arXiv subjects

J. Salfi

Publications and source records attributed to J. Salfi.

18 recordsLinked to original sources

arXiv2025

Nuclear spin-free 70Ge/28Si70Ge quantum well heterostructures grown on industrial SiGe-buffered wafers

The coherence of hole spin qubits in germanium planar heterostructures is limited by the hyperfine coupling to the nuclear spin bath due to 29Si and 73Ge isotopes. Thus, removing these nuclear spin-full isotopes is essential to extend the hyperfine-limited coherence times needed to implement robust quantum processors. This work demonstrates the epitaxial growth of device-grade nuclear spin-free 70Ge/28Si70Ge heterostructures on industrial SiGe buffers while minimizing the amounts of highly purified 70GeH4 and 28SiH4 used. The obtained 70Ge/28Si70Ge heterostructures exhibit a dislocation density of 5.3 x 10e6 cm-2 and an isotopic purity exceeding 99.99%, with carbon and oxygen impurities below the detection sensitivity, as revealed by atom probe tomography. Magnetotransport measurements on gated Hall bars demonstrate effective gate control of hole density in nuclear spin-free quantum wells. Negative threshold gate voltages confirm the absence of intentional doping in the wells, while Hall and Shubnikov-de Haas analyses yield consistent carrier densities (1.4 x 10e11 cm-2) and high mobilities (2.4 x 10e5 cm2/Vs). Mobility trends reveal interfacetrap- limited scattering and percolation concentration below 7 x 10e10 cm-2. These analyses, along with atomic-level studies, confirm the high quality of epitaxial 70Ge/28Si70Ge heterostructures and their relevance as a platform for long-coherence spin qubits.

cond-mat.mtrl-sci↗

arXiv2024

Cryogenic Optical-to-Microwave Conversion Using Si Photonic Integrated Circuit Ge Photodiodes

Integrated circuit technology enables the scaling of circuit complexity and functionality while maintaining manufacturability and reliability. Integration is expected to play an important role in quantum information technologies, including in the highly demanding task of producing the classical signals to control and measure quantum circuits at scales needed for fault-tolerant quantum computation. Here we experimentally characterize the cryogenic performance of a miniaturized photonic integrated circuit fabricated by a commercial foundry that down-converts classical optical signals to microwave signals. The circuit consists of waveguide-integrated germanium PIN photodiodes packaged using a scalable photonic wire bonding approach to a multi-channel optical fiber array that provides the optical excitation. We find the peak optical-to-microwave conversion response to be $\sim 150 \pm 13$ mA/W in the O-band at 4.2 K, well below the temperature the circuit was designed for and tested at in the past, for two different diode designs. The second diode design operates to over 6 GHz of 3 dB bandwidth making it suitable for controlling quantum circuits, with improvements in bandwidth and response expected from improved packaging. The demonstrated miniaturization and integration offers new perspectives for wavelength-division multiplexed control of microwave quantum circuits and scalable processors using light delivered by optical fiber arrays.

physics.optics↗

arXiv2024

Kinetic Inductance Parametric Converter

Parametric converters are parametric amplifiers that mix two spatially separate nondegenerate modes and are commonly used for amplifying and squeezing microwave signals in quantum computing and sensing. In Josephson parametric converters, the strong localized nonlinearity of the Josephson Junction limits the amplification and squeezing, as well as the dynamic range, in current devices. In contrast, a weak distributed nonlinearity can provide higher gain and dynamic range, when implemented as a kinetic inductance (KI) nanowire of a dirty superconductor, and has additional benefits such as resilience to magnetic field, higher-temperature operation, and simplified fabrication. Here, we propose, demonstrate, and analyze the performance of a KI parametric converter that relies on the weak distributed nonlinearity of a KI nanowire. The device utilizes three-wave mixing induced by a DC current bias. We demonstrate its operation as a nondegenerate parametric amplifier with high phase-sensitive gain, reaching two-mode amplification and deamplification of $\sim$30 dB for two resonances separated by 0.8 GHz, in excellent agreement with our theory of the device. We observe a dynamic range of -108~dBm at 30 dB gain. Our device can significantly broaden applications of quantum-limited signal processing devices including phase-preserving amplification and two-mode squeezing.

quant-ph↗

arXiv2022

Nonlinearity and Parametric Amplification of Superconducting Nanowire Resonators in Magnetic Field

