40 research outputs found
Full control of qubit rotations in a voltage-biased superconducting flux qubit
We study a voltage-controlled version of the superconducting flux qubit
[Chiorescu et al., Science 299, 1869 (2003)] and show that full control of
qubit rotations on the entire Bloch sphere can be achieved. Circuit graph
theory is used to study a setup where voltage sources are attached to the two
superconducting islands formed between the three Josephson junctions in the
flux qubit. Applying a voltage allows qubit rotations about the y axis, in
addition to pure x and z rotations obtained in the absence of applied voltages.
The orientation and magnitude of the rotation axis on the Bloch sphere can be
tuned by the gate voltages, the external magnetic flux, and the ratio alpha
between the Josephson energies via a flux-tunable junction. We compare the
single-qubit control in the known regime alpha<1 with the unexplored range
alpha>1 and estimate the decoherence due to voltage fluctuations.Comment: 12 pages, 12 figures, 1 tabl
Theory of integer quantum Hall polaritons in graphene
We present a theory of the cavity quantum electrodynamics of the graphene
cyclotron resonance. By employing a canonical transformation, we derive an
effective Hamiltonian for the system comprised of two neighboring Landau levels
dressed by the cavity electromagnetic field (integer quantum Hall polaritons).
This generalized Dicke Hamiltonian, which contains terms that are quadratic in
the electromagnetic field and respects gauge invariance, is then used to
calculate thermodynamic properties of the quantum Hall polariton system.
Finally, we demonstrate that the generalized Dicke description fails when the
graphene sheet is heavily doped, i.e. when the Landau level spectrum of 2D
massless Dirac fermions is approximately harmonic. In this case we `integrate
out' the Landau levels in valence band and obtain an effective Hamiltonian for
the entire stack of Landau levels in conduction band, as dressed by strong
light-matter interactions.Comment: 20 pages, 7 figure
Strain-induced bound states in transition-metal dichalcogenide bubbles
This is an author-created, un-copyedited version of an article published in 2D Materials. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/2053-1583/ab0113We theoretically study the formation of single-particle bound states confined by strain at the center of bubbles in monolayers of transition-metal dichalcogenides (TMDs). Bubbles ubiquitously form in two-dimensional crystals on top of a substrate by the competition between van der Waals forces and the hydrostatic pressure exerted by trapped fluid. This leads to strong strain at the center of the bubble that reduces the bangap locally, creating potential wells for the electrons that confine states inside. We simulate the spectrum versus the bubble radius for the four semiconducting group VI TMDs, MoS2, WSe2, WS2 and MoSe2, and find an overall Fock-Darwin spectrum of bubble bound states, characterised by small deviations compatible with Berry curvature effects. We analyse the density of states, the state degeneracies, orbital structure and optical transition rules. Our results show that elastic bubbles in these materials are remarkably efficient at confining photocarriersWe acknowledge funding from the Graphene Flagship, contract CNECTICT-604391, from the Comunidad de Madrid through Grant MAD2D-CM, S2013/MIT-3007, from the Spanish Ministry of Economy and Competitiveness through Grants No. RYC-2011-09345, RYC-2016-20663, FIS2015-65706-P, FIS2016-80434-P (AEI/FEDER, EU) and the MarÃa de Maeztu Programme for Units of Excellence in R&D (MDM-2014-0377
Electronic implementations of Interaction-Free Measurements
Three different implementations of interaction-free measurements (IFMs) in
solid-state nanodevices are discussed. The first one is based on a series of
concatenated Mach-Zehnder interferometers, in analogy to optical-IFM setups.
The second one consists of a single interferometer and concatenation is
achieved in the time domain making use of a quantized electron emitter. The
third implementation consists of an asymmetric Aharonov-Bohm ring. For all
three cases we show that the presence of a dephasing source acting on one arm
of the interferometer can be detected without degrading the coherence of the
measured current. Electronic implementations of IFMs in nanoelectronics may
play a fundamental role as very accurate and noninvasive measuring schemes for
quantum devices.Comment: 12 pages, 10 figure
Coherent edge mixing and interferometry in quantum Hall bilayers
We discuss the implementation of a beam splitter for electron waves in a quantum Hall bilayer. Our architecture exploits interlayer tunneling to mix edge states belonging to different layers. We discuss the basic working principle of the proposed coherent edge mixer, possible interferometric implementations based on existing semiconductor-heterojunction technologies, and advantages with respect to canonical quantum Hall interferometers based on quantum point contacts
Coherent detection of electron dephasing
We show that an Aharonov-Bohm (AB) ring with asymmetric electron injection
can act as a coherent detector of electron dephasing. The presence of a
dephasing source in one of the two arms of a moderately-to-highly asymmetric
ring changes the response of the system from total reflection to complete
transmission while preserving the coherence of the electrons propagating from
the ring, even for strong dephasing. We interpret this phenomenon as an
implementation of an interaction-free measurement.Comment: 4 pages, 3 figure