395 research outputs found
Transmission lines and resonators based on quantum Hall plasmonics: electromagnetic field, attenuation and coupling to qubits
Quantum Hall edge states have some characteristic features that can prove
useful to measure and control solid state qubits. For example, their high
voltage to current ratio and their dissipationless nature can be exploited to
manufacture low-loss microwave transmission lines and resonators with a
characteristic impedance of the order of the quantum of resistance . The high value of the impedance guarantees that the
voltage per photon is high and for this reason high impedance resonators can be
exploited to obtain larger values of coupling to systems with a small charge
dipole, e.g. spin qubits. In this paper, we provide a microscopic analysis of
the physics of quantum Hall effect devices capacitively coupled to external
electrodes. The electrical current in these devices is carried by edge
magnetoplasmonic excitations and by using a semiclassical model, valid for a
wide range of quantum Hall materials, we discuss the spatial profile of the
electromagnetic field in a variety of situations of interest. Also, we perform
a numerical analysis to estimate the lifetime of these excitations and, from
the numerics, we extrapolate a simple fitting formula which quantifies the
factor in quantum Hall resonators. We then explore the possibility of reaching
the strong photon-qubit coupling regime, where the strength of the interaction
is higher than the losses in the system. We compute the Coulomb coupling
strength between the edge magnetoplasmons and singlet-triplet qubits, and we
obtain values of the coupling parameter of the order ;
comparing these values to the estimated attenuation in the resonator, we find
that for realistic qubit designs the coupling can indeed be strong
Self impedance matched Hall-effect gyrators and circulators
We present a model study of an alternative implementation of a two-port
Hall-effect microwave gyrator. Our set-up involves three electrodes, one of
which acts as a common ground for the others. Based on the capacitive-coupling
model of Viola and DiVincenzo, we analyze the performance of the device and we
predict that ideal gyration can be achieved at specific frequencies.
Interestingly, the impedance of the three-terminal gyrator can be made
arbitrarily small for certain coupling strengths, so that no auxiliary
impedance matching is required. Although the bandwidth of the device shrinks as
the impedance decreases, it can be improved by reducing the magnetic field; it
can be realistically increased up to at
by working at filling factor . We examine also the effects of the
parasitic capacitive coupling between electrodes and we find that, although in
general they strongly influence the response of device, their effect is
negligible at low impedance. Finally, we analyze an interferometric
implementation of a circulator, which incorporates the gyrator in a
Mach-Zender-like construction. Perfect circulation in both directions can be
achieved, depending on frequency and on the details of the interferometer
A model study of present-day Hall-effect circulators
Stimulated by the recent implementation of a three-port Hall-effect microwave
circulator of Mahoney et al. (MEA), we present model studies of the performance
of this device. Our calculations are based on the capacitive-coupling model of
Viola and DiVincenzo (VD). Based on conductance data from a typical Hall-bar
device obtained from a two-dimensional electron gas (2DEG) in a magnetic field,
we numerically solve the coupled field-circuit equations to calculate the
expected performance of the circulator, as determined by the parameters of
the device when coupled to 50 ports, as a function of frequency and
magnetic field. Above magnetic fields of 1.5T, for which a typical 2DEG enters
the quantum Hall regime (corresponding to a Landau-level filling fraction
of 20), the Hall angle always
remains close to , and the parameters are close to the analytic
predictions of VD for . As anticipated by VD, MEA find the
device to have rather high (k) impedance, and thus to be extremely
mismatched to , requiring the use of impedance matching. We
incorporate the lumped matching circuits of MEA in our modeling and confirm
that they can produce excellent circulation, although confined to a very small
bandwidth. We predict that this bandwidth is significantly improved by working
at lower magnetic field when the Landau index is high, e.g. , and the
impedance mismatch is correspondingly less extreme. Our modeling also confirms
the observation of MEA that parasitic port-to-port capacitance can produce very
interesting countercirculation effects
Andrew Menard, Sight Unseen: How Frémont’s First Expedition Changed the American Landscape.
How did the idea of America as ‘the nation of futurity’ relate to the need of territorial expansion across the continent in the mid-19th century? How did the notion of Manifest Destiny (first formulated by the journalist John O’Sullivan) come to be firmly connected with the progress across space besides that across time? How did Americans solve the dilemma between the moral imperative (of Puritanical origin) to civilize the continent and the theoretical need (expressed by Thomas Jefferson, am..
José Barreiro, Tim Johnson, eds.America is Indian Country: Opinions and Perspectives from Indian Country Today.
Indian Country Today is—or, better, was—the former name of what is now Indian Country Today Media Network, an online website and weekly newsletter that provides Native people across North America with an easily accessible news source over a variety of topics affecting American Indian people in the U.S., from politics and business to sports and environment. Starting in 1981, Indian Country Today remained a print weekly magazine until 2013 when it went online-only, after having moved its headqu..