Nonlinear superconducting devices, typically based on Josephson Junction (JJ) nonlinearities, are the basis for superconducting quantum electronics, enabling, e.g., the formation of isolated two-level superconducting qubits and amplifiers. While emerging spin, hybrid spin/superconducting (including Majorana), and nano-magneto-optical quantum systems could benefit tremendously from superconducting nonlinearities, the presence of strong magnetic fields in these systems are incompatible with conventional JJ devices, which are highly sensitive to applied magnetic fields. One potential solution is the use of kinetic inductance (KI) nonlinearity. To date, only linear kinetic inductance (KI) devices have been shown to operate in high magnetic fields, while nonlinear KI device operation in high magnetic fields has received virtually no attention. Here, we study the nonlinearity of superconducting nanowire (NW) KI resonators and their performance as parametric amplifiers for in-plane magnetic fields. We study the Kerr coefficients of NW KI resonators made from 10 nm-thin NbTiN films with characteristic impedance up to 3 k$Ω$, and demonstrate both nondegenerate and degenerate parametric amplification, at magnetic fields up to 2 T, for the first time. We find that narrow KI resonators of width 0.1 $μ$m are robust, in terms of gain, dynamic range and noise, to magnetic fields up to $\sim$2 T, while wider KI resonators of width 1 $μ$m suffer significant suppression in the gain around at fields well below 2 T. Around 8 dB deamplification is observed for coherent states for a 0.1 $μ$m KI resonator, implying the capability of noise squeezing. These results open a new pathway to developing nonlinear quantum devices that operate in or generate high magnetic fields such as spin, hybrid spin/superconducting, and magneto-opto-mechanical devices.

cond-mat.mes-hall↗

arXiv2021

Valley population of donor states in highly strained silicon

Strain is extensively used to controllably tailor the electronic properties of materials. In the context of indirect band-gap semiconductors such as silicon, strain lifts the valley degeneracy of the six conduction band minima, and by extension the valley states of electrons bound to phosphorus donors. Here, single phosphorus atoms are embedded in an engineered thin layer of silicon strained to 0.8% and their wave function imaged using spatially resolved spectroscopy. A prevalence of the out-of-plane valleys is confirmed from the real-space images, and a combination of theoretical modelling tools is used to assess how this valley repopulation effect can yield isotropic exchange and tunnel interactions in the $xy$-plane relevant for atomically precise donor qubit devices. Finally, the residual presence of in-plane valleys is evidenced by a Fourier analysis of both experimental and theoretical images, and atomistic calculations highlight the importance of higher orbital excited states to obtain a precise relationship between valley population and strain. Controlling the valley degree of freedom in engineered strained epilayers provides a new competitive asset for the development of donor-based quantum technologies in silicon.

cond-mat.mes-hall↗

arXiv2021

Novel characterisation of dopant-based qubits

Silicon is a leading qubit platform thanks to the exceptional coherence times that can be achieved and to the available commercial manufacturing platform for integration. Building scalable quantum processing architectures relies on accurate quantum state manipulation, which can only be achieved through a complete understanding of the underlying quantum state properties. This article reviews the electrical methods that have been developed to probe the quantum states encoded in individual and interacting atom qubits in silicon, from the pioneering single electron tunneling spectroscopy framework in nanoscale transistors, to radio frequency reflectometry to probe coherence properties and scanning tunneling microscopy to directly image the wave function at the atomic scale. Together with the development of atomistic simulations of realistic devices, these methods are today applied to other emerging dopant and optically addressable defect states to accelerate the engineering of quantum technologies in silicon.

cond-mat.mes-hall↗

arXiv2021

Valley interference and spin exchange at the atomic scale in silicon

Tunneling is a fundamental quantum process with no classical equivalent, which can compete with Coulomb interactions to give rise to complex phenomena. Phosphorus dopants in silicon can be placed with atomic precision to address the different regimes arising from this competition. However, they exploit wavefunctions relying on crystal band symmetries, which tunneling interactions are inherently sensitive to. Here we directly image lattice-aperiodic valley interference between coupled atoms in silicon using scanning tunneling microscopy. Our atomistic analysis unveils the role of envelope anisotropy, valley interference and dopant placement on the Heisenberg spin exchange interaction. We find that the exchange can become immune to valley interference by engineering in-plane dopant placement along specific crystallographic directions. A vacuum-like behaviour is recovered, where the exchange is maximised to the overlap between the donor orbitals, and pair-to-pair variations limited to a factor of less than 10 considering the accuracy in dopant positioning. This robustness remains over a large range of distances, from the strongly Coulomb interacting regime relevant for high-fidelity quantum computation to strongly coupled donor arrays of interest for quantum simulation in silicon.

