73 research outputs found
Andreev Level Qubit
We investigate the dynamics of a two-level Andreev bound state system in a
transmissive quantum point contact embedded in an rf-SQUID. Coherent coupling
of the Andreev levels to the circulating supercurrent allows manipulation and
read out of the level states. The two-level Hamiltonian for the Andreev levels
is derived, and the effect of interaction with the quantum fluctuations of the
induced flux is studied. We also consider an inductive coupling of qubits, and
discuss the relevant SQUID parameters for qubit operation and read out.Comment: 4 pages, 1 figur
SegSlice: Towards a New Class of Secure Programming Primitives for Trustworthy Platforms
Abstract. The TPM is a fairly passive entity. As a result, it can be difficult to involve the TPM in measurements of software trustworthiness beyond simple load-time hashing of static program code. We suggest an approach to dynamic, runtime measurement of software trustworthiness properties as they relate to code-data owernship relationships. We outline a system, SegSlice, that actively involves the TPM in fine-grained labeling and measurement of code slices and the data that these slices operate on. SegSlice requires no changes to x86 hardware, and it relies on the relatively underused x86 segmentation mechanism to mediate access to data events.
DC current through a superconducting two-barrier system
We analyze the influence of the structure within a SNS junction on the
multiple Andreev resonances in the subgap I-V characteristics. Coherent
interference processes and incoherent propagation in the normal region are
considered. The detailed geometry of the normal region where the voltage drops
in superconducting contacts can lead to observable effects in the conductance
at low voltages.Comment: 11 pages, including 7 postscript file
Theory of AC Josepson Effect in Superconducting Constrictions
We have developed a microscopic theory of ac Josephson effect in short
ballistic superconducting constrictions with arbitrary electron transparency
and in constrictions with diffusive electron transport. The theory is valid for
arbitrary miscroscopic structure of the superconducting electrodes of the
constriction. As applications of the theory we study smearing of the subgap
current singularities by pair-breaking effects and also the structure of these
singularities in the constrictions between the composite S/N electrodes with
the proximity-induced gap in the normal layer.Comment: 11 pages, RevTex, 3 figures available on reques
Circuit theory of multiple Andreev reflections in diffusive SNS junctions: the incoherent case
The incoherent regime of Multiple Andreev Reflections (MAR) is studied in
long diffusive SNS junctions at applied voltages larger than the Thouless
energy. Incoherent MAR is treated as a transport problem in energy space by
means of a circuit theory for an equivalent electrical network. The current
through NS interfaces is explained in terms of diffusion flows of electrons and
holes through tunnel and Andreev resistors. These resistors in diffusive
junctions play roles analogous to the normal and Andreev reflection
coefficients in OTBK theory for ballistic junctions. The theory is applied to
the subharmonic gap structure (SGS); simple analytical results are obtained for
the distribution function and current spectral density for the limiting cases
of resistive and transparent NS interfaces. In the general case, the exact
solution is found in terms of chain-fractions, and the current is calculated
numerically. SGS shows qualitatively different behavior for even and odd
subharmonic numbers, and the maximum slopes of the differential resistance
correspond to the gap subharmonics. The influence of inelastic scattering on
the subgap anomalies of the differential resistance is analyzed.Comment: 14 pages, 9 figures, title and text revised, to appear in PR
ac Josephson effect in superconducting d-wave junctions
We study theoretically the ac Josephson effect in superconducting planar
d-wave junctions. The insulating barrier assumed to be present between the two
superconductors may have arbitrary strength. Many properties of this system
depend on the orientation of the d-wave superconductor: we calculate the ac
components of the Josephson current. In some arrangements there is substantial
negative differential conductance due to the presence of mid-gap states. We
study how robust these features are to finite temperature and also comment on
how the calculated current-voltage curves compare with experiments. For some
other configurations (for small barrier strength) we find zero-bias conductance
peaks due to multiple Andreev reflections through midgap states. Moreover, the
odd ac components are strongly suppressed and even absent in some arrangements.
This absence will lead to a doubling of the Josephson frequency. All these
features are due to the d-wave order parameter changing sign when rotated
. Recently, there have been several theoretical reports on parallel
current in the d-wave case for both the stationary Josephson junction and for
the normal metal-superconductor junction. Also in our case there may appear
current density parallel to the junction, and we present a few examples when
this takes place. Finally, we give a fairly complete account of the method used
and also discuss how numerical calculations should be performed in order to
produce current-voltage curves
Subharmonic Shapiro steps and assisted tunneling in superconducting point contacts
We analyze the current in a superconducting point contact of arbitrary
transmission in the presence of a microwave radiation. The interplay between
the ac Josephson current and the microwave signal gives rise to Shapiro steps
at voltages V = (m/n) \hbar \omega_r/2e, where n,m are integer numbers and
\omega_r is the radiation frequency. The subharmonic steps (n different from 1)
are a consequence of the ocurrence of multiple Andreev reflections (MAR) and
provide an unambiguous signature of the peculiar ac Josephson effect at high
transmission. Moreover, the dc current exhibits a rich subgap structure due to
photon-assisted MARs.Comment: Revtex, 4 pages, 4 figure
LMS-Verify: abstraction without regret for verified systems programming
Performance critical software is almost always developed in C, as programmers do not trust high-level languages to deliver the same reliable performance. This is bad because low-level code in unsafe languages attracts security vulnerabilities and because development is far less productive, with PL advances mostly lost on programmers operating under tight performance constraints. High-level languages provide memory safety out of the box, but they are deemed too slow and unpredictable for serious system software.
Recent years have seen a surge in staging and generative programming: the key idea is to use high-level languages and their abstraction power as glorified macro systems to compose code fragments in first-order, potentially domain-specific, intermediate languages, from which fast C can be emitted. But what about security? Since the end result is still C code, the safety guarantees of the high-level host language are lost.
In this paper, we extend this generative approach to emit ACSL specifications along with C code. We demonstrate that staging achieves ``abstraction without regret'' for verification: we show how high-level programming models, in particular higher-order composable contracts from dynamic languages, can be used at generation time to compose and generate first-order specifications that can be statically checked by existing tools. We also show how type classes can automatically attach invariants to data types, reducing the need for repetitive manual annotations.
We evaluate our system on several case studies that varyingly exercise verification of memory safety, overflow safety, and functional correctness. We feature an HTTP parser that is (1) fast (2) high-level: implemented using staged parser combinators (3) secure: with verified memory safety. This result is significant, as input parsing is a key attack vector, and vulnerabilities related to HTTP parsing have been documented in all widely-used web servers.</jats:p
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