Quantum Control
Atom-scale impurities in solids can trap individual electrons, forming analogs of molecules in free space. Particularly in wide-bandgap semiconductors like diamond, the quantum coherence of such trapped electron spins can be remarkably long lived, even at room temperature, and individual defect spins can be controlled using optics and electronics. A prime example is the nitrogen-vacancy (NV) center in diamond, which shows promise as an optically-addressable quantum bit in future technologies for quantum information processing and nanoscale sensing. Harnessing the versatility of semiconductor fabrication technology, we aim to realize practical implementations of quantum technologies based on such systems.
 | Hopper, D A; Lauigan, J D; Huang, T -Y; Bassett, L C Real-Time Charge Initialization of Diamond Nitrogen-Vacancy Centers for Enhanced Spin Readout Journal Article Phys. Rev. Applied, 13 , pp. 024016, 2020. Abstract | Links | BibTeX @article{Hopper2019,
title = {Real-Time Charge Initialization of Diamond Nitrogen-Vacancy Centers for Enhanced Spin Readout},
author = {D A Hopper and J D Lauigan and T -Y Huang and L C Bassett},
url = {https://arxiv.org/abs/1907.08741
https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.13.024016
https://medium.com/penn-engineering/penn-engineers-ensure-quantum-experiments-get-off-to-the-right-start-bbe6f3382cb},
year = {2020},
date = {2020-02-07},
journal = {Phys. Rev. Applied},
volume = {13},
pages = {024016},
abstract = {Selected as an Editor's Suggestion.
A common impediment to qubit performance is imperfect state initialization. In the case of the diamond nitrogen-vacancy (NV) center, the initialization fidelity is limited by fluctuations in the defect's charge state during optical pumping. Here, we use real-time control to deterministically initialize the NV center's charge state at room temperature. We demonstrate a maximum charge initialization fidelity of 99.4±0.1% and present a quantitative model of the initialization process that allows for systems-level optimization of the spin-readout signal-to-noise ratio. Even accounting for the overhead associated with the initialization sequence, increasing the charge initialization fidelity from the steady-state value of 75% near to unity allows for a factor-of-two speedup in experiments while maintaining the same signal-to-noise-ratio. In combination with high-fidelity readout based on spin-to-charge conversion, real-time initialization enables a factor-of-20 speedup over traditional methods, resulting in an ac magnetic sensitivity of 1.3 nT/Hz1/2 for our single NV-center spin. The real-time control method is immediately beneficial for quantum sensing applications with NV centers as well as probing charge-dependent physics, and it will facilitate protocols for quantum feedback control over multi-qubit systems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
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Selected as an Editor's Suggestion.
A common impediment to qubit performance is imperfect state initialization. In the case of the diamond nitrogen-vacancy (NV) center, the initialization fidelity is limited by fluctuations in the defect's charge state during optical pumping. Here, we use real-time control to deterministically initialize the NV center's charge state at room temperature. We demonstrate a maximum charge initialization fidelity of 99.4±0.1% and present a quantitative model of the initialization process that allows for systems-level optimization of the spin-readout signal-to-noise ratio. Even accounting for the overhead associated with the initialization sequence, increasing the charge initialization fidelity from the steady-state value of 75% near to unity allows for a factor-of-two speedup in experiments while maintaining the same signal-to-noise-ratio. In combination with high-fidelity readout based on spin-to-charge conversion, real-time initialization enables a factor-of-20 speedup over traditional methods, resulting in an ac magnetic sensitivity of 1.3 nT/Hz1/2 for our single NV-center spin. The real-time control method is immediately beneficial for quantum sensing applications with NV centers as well as probing charge-dependent physics, and it will facilitate protocols for quantum feedback control over multi-qubit systems. |
 | Hopper, D A; Shulevitz, H J; Bassett, L C Spin readout techniques of the nitrogen-vacancy center in diamond Journal Article Micromachines, 9 , pp. 437, 2018. Abstract | Links | BibTeX @article{Hopper2018,
title = {Spin readout techniques of the nitrogen-vacancy center in diamond},
author = {D A Hopper and H J Shulevitz and L C Bassett},
url = {https://www.mdpi.com/2072-666X/9/9/437},
year = {2018},
date = {2018-08-30},
journal = {Micromachines},
volume = {9},
pages = {437},