cond-mat.mes-hall↗

arXiv2018

Engineering long spin coherence times of spin-orbit systems

Spin-orbit coupling fundamentally alters spin qubits, opening pathways to improve the scalability of quantum computers via long distance coupling mediated by electric fields, photons, or phonons. It also allows for new engineered hybrid and topological quantum systems. However, spin qubits with intrinsic spin-orbit coupling are not yet viable for quantum technologies due to their short ($\sim1~μ$s) coherence times $T_2$, while qubits with long $T_2$ have weak spin-orbit coupling making qubit coupling short-ranged and challenging for scale-up. Here we show that an intrinsic spin-orbit coupled "generalised spin" with total angular momentum $J=\tfrac{3}{2}$, which is defined by holes bound to boron dopant atoms in strained $^{28}\mathrm{Si}$, has $T_2$ rivalling the electron spins of donors and quantum dots in $^{28}\mathrm{Si}$. Using pulsed electron paramagnetic resonance, we obtain $0.9~\mathrm{ms}$ Hahn-echo and $9~\mathrm{ms}$ dynamical decoupling $T_2$ times, where strain plays a key role to reduce spin-lattice relaxation and the longitudinal electric coupling responsible for decoherence induced by electric field noise. Our analysis shows that transverse electric dipole can be exploited for electric manipulation and qubit coupling while maintaining a weak longitudinal coupling, a feature of $J=\tfrac{3}{2}$ atomic systems with a strain engineered quadrupole degree of freedom. These results establish single-atom hole spins in silicon with quantised total angular momentum, not spin, as a highly coherent platform with tuneable intrinsic spin-orbit coupling advantageous to build artificial quantum systems and couple qubits over long distances.

cond-mat.mes-hall↗

arXiv2018

Valley filtering and spatial maps of coupling between silicon donors and quantum dots

Exchange coupling is a key ingredient for spin-based quantum technologies since it can be used to entangle spin qubits and create logical spin qubits. However, the influence of the electronic valley degree of freedom in silicon on exchange interactions is presently the subject of important open questions. Here we investigate the influence of valleys on exchange in a coupled donor/quantum dot system, a basic building block of recently proposed schemes for robust quantum information processing. Using a scanning tunneling microscope tip to position the quantum dot with sub-nm precision, we find a near monotonic exchange characteristic where lattice-aperiodic modulations associated with valley degrees of freedom comprise less than 2~\% of exchange. From this we conclude that intravalley tunneling processes that preserve the donor's $\pm x$ and $\pm y$ valley index are filtered out of the interaction with the $\pm z$ valley quantum dot, and that the $\pm x$ and $\pm y$ intervalley processes where the electron valley index changes are weak. Complemented by tight-binding calculations of exchange versus donor depth, the demonstrated electrostatic tunability of donor/QD exchange can be used to compensate the remaining intravalley $\pm z$ oscillations to realise uniform interactions in an array of highly coherent donor spins.

cond-mat.mes-hall↗

arXiv2017

Towards visualisation of central-cell-effects in scanning-tunnelling-microscope images of subsurface dopant qubits in silicon

Atomic-scale understanding of phosphorous donor wave functions underpins the design and optimisation of silicon based quantum devices. The accuracy of large-scale theoretical methods to compute donor wave functions is dependent on descriptions of central-cell-corrections, which are empirically fitted to match experimental binding energies, or other quantities associated with the global properties of the wave function. Direct approaches to understanding such effects in donor wave functions are of great interest. Here, we apply a comprehensive atomistic theoretical framework to compute scanning tunnelling microscopy (STM) images of subsurface donor wave functions with two central-cell-correction formalisms previously employed in the literature. The comparison between central-cell models based on real-space image features and the Fourier transform profiles indicate that the central-cell effects are visible in the simulated STM images up to ten monolayers below the silicon surface. Our study motivates a future experimental investigation of the central-cell effects via STM imaging technique with potential of fine tuning theoretical models, which could play a vital role in the design of donor-based quantum systems in scalable quantum computer architectures.