Transmission Lines and Meta-Materials based on Quantum Hall Plasmonics
The characteristic impedance of a microwave transmission line is typically
constrained to a value = 50 , in-part because of the low
impedance of free space and the limited range of permittivity and permeability
realizable with conventional materials. Here we suggest the possibility of
constructing high-impedance transmission lines by exploiting the plasmonic
response of edge states associated with the quantum Hall effect in gated
devices. We analyze various implementations of quantum Hall transmission lines
based on distributed networks and lumped-element circuits, including a detailed
account of parasitic capacitance and Coulomb drag effects, which can modify
device performance. We additionally conceive of a meta-material structure
comprising arrays of quantum Hall droplets and analyze its unusual properties.
The realization of such structures holds promise for efficiently wiring-up
quantum circuits on chip, as well as engineering strong coupling between
semiconductor qubits and microwave photons
Hole spin qubits in thin curved quantum wells
Hole spin qubits are frontrunner platforms for scalable quantum computers
because of their large spin-orbit interaction which enables ultrafast
all-electric qubit control at low power. The fastest spin qubits to date are
defined in long quantum dots with two tight confinement directions, when the
driving field is aligned to the smooth direction. However, in these systems the
lifetime of the qubit is strongly limited by charge noise, a major issue in
hole qubits. We propose here a different, scalable qubit design, compatible
with planar CMOS technology, where the hole is confined in a curved germanium
quantum well surrounded by silicon. This design takes full advantage of the
strong spin-orbit interaction of holes, and at the same time suppresses charge
noise in a wide range of configurations, enabling highly coherent, ultrafast
qubit gates. While here we focus on a Si/Ge/Si curved quantum well, our design
is also applicable to different semiconductors. Strikingly, these devices allow
for ultrafast operations even in short quantum dots by a transversal electric
field. This additional driving mechanism relaxes the demanding design
constraints, and opens up a new way to reliably interface spin qubits in a
single quantum dot to microwave photons. By considering state-of-the-art
high-impedance resonators and realistic qubit designs, we estimate interaction
strengths of a few hundreds of MHz, largely exceeding the decay rate of spins
and photons. Reaching such a strong coupling regime in hole spin qubits will be
a significant step towards high-fidelity entangling operations between distant
qubits, as well as fast single-shot readout, and will pave the way towards the
implementation of a large-scale semiconducting quantum processor
High-fidelity spin qubit shuttling via large spin-orbit interaction
Shuttling spins with high fidelity is a key requirement to scale up
semiconducting quantum computers, enabling qubit entanglement over large
distances and favoring the integration of control electronics on-chip. To
decouple the spin from the unavoidable charge noise, state-of-the-art spin
shuttlers try to minimize the inhomogeneity of the Zeeman field. However, this
decoupling is challenging in otherwise promising quantum computing platforms
such as hole spin qubits in silicon and germanium, characterized by a large
spin-orbit interaction and electrically-tunable qubit frequency. In this work,
we show that, surprisingly, the large inhomogeneity of the Zeeman field
stabilizes the coherence of a moving spin state, thus enabling high-fidelity
shuttling also in these systems. We relate this enhancement in fidelity to the
deterministic dynamics of the spin which filters out the dominant low-frequency
contributions of the charge noise. By simulating several different scenarios
and noise sources, we show that this is a robust phenomenon generally occurring
at large field inhomogeneity. By appropriately adjusting the motion of the
quantum dot, we also design realistic protocols enabling faster and more
coherent spin shuttling. Our findings are generally applicable to a wide range
of setups and could pave the way toward large-scale quantum processors
Hole spin qubits in Si FinFETs with fully tunable spin-orbit coupling and sweet spots for charge noise
The strong spin-orbit coupling in hole spin qubits enables fast and
electrically tunable gates, but at the same time enhances the susceptibility of
the qubit to charge noise. Suppressing this noise is a significant challenge in
semiconductor quantum computing. Here, we show theoretically that hole Si
FinFETs are not only very compatible with modern CMOS technology, but they
present operational sweet spots where the charge noise is completely removed.
The presence of these sweet spots is a result of the interplay between the
anisotropy of the material and the triangular shape of the FinFET
cross-section, and it does not require an extreme fine-tuning of the
electrostatics of the device. We present how the sweet spots appear in FinFETs
grown along different crystallographic axes and we study in detail how the
behaviour of these devices change when the cross-section area and aspect ratio
are varied. We identify designs that maximize the qubit performance and could
pave the way towards a scalable spin-based quantum computer
Anomalous zero-field splitting for hole spin qubits in Si and Ge quantum dots
An anomalous energy splitting of spin triplet states at zero magnetic field
has recently been measured in germanium quantum dots. This zero-field splitting
could crucially alter the coupling between tunnel-coupled quantum dots, the
basic building blocks of state-of-the-art spin-based quantum processors, with
profound implications for semiconducting quantum computers. We develop an
analytical model linking the zero-field splitting to spin-orbit interactions
that are cubic in momentum. Such interactions naturally emerge in hole
nanostructures, where they can also be tuned by external electric fields, and
we find them to be particularly large in silicon and germanium, resulting in a
significant zero-field splitting in the eV range. We confirm our
analytical theory by numerical simulations of different quantum dots, also
including other possible sources of zero-field splitting. Our findings are
applicable to a broad range of current architectures encoding spin qubits and
provide a deeper understanding of these materials, paving the way towards the
next generation of semiconducting quantum processors
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