Session F11: Defects in Semiconductors -- Quantum Defects

High frequency electrometry and imaging with defects in silicon carbide

Optically active defects in wide bandgap host materials are promising sensors of local properties such as magnetic fields, electric fields, temperature and mechanical strain [1]. While magnetometry has received considerable interest owing to the spin properties of these defects, electrometry and strain sensing have been significantly more challenging due to the weak sensitivity of the ground spin state.

Here we demonstrate that the charge state of defects in silicon carbide (SiC), including divacancies and silicon vacancies, can be used to sense high frequency (MHz – GHz) electric fields and acoustics [2]. Optical excitation can convert the defect from a bright (photoluminescent) charge state to a dark charge state, and vice-versa [3]. This conversion rate depends directly on local high frequency electric fields and mechanical strains, resulting in direct modulation of the photoluminescence intensity and enabling sensing protocols. We further develop methods for spectroscopy (phase and frequency resolution) and vector measurements.

We explore electric field and strain sensing using both this novel charge technique and spin measurements in various electromechanical devices. We map the electric fields from lithographically patterned capacitors and observe mechanical modes in a surface acoustic wave resonator and in clamped membranes. We obtain 3D spatial and high frequency information typically challenging to achieve with other sensing techniques. This demonstrates the potential of optically active defects for in-situ electrical and micromechanical systems characterization in commercial materials such as SiC.

[1] F. J. Heremans et. al., Proc. IEEE 104, 10 (2016).
[2] G. Wolfowicz et. al., Proc. Natl. Acad. Sci. 115, 31 7879-7883 (2018).
[3] G. Wolfowicz et. al., Nat. Com. 8, 1876 (2017).

In collaboration with S. J. Whiteley, C. P. Anderson, A. L. Yeats, P.-L. Yu, S. Bhave, F. J. Heremans, D. D. Awschalom.   

Optimizing Spin Readout of the Nitrogen-Vacancy Center in Diamond with Spin-to-Charge Conversion

The nitrogen-vacancy center in diamond is a mature platform for quantum technology, enabling sophisticated quantum information protocols as well as versatile quantum sensors operating in previously unreachable size and field regimes. The standard photoluminescence-based spin readout is fast (300 ns) but typical measurements yield only a few hundredths of a photon on average, necessitating tens of thousands of repeats to overcome shot noise in detecting the spin state. Spin-to-charge conversion (SCC) offers an alternative readout with significantly improved single-shot information. However, this benefit comes at the expense of orders of magnitude longer readout durations. Here, we present a framework for optimizing the SCC readout parameters that leads to dramatic reductions in overall measurement acquisition times [1]. The improvements arise from the combination of an analytical charge readout model with numerical optimization of the overhead durations. We discuss relevant applications such as T₁ relaxometry and control of nuclear registers and outline how other spin readout methods can benefit from this framework.
[1] D.A. Hopper et al. Micromachines 9, 437 (2018)
  

Coupled defect centers for diamond quantum nodes

Nitrogen-vacancy (NV) defect centers in diamond are well-suited to realize quantum networks owing to a coherent spin ground state, nuclear quantum memories and an optical transition for remote entanglement. State-of-the-art experiments with single NV centers used ¹³C atoms as quantum memories to perform entanglement distillation and quantum error correction [Kalb et. al., Science, 2017, Cramer et al., Nat. Commun., 2015]. Such experiments involve an intrinsic trade-off: reducing the coupling to the NV center improves ¹³C quantum memories, but reduces gate speeds.

In this work, we aim to extend the possibilities of quantum networks by creating systems of two or more coupled defects centers in diamond. Such coupled defects, hold the promise to combine fast gates with well-isolated quantum nuclear memories. I will present recent progress towards this goal.

First, we realized a long coherence time, exceeding two seconds, of the host nitrogen spin of a single NV center, demonstrating its potential as a quantum memory. Second, I will discuss our recent progress using ion implantation and other techniques to realize coupled defects.   