cond-mat.mtrl-sci↗

arXiv2017

Spin-orbit dynamics of single acceptor atoms in silicon

Two-level quantum systems with strong spin-orbit coupling allow for all-electrical qubit control and long-distance qubit coupling via microwave and phonon cavities, making them of particular interest for scalable quantum information technologies. In silicon, a strong spin-orbit coupling exists within the spin-3/2 system of acceptor atoms and their energy levels and properties are expected to be highly tunable. Here we show the influence of local symmetry tuning on the acceptor spin-dynamics, measured in the single-atom regime. Spin-selective tunneling between two coupled boron atoms in a commercial CMOS transistor is utilised for spin-readout, which allows for the probing of the two-hole spin relaxation mechanisms. A relaxation-hotspot is measured and explained by the mixing of acceptor heavy and light hole states. Furthermore, excited state spectroscopy indicates a magnetic field controlled rotation of the quantization axes of the atoms. These observations demonstrate the tunability of the spin-orbit states and dynamics of this spin-3/2 system.

cond-mat.mes-hall↗

arXiv2017

Charge-insensitive single-atom spin-orbit qubit in silicon

High fidelity entanglement of an on-chip array of spin qubits poses many challenges. Spin-orbit coupling (SOC) can ease some of these challenges by enabling long-ranged entanglement via electric dipole-dipole interactions, microwave photons, or phonons. However, SOC exposes conventional spin qubits to decoherence from electrical noise. Here we propose an acceptor-based spin-orbit qubit in silicon offering long-range entanglement at a sweet spot where the qubit is protected from electrical noise. The qubit relies on quadrupolar SOC with the interface and gate potentials. As required for surface codes, $10^5$ electrically mediated single-qubit and $10^4$ dipole-dipole mediated two-qubit gates are possible in the predicted spin lifetime. Moreover, circuit quantum electrodynamics with single spins is feasible, including dispersive readout, cavity-mediated entanglement, and spin-photon entanglement. An industrially relevant silicon-based platform is employed.

cond-mat.mes-hall↗

arXiv2016

Quantum Simulation of the Hubbard Model with Dopant Atoms in Silicon

In quantum simulation, many-body phenomena are probed in controllable quantum systems. Recently, simulation of Bose-Hubbard Hamiltonians using cold atoms revealed previously hidden local correlations. However, fermionic many-body Hubbard phenomena such as unconventional superconductivity and spin liquids are more difficult to simulate using cold atoms. To date the required single-site measurements and cooling remain problematic, while only ensemble measurements have been achieved. Here we simulate a two-site Hubbard Hamiltonian at low effective temperatures with single-site resolution using subsurface dopants in silicon. We measure quasiparticle tunneling maps of spin-resolved states with atomic resolution, finding interference processes from which the entanglement entropy and Hubbard interactions are quantified. Entanglement, determined by spin and orbital degrees of freedom, increases with increasing covalent bond length. We find separation-tunable Hubbard interaction strengths that are suitable for simulating strongly correlated phenomena in larger arrays of dopants, establishing dopants as a platform for quantum simulation of the Hubbard model.

cond-mat.mes-hall↗

arXiv2015

Donor Wavefunctions in Si Gauged by STM Images

The triumph of effective mass theory in describing the energy spectrum of dopants does not guarantee that the model wavefunctions will withstand an experimental test. Such wavefunctions have recently been probed by scanning tunneling spectroscopy, revealing localized patterns of resonantly enhanced tunneling currents. We show that the shape of the conducting splotches resemble a cut through Kohn-Luttinger (KL) hydrogenic envelopes, which modulate the interfering Bloch states of conduction electrons. All the non-monotonic features of the current profile are consistent with the charge density fluctuations observed between successive {001} atomic planes, including a counterintuitive reduction of the symmetry - a heritage of the lowered point group symmetry at these planes. A model-independent analysis of the diffraction figure constrains the value of the electron wavevector to $k_0 = (0.82 \pm 0.03)(2π/a_{Si})$. Unlike prior measurements, averaged over a sizeable density of electrons, this estimate is obtained directly from isolated electrons. We further investigate the model-specific anisotropy of the wave function envelope, related to the effective mass anisotropy. This anisotropy appears in the KL variational wave function envelope as the ratio between Bohr radii b=a. We demonstrate that the central cell corrected estimates for this ratio are encouragingly accurate, leading to the conclusion that the KL theory is a valid model not only for energies but for wavefunctions as well.