abstract = {The diamond nitrogen-vacancy (NV) center is a leading platform for quantum information science due to its optical addressability and room-temperature spin coherence. However, measurements of the NV center’s spin state typically require averaging over many cycles to overcome noise. Here, we review several approaches to improve the readout performance and highlight future avenues of research that could enable single-shot electron-spin readout at room temperature.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The diamond nitrogen-vacancy (NV) center is a leading platform for quantum information science due to its optical addressability and room-temperature spin coherence. However, measurements of the NV center’s spin state typically require averaging over many cycles to overcome noise. Here, we review several approaches to improve the readout performance and highlight future avenues of research that could enable single-shot electron-spin readout at room temperature. |
Sensing and Imaging
Highly localized, optically controllable spins can serve as remote sensors of their nanoscale environment, providing detailed information about their local conditions such as magnetic and electric fields, crystal strain, and temperature. This capability suggests many exciting applications, particularly in materials science, chemistry, and biology, where the ability to monitor fields and their dynamics on nanometer length scales could lead to breakthroughs in understanding protein dynamics, cellular communication, and complex quantum materials. We are developing quantum sensing applications and integrated quantum sensing platforms based on spins in solids.
 | Hopper, D A; Grote, R R; Parks, S M; Bassett, L C Amplified sensitivity of nitrogen-vacancy spins in nanodiamonds using all-optical charge readout Journal Article ACS Nano, 12 , pp. 4678-4686, 2018. Abstract | Links | BibTeX @article{Hopper2018b,
title = {Amplified sensitivity of nitrogen-vacancy spins in nanodiamonds using all-optical charge readout},
author = {D A Hopper and R R Grote and S M Parks and L C Bassett},
url = {https://pubs.acs.org/doi/10.1021/acsnano.8b01265
https://arxiv.org/abs/1712.03882},
year = {2018},
date = {2018-04-13},
journal = {ACS Nano},
volume = {12},
pages = {4678-4686},
abstract = {Nanodiamonds containing nitrogen-vacancy (NV) centers offer a versatile platform for sensing applications spanning from nanomagnetism to in vivo monitoring of cellular processes. In many cases, however, weak optical signals and poor contrast demand long acquisition times that prevent the measurement of environmental dynamics. Here, we demonstrate the ability to perform fast, high-contrast optical measurements of charge distributions in ensembles of NV centers in nanodiamonds and use the technique to improve the spin-readout signal-to-noise ratio through spin-to-charge conversion. A study of 38 nanodiamonds with sizes ranging between 20 and 70 nm, each hosting a small ensemble of NV centers, uncovers complex, multiple time scale dynamics due to radiative and nonradiative ionization and recombination processes. Nonetheless, the NV-containing nanodiamonds universally exhibit charge-dependent photoluminescence contrasts and the potential for enhanced spin readout using spin-to-charge conversion. We use the technique to speed up a T1 relaxometry measurement by a factor of 5.},
keywords = {},
pubstate = {published},
tppubtype = {article}
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Nanodiamonds containing nitrogen-vacancy (NV) centers offer a versatile platform for sensing applications spanning from nanomagnetism to in vivo monitoring of cellular processes. In many cases, however, weak optical signals and poor contrast demand long acquisition times that prevent the measurement of environmental dynamics. Here, we demonstrate the ability to perform fast, high-contrast optical measurements of charge distributions in ensembles of NV centers in nanodiamonds and use the technique to improve the spin-readout signal-to-noise ratio through spin-to-charge conversion. A study of 38 nanodiamonds with sizes ranging between 20 and 70 nm, each hosting a small ensemble of NV centers, uncovers complex, multiple time scale dynamics due to radiative and nonradiative ionization and recombination processes. Nonetheless, the NV-containing nanodiamonds universally exhibit charge-dependent photoluminescence contrasts and the potential for enhanced spin readout using spin-to-charge conversion. We use the technique to speed up a T1 relaxometry measurement by a factor of 5. |
New Materials
Point defects in wide bandgap semiconductors, like the NV center in diamond, have emerged as leading platforms for quantum technologies, providing optically and electronically addressable spin states that are robust to environmental noise. However, identification of new defect systems in new host materials is generally a slow, ad hoc process. Because the desired operating parameters for these defect systems are application-specific and depend heavily on the materials involved, we are focused on combining new computational and experimental techniques to accelerate the discovery of defect-host systems that are optimized for spin-light quantum interfaces.