Linewidth of NV-detected Electron Spin Resonance

A nitrogen-vacancy (NV) center in diamond possesses unique electronic, magnetic and quantum properties enabling magnetic field sensing through optically detected magnetic resonance (ODMR). Due to the extreme sensitivity of an NV center to magnetic fields, the NV is a promising candidate for applications of electron spin resonance (ESR) with single spin sensitivity. In addition to the sensitivity, ESR using a single NV has a significant advantage in spatial resolution of the detection volume over conventional ESR because of the nanometer sensing distance on single NV-detected ESR. In this presentation, we discuss the linewidth of NV-detected ESR. In the experiment, we study NV-detected ESR on substitutional nitrogen centers in diamond and identify components partially contributing to the linewidth. We also discuss differences of the ESR linewidth between conventional and NV-based detection.   

Ab-initio photoluminescence spectrum of NV− centres by Time-Dependent Density Functional Theory

Quantum information and communication require single photons on demand, however current state of the art emitters are probabilistic. Defect centres in nanomaterials have shown promise as deterministic single photon sources. To investigate these materials, first principle models are required. Purely Density Functional Theory (DFT) models have met with much success, however, DFT is a ground state theory and excited states can only be calculated under strict assumptions. Here we demonstrate a method to calculate the ab-initio photoluminescence spectrum for NV^- centres using Time-Dependent Density Functional Theory (TD-DFT). Ground state properties are calculated using DFT. Excited state energies and transition dipole moments are calculated with Linear Response TD-DFT. Excited vibrational modes are given as normal modes the TD-DFT energy second derivatives. The emission rate is calculated from the transition dipole moment and Franck-Condon overlaps. Our technique can be extended to more general defects and is especially useful for edge effects like with small clusters or defects with Jahn-Tellar distortions. The ability to recreate experimental photoluminescence spectra marks a step forward in understanding and controlling single photon emission for defect emitters.   

Simulation Driven Search for Promising Quantum Defects in Diamond

Current research into quantum memories in diamond has mainly focused on the NV^- and SiV^- color centers, however both of these have clear limitations in spectral efficiency and spin coherence, respectively. More recent work has characterised the new group-IV centers GeV^-, SnV^- and PbV^-, as well as the neutral SiV, though other as yet unstudied defects in diamond are also possible. To efficiently screen these new defects, we use first-principles density-functional theory to predict important defect properties, including orbital structure, structural and charge stability, as well as vibrational effects on the emission profile. Promising defects are also investigated for possible Jahn-Teller distortions, and we discuss these implications. Overall, our results provide a roadmap toward the discovery of novel centers for quantum information.   

Optimizing the Production of Single Group IV Color Centers in Diamond

The excitement of color centers, like the NV vacancy center in diamond, has been motivated by their use as single-photon emitters for applications in quantum information technology. Recent work has shown that a number of color centers in diamond using other Group IV elements (e.g. SiV, GeV, SnV and PbV)¹ may also be good candidates for quantum emitters. In this work we use density-functional theory (DFT) to study the thermodynamics and kinetics of these Group IV color centers in diamond. We find that for p-type diamond the production of isolated color centers will be enhanced. We further investigate the stability of complexes of these Group IV color centers in diamond with carbon vacancies which are present in diamond in abundance after implantation. We believe that this work may lead to optimal experimental conditions which lead to longer coherence times for color centers as previously discussed in the literature.²

[1] G. Thiering and A. Gali, Phys. Rev. X, 8, 021063, (2018).
[2] B. Naydenov, F. Reinhard, A. Lämmle, V. Richter, R. Kalish, U. F. D’Haenens-Johansson, M. Newton, F. Jelezko, and J. Wrachtrup, App. Phys. Lett. 97, 242511 (2010).   

Spin coherence properties of shallow donor-bound electrons in ZnO

Defects in crystals are leading candidates for photon-based quantum technologies, but progress in developing practical devices critically depends on improving defect optical and spin properties. Motivated by this need, we study a new defect qubit candidate, the shallow donor in ZnO. We demonstrate all-optical control of the electron spin state of the donor qubits and measure the spin coherence properties. We find a longitudinal relaxation time T1 exceeding 100 ms, an inhomogeneous dephasing time T2* of 17 ns, and a Hahn spin-echo time T2 of 50 us. The magnitude of T2* is consistent with the inhomogeneity of the nuclear hyperfine field in natural ZnO. Possible mechanisms limiting T2 include instantaneous diffusion and nuclear spin diffusion (spectral diffusion). These results are comparable to the phosphorous donor system in natural silicon, suggesting that with isotope and chemical purification long qubit coherence times can be obtained for donor spins in a direct band gap semiconductor.
  