cond-mat.mes-hall↗

arXiv2015

Spatially Resolving Valley Quantum Interference of a Donor in Silicon

Electron and nuclear spins of donor ensembles in isotopically pure silicon experience a vacuum-like environment, giving them extraordinary coherence. However, in contrast to a real vacuum, electrons in silicon occupy quantum superpositions of valleys in momentum space. Addressable single-qubit and two-qubit operations in silicon require that qubits are placed near interfaces, modifying the valley degrees of freedom associated with these quantum superpositions and strongly influencing qubit relaxation and exchange processes. Yet to date, spectroscopic measurements only indirectly probe wavefunctions, preventing direct experimental access to valley population, donor position, and environment. Here we directly probe the probability density of single quantum states of individual subsurface donors, in real space and reciprocal space, using scanning tunneling spectroscopy. We directly observe quantum mechanical valley interference patterns associated with linear superpositions of valleys in the donor ground state. The valley population is found to be within $5 \%$ of a bulk donor when $2.85\pm0.45$ nm from the interface, indicating that valley perturbation-induced enhancement of spin relaxation will be negligible for depths $>3$ nm. The observed valley interference will render two-qubit exchange gates sensitive to atomic-scale variations in positions of subsurface donors. Moreover, these results will also be of interest to emerging schemes proposing to encode information directly in valley polarization.

cond-mat.mes-hall↗

arXiv2015

Interface-induced heavy-hole/light-hole splitting of acceptors in silicon

The energy spectrum of spin-orbit coupled states of individual sub-surface boron acceptor dopants in silicon have been investigated using scanning tunneling spectroscopy (STS) at cryogenic temperatures. The spatially resolved tunnel spectra show two resonances which we ascribe to the heavy- and light-hole Kramers doublets. This type of broken degeneracy has recently been argued to be advantageous for the lifetime of acceptor-based qubits [Phys. Rev. B 88 064308 (2013)]. The depth dependent energy splitting between the heavy- and light-hole Kramers doublets is consistent with tight binding calculations, and is in excess of 1 meV for all acceptors within the experimentally accessible depth range (< 2 nm from the surface). These results will aid the development of tunable acceptor-based qubits in silicon with long coherence times and the possibility for electrical manipulation.

cond-mat.mes-hall↗

arXiv2015

Spatially resolved resonant tunneling on single atoms in silicon

The ability to control single dopants in solid-state devices has opened the way towards reliable quantum computation schemes. In this perspective it is essential to understand the impact of interfaces and electric fields, inherent to address coherent electronic manipulation, on the dopants atomic scale properties. This requires both fine energetic and spatial resolution of the energy spectrum and wave-function, respectively. Here we present an experiment fulfilling both conditions: we perform transport on single donors in silicon close to a vacuum interface using a scanning tunneling microscope (STM) in the single electron tunneling regime. The spatial degrees of freedom of the STM tip provide a versatility allowing a unique understanding of electrostatics. We obtain the absolute energy scale from the thermal broadening of the resonant peaks, allowing to deduce the charging energies of the donors. Finally we use a rate equations model to derive the current in presence of an excited state, highlighting the benefits of the highly tunable vacuum tunnel rates which should be exploited in further experiments. This work provides a general framework to investigate dopant-based systems at the atomic scale.

cond-mat.mes-hall↗

arXiv2013

Interplay between quantum confinement and dielectric mismatch for ultra-shallow dopants

Understanding the electronic properties of dopants near an interface is a critical challenge for nano-scale devices. We have determined the effect of dielectric mismatch and quantum confinement on the ionization energy of individual acceptors beneath a hydrogen passivated silicon (100) surface. Whilst dielectric mismatch between the vacuum and the silicon at the interface results in an image charge which enhances the binding energy of sub-surface acceptors, quantum confinement is shown to reduce the binding energy. Using scanning tunneling spectroscopy we measure resonant transport through the localized states of individual acceptors. Thermal broadening of the conductance peaks provides a direct measure for the absolute energy scale. Our data unambiguously demonstrates that these two independent effects compete with the result that the ionization energy is less than 5 meV lower than the bulk value for acceptors less than a Bohr radius from the interface.

cond-mat.mes-hall↗