Quantum-mechanical effects become progressively more important in materials with reduced dimensions, offering both a challenge for the continued miniaturization of traditional electronics and an opportunity for developing new quantum technologies. Our group combines optical and electrical techniques to study new materials systems in both bulk and low-dimensional (0D, 1D, 2D) samples, particularly in semiconducting “beyond-carbon” materials such as boron nitride. These emerging materials offer many of the appealing features of their carbon counterparts but with additional advantages, particularly for optical measurements. We are interested in both the fundamental physics of these materials and their potential as electrical & optical components, sensors, and in quantum information technology.
 | Breitweiser, S A; Exarhos, A L; Patel, R N; Saouaf, J; Porat, B; Hopper, D A; Bassett, L C Efficient optical quantification of heterogeneous emitter ensembles Journal Article ACS Photonics, 7 , pp. 288-295, 2019. Abstract | Links | BibTeX @article{Breitweiser2019,
title = {Efficient optical quantification of heterogeneous emitter ensembles},
author = {S A Breitweiser and A L Exarhos and R N Patel and J Saouaf and B Porat and D A Hopper and L C Bassett},
url = {https://arxiv.org/abs/1909.12726
https://pubs.acs.org/doi/10.1021/acsphotonics.9b01707},
year = {2019},
date = {2019-12-17},
journal = {ACS Photonics},
volume = {7},
pages = {288-295},
abstract = {Defect-based quantum emitters in solid state materials offer a promising platform for quantum communication and sensing. Confocal fluorescence microscopy techniques have revealed quantum emitters in a multitude of host materials. In some materials, however, optical properties vary widely between emitters, even within the same sample. In these cases, traditional ensemble fluorescence measurements are confounded by heterogeneity, whereas individual defect-by-defect studies are impractical. Here, we develop a method to quantitatively and systematically analyze the properties of heterogeneous emitter ensembles using large-area photoluminescence maps. We apply this method to study the effects of sample treatments on emitters in hexagonal boron nitride, and we find that low-energy (3 keV) electron irradiation creates emitters, whereas high-temperature (850 ∘C) annealing in an inert gas environment brightens emitters.},
keywords = {},
pubstate = {published},
tppubtype = {article}
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Defect-based quantum emitters in solid state materials offer a promising platform for quantum communication and sensing. Confocal fluorescence microscopy techniques have revealed quantum emitters in a multitude of host materials. In some materials, however, optical properties vary widely between emitters, even within the same sample. In these cases, traditional ensemble fluorescence measurements are confounded by heterogeneity, whereas individual defect-by-defect studies are impractical. Here, we develop a method to quantitatively and systematically analyze the properties of heterogeneous emitter ensembles using large-area photoluminescence maps. We apply this method to study the effects of sample treatments on emitters in hexagonal boron nitride, and we find that low-energy (3 keV) electron irradiation creates emitters, whereas high-temperature (850 ∘C) annealing in an inert gas environment brightens emitters. |
 | Exarhos, A L; Hopper, D A; Patel, R N; Doherty, M W; Bassett, L C Magnetic-field-dependent quantum emission in hexagonal boron nitride at room temperature Journal Article Nature Communications, 10 (222), 2019. Abstract | Links | BibTeX @article{Exarhos2019,
title = {Magnetic-field-dependent quantum emission in hexagonal boron nitride at room temperature},
author = {A L Exarhos and D A Hopper and R N Patel and M W Doherty and L C Bassett},
url = {https://www.nature.com/articles/s41467-018-08185-8
https://www.nature.com/collections/rcdhyvxytb
https://spectrum.ieee.org/tech-talk/semiconductors/nanotechnology/qubits-and-nanosensors-in-a-2d-material
https://medium.com/penn-engineering/penn-engineers-develop-room-temperature-two-dimensional-platform-for-quantum-technology-cae3a5c0d8f9},
year = {2019},
date = {2019-01-15},
journal = {Nature Communications},
volume = {10},
number = {222},
abstract = {Selected as Editor's Highlight (linked)
