on a sub-micrometer thin Ge layer. The Ge layer has been grown on Si substrate by utilizing thin virtual buffer, so it becomes stress free but with high threading dislocation density. We show that such forward biased diode generates strong emission, caused by direct band to band transition in Ge. Using an InSb based detector we were able to analyze the emission spectrum in a broad energy range. We show that at low and moderate currents, features belonging to the direct and the indirect band to band electronic transitions are present which are characteristic for Ge. Clearly dominating is the direct transition related peak. Due to the missing stress-related red shift this peak appears close to the desired communication wave length of 1.55 μm. The dependence of radiation intensity on the excitation current follows a power low with exponent of 1.7, indicating that the recombination rate of the competitive nonradiative processes is relatively low. At high excitation currents features appear in the low energetic part of the spectrum. All results presented here are discussed in view of the outcome from measurements on Ge high quality bulk material. The role of the dislocation in the Ge films is discussed.

Room temperature luminescence from Germanium

We analyze the band to band radiative transitions in germanium thin films deposited on silicon and compare them with those in bulk material. A significant down shift of the direct transition related peak was observed from the thin films samples, caused by the preparation formed tensile strain in the film. A comparison between the ratios of the direct to indirect transitions peak intensities showed that those are very similar for the thin films and the bulk material, when the self absorption of the emission is accounted for. The observed similarity in the spectral shape indicates that the strong direct transition luminescence detected in strained germanium films is mainly due to the improved light extraction in the energy range of about 0.8 eV, rather than an increase in the probability of band to band direct transition. We find a feature at around 0.72 eV in the spectrum of the germanium luminescence, which correlates with the presence of dislocations in the crystal. We discuss the origin of this feature in view of one dimensional dislocation bands, split off from the Gamma valley of the conduction band due to the dislocations local strain field.

A photonic platform for donor spin qubits in silicon

Chalcogen donors in silicon enable a scalable photonic cavity quantum electrodynamics solution for universal quantum computing. Donor spins in silicon are highly competitive qubits for upcoming quantum technologies, offering complementary metal-oxide semiconductor compatibility, coherence (T2) times of minutes to hours, and simultaneous initialization, manipulation, and readout fidelities near ~99.9%. This allows for many quantum error correction protocols, which will be essential for scale-up. However, a proven method of reliably coupling spatially separated donor qubits has yet to be identified. We present a scalable silicon-based platform using the unique optical properties of “deep” chalcogen donors. For the prototypical 77Se+ donor, we measure lower bounds on the transition dipole moment and excited-state lifetime, enabling access to the strong coupling limit of cavity quantum electrodynamics using known silicon photonic resonator technology and integrated silicon photonics. We also report relatively strong photon emission from this same transition. These results unlock clear pathways for silicon-based quantum computing, spin-to-photon conversion, photonic memories, integrated single-photon sources, and all-optical switches.