A first-principles study of the electronic structure of deterministically implanted donor arrays in silicon: multi-valley effects

Deterministically implanted donors in silicon provide an important route to develop quantum gates, analogue quantum simulators, and other atomic-scale devices [1]. We have computed the optical properties of a series of donor lines with up to 10 atoms in the sphereical-band approximation (single isotropic valley) [2]. Our calculations show charge-transfer excitations play an important role, dominating the transition for separation ~5nm and dropping down to 10 meV. We have also performed multi-valley calculations (with conduction-band anisotropy) for a donor pair and a three-donor linear cluster, which show distinct features in the excitation spectra arising from valley interaction. One consequence is to open up a gap between the ionic-state and the 1s→2p intra-atom transition [3]. Single-valley calculations can be useful to understand the excited states of valley-polarized electrons [4]. Our calculations thus provide solid theoretical foundation to many promising applications for donor arrays, including valleytronics, quantum terahertz cascade laser devices, and quantum information technology.

[1] J. Salfi, et. al., Nat. Commun. 7, 11342 (2016).
[2] Wei Wu, et. al., Phys. Rev. B, 97, 035205 (2018).
[3] Wei Wu, et. al., In preparation.
[4] J. Salfi, et. al., Nat. Mat., 13, 605 (2014).   

Creation and coherent control of Cr4+ spin ensembles in commercial silicon carbide

Optically active defect spins in solid-state materials, such as the nitrogen-vacancy center in diamond and vacancy complexes in silicon carbide (SiC), are an important resource for quantum technologies. Their electronic spins show promise as long-lived qubits with optical addressability. Recent work in 4H-SiC demonstrates the potential of transition metal defects [1]. In particular, Cr⁴⁺ has a spin-1 electronic ground state with long T₁ times at cryogenic temperatures. High zero phonon line emission (73%), and narrow inhomogeneous optical ensemble linewidths (<7 GHz) of Cr⁴⁺ enable optical spin initialization as well as readout using resonant near-infrared excitation. Here, we demonstrate that ion implantation followed by annealing at temperatures up to 1900 ^oC results in Cr⁴⁺ ensembles with spectral-hole linewidths that are an order of magnitude narrower compared to as-grown samples. Improvements in material preparation and photon collection allow for coherent control and measurement of the spin properties of Cr⁴⁺ defects, highlighting their potential for quantum information processing.

[1] W. F. Koehl, et. al. Phys. Rev. B 95, 035207 (2017)   

Electric coupling and long dephasing times of single defect spins in commercial 4H-SiC

Divacancies (VV) in silicon carbide (SiC) are a promising platform for quantum communication owing to their long-lived spin coherence [1] and high-fidelity spin-to-photon interface [2] in a wafer-scale host material. Here, we investigate the properties of single basal kh VV in commercially available 4H-SiC. We report an electronic ground-state spin dephasing time (T₂*) exceeding 60 µs for a single kh VV at 4 K, which is among the longest reported in a naturally abundant host. Furthermore, the C_1h symmetry of kh VV quenches dynamic Jahn-Teller distortions, leading to long optical coherence and excited-state energy level coupling with ac electric fields. We observe optical Rabi oscillations with coherence times approaching the lifetime limit, permitting high-visibility quantum interference of emitted photons. We demonstrate coupling between excited-state energy levels and ac electric fields through the observation of a Floquet-dressed optical spectrum. These robust spin and optical properties make the kh VV a versatile candidate for quantum information processing and hybrid system applications.

[1] Seo, H. et al. Nat. Commun. 7, 12935 (2016).
[2] Christle, D. et al. Phys. Rev. X. 7, 021046 (2017).