Press coverage in IEEE Spectrum and Penn Medium (linked)
Optically addressable spins associated with defects in wide-bandgap semiconductors are versatile platforms for quantum information processing and nanoscale sensing, where spin-dependent inter-system crossing transitions facilitate optical spin initialization and readout. Recently, the van der Waals material hexagonal boron nitride (h-BN) has emerged as a robust host for quantum emitters, promising efficient photon extraction and atom-scale engineering, but observations of spin-related effects have remained thus far elusive. Here, we report room-temperature observations of strongly anisotropic photoluminescence patterns as a function of applied magnetic field for select quantum emitters in h-BN. Field-dependent variations in the steady-state photoluminescence and photon emission statistics are consistent with an electronic model featuring a spin-dependent inter-system crossing between triplet and singlet manifolds, indicating that optically-addressable spin defects are present in h-BN.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Selected as Editor's Highlight (linked)
Press coverage in IEEE Spectrum and Penn Medium (linked)
Optically addressable spins associated with defects in wide-bandgap semiconductors are versatile platforms for quantum information processing and nanoscale sensing, where spin-dependent inter-system crossing transitions facilitate optical spin initialization and readout. Recently, the van der Waals material hexagonal boron nitride (h-BN) has emerged as a robust host for quantum emitters, promising efficient photon extraction and atom-scale engineering, but observations of spin-related effects have remained thus far elusive. Here, we report room-temperature observations of strongly anisotropic photoluminescence patterns as a function of applied magnetic field for select quantum emitters in h-BN. Field-dependent variations in the steady-state photoluminescence and photon emission statistics are consistent with an electronic model featuring a spin-dependent inter-system crossing between triplet and singlet manifolds, indicating that optically-addressable spin defects are present in h-BN. |
Quantum Photonics
The atomic scales and high refractive index environments of solid state quantum emitters prevent their detection without bulky free-space optical setups. Condensed optical tools will be required to replace these traditional setups in integrated devices that rely on solid state quantum emitters. An example of such a tool is the immersion metalens that our lab has fabricated and is now improving through fabrication-constrained inverse design techniques.
 | Huang, T -Y; Grote, R R; Mann, S A; Hopper, D A; Exarhos, A L; Lopez, G G; Klein, A R; Garnett, E C; Bassett, L C Imaging a nitrogen-vacancy center with a diamond immersion metalens Journal Article Nature Communications, 10 (2392), 2019. Abstract | Links | BibTeX @article{Huang2019,
title = {Imaging a nitrogen-vacancy center with a diamond immersion metalens},
author = {T -Y Huang and R R Grote and S A Mann and D A Hopper and A L Exarhos and G G Lopez and A R Klein and E C Garnett and L C Bassett},
url = {https://www.nature.com/articles/s41467-019-10238-5
https://medium.com/penn-engineering/penn-engineers-design-nanostructured-diamond-metalens-for-compact-quantum-technologies-271adddf69ba},
year = {2019},
date = {2019-06-03},
journal = {Nature Communications},
volume = {10},
number = {2392},
abstract = {Quantum emitters such as the diamond nitrogen-vacancy (NV) center are the basis for a wide range of quantum technologies. However, refraction and reflections at material interfaces impede photon collection, and the emitters’ atomic scale necessitates the use of free space optical measurement setups that prevent packaging of quantum devices. To overcome these limitations, we design and fabricate a metasurface composed of nanoscale diamond pillars that acts as an immersion lens to collect and collimate the emission of an individual NV center. The metalens exhibits a numerical aperture greater than 1.0, enabling efficient fiber-coupling of quantum emitters. This flexible design will lead to the miniaturization of quantum devices in a wide range of host materials and the development of metasurfaces that shape single-photon emission for coupling to optical cavities or route photons based on their quantum state.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Quantum emitters such as the diamond nitrogen-vacancy (NV) center are the basis for a wide range of quantum technologies. However, refraction and reflections at material interfaces impede photon collection, and the emitters’ atomic scale necessitates the use of free space optical measurement setups that prevent packaging of quantum devices. To overcome these limitations, we design and fabricate a metasurface composed of nanoscale diamond pillars that acts as an immersion lens to collect and collimate the emission of an individual NV center. The metalens exhibits a numerical aperture greater than 1.0, enabling efficient fiber-coupling of quantum emitters. This flexible design will lead to the miniaturization of quantum devices in a wide range of host materials and the development of metasurfaces that shape single-photon emission for coupling to optical cavities or route photons based on their quantum state. |
 | Grote, R R; Bassett, L C Single-mode optical waveguides on native high-refractive-index substrates Journal Article APL Photonics, 1 , pp. 071302, 2016. Abstract | Links | BibTeX @article{Grote2016,
title = {Single-mode optical waveguides on native high-refractive-index substrates},
author = {R R Grote and L C Bassett},
url = {https://aip.scitation.org/doi/10.1063/1.4955065},
year = {2016},
date = {2016-08-01},
journal = {APL Photonics},
volume = {1},
pages = {071302},
abstract = {High-refractive-index semiconductor optical waveguides form the basis for modern photonic integrated circuits (PICs). However, conventional methods for achieving optical confinement require a thick lower-refractive-index support layer that impedes large-scale co-integration with electronics and limits the materials on which PICs can be fabricated. To address this challenge, we present a general architecture for single-mode waveguides that confine light in a high-refractive-index material on a native substrate. The waveguide consists of a high-aspect-ratio fin of the guiding material surrounded by lower-refractive-index dielectrics and is compatible with standard top-down fabrication techniques. This letter describes a physically intuitive, semi-analytical, effective index model for designing fin waveguides, which is confirmed with fully vectorial numerical simulations. Design examples are presented for diamond and silicon at visible and telecommunications wavelengths, respectively, along with calculations of propagation loss due to bending, scattering, and substrate leakage. Potential methods of fabrication are also discussed. The proposed waveguide geometry allows PICs to be fabricated alongside silicon CMOS electronics on the same wafer, removes the need for heteroepitaxy in III-V PICs, and will enable wafer-scale photonic integration on emerging material platforms such as diamond and SiC.},
keywords = {},
pubstate = {published},
tppubtype = {article}
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High-refractive-index semiconductor optical waveguides form the basis for modern photonic integrated circuits (PICs). However, conventional methods for achieving optical confinement require a thick lower-refractive-index support layer that impedes large-scale co-integration with electronics and limits the materials on which PICs can be fabricated. To address this challenge, we present a general architecture for single-mode waveguides that confine light in a high-refractive-index material on a native substrate. The waveguide consists of a high-aspect-ratio fin of the guiding material surrounded by lower-refractive-index dielectrics and is compatible with standard top-down fabrication techniques. This letter describes a physically intuitive, semi-analytical, effective index model for designing fin waveguides, which is confirmed with fully vectorial numerical simulations. Design examples are presented for diamond and silicon at visible and telecommunications wavelengths, respectively, along with calculations of propagation loss due to bending, scattering, and substrate leakage. Potential methods of fabrication are also discussed. The proposed waveguide geometry allows PICs to be fabricated alongside silicon CMOS electronics on the same wafer, removes the need for heteroepitaxy in III-V PICs, and will enable wafer-scale photonic integration on emerging material platforms such as diamond and SiC. |
