584 research outputs found
Joint Entanglement of Topology and Polarization Enables Error-Protected Quantum Registers
Linear-optical systems can implement photonic quantum walks that simulate
systems with nontrivial topological properties. Here, such photonic walks are
used to jointly entangle polarization and winding number. This joint
entanglement allows information processing tasks to be performed with
interactive access to a wide variety of topological features. Topological
considerations are used to suppress errors, with polarization allowing easy
measurement and manipulation of qubits. We provide three examples of this
approach: production of two-photon systems with entangled winding number
(including topological analogs of Bell states), a topologically error-protected
optical memory register, and production of entangled topologicallyprotected
boundary states. In particular it is shown that a pair of quantum memory
registers, entangled in polarization and winding number, with
topologically-assisted error suppression can be made with qubits stored in
superpositions of winding numbers; as a result, information processing with
winding number-based qubits is a viable possibility
Directionally-unbiased unitary optical devices in discrete-time quantum walks
The optical beam splitter is a widely-used device in photonics-based quantum information processing. Specifically, linear optical networks demand large numbers of beam splitters for unitary matrix realization. This requirement comes from the beam splitter property that a photon cannot go back out of the input ports, which we call “directionally-biased”. Because of this property, higher dimensional information processing tasks suffer from rapid device resource growth when beam splitters are used in a feed-forward manner. Directionally-unbiased linear-optical devices have been introduced recently to eliminate the directional bias, greatly reducing the numbers of required beam splitters when implementing complicated tasks. Analysis of some originally directional optical devices and basic principles of their conversion into directionally-unbiased systems form the base of this paper. Photonic quantum walk implementations are investigated as a main application of the use of directionally-unbiased systems. Several quantum walk procedures executed on graph networks constructed using directionally-unbiased nodes are discussed. A significant savings in hardware and other required resources when compared with traditional directionally-biased beam-splitter-based optical networks is demonstrated.Accepted manuscriptPublished versio
Experimental demonstration of a directionally-unbiased linear-optical multiport
All existing optical quantum walk approaches are based on the use of
beamsplitters and multiple paths to explore the multitude of unitary
transformations of quantum amplitudes in a Hilbert space. The beamsplitter is
naturally a directionally biased device: the photon cannot travel in reverse
direction. This causes rapid increases in optical hardware resources required
for complex quantum walk applications, since the number of options for the
walking particle grows with each step. Here we present the experimental
demonstration of a directionally-unbiased linear-optical multiport, which
allows reversibility of photon direction. An amplitude-controllable probability
distribution matrix for a unitary three-edge vertex is reconstructed with only
linear-optical devices. Such directionally-unbiased multiports allow direct
execution of quantum walks over a multitude of complex graphs and in tensor
networks. This approach would enable simulation of complex Hamiltonians of
physical systems and quantum walk applications in a more efficient and compact
setup, substantially reducing the required hardware resources
Quantum simulation of topologically protected states using directionally unbiased linear-optical multiports
It is shown that quantum walks on one-dimensional arrays of special
linear-optical units allow the simulation of discrete-time Hamiltonian systems
with distinct topological phases. In particular, a slightly modified version of
the Su-Schrieffer-Heeger (SSH) system can be simulated, which exhibits states
of nonzero winding number and has topologically protected boundary states. In
the large-system limit this approach uses quadratically fewer resources to
carry out quantum simulations than previous linear-optical approaches and can
be readily generalized to higher-dimensional systems. The basic optical units
that implement this simulation consist of combinations of optical multiports
that allow photons to reverse direction
Quantum simulation of discrete-time Hamiltonians using directionally unbiased linear optical multiports
Recently, a generalization of the standard optical multiport was proposed [Phys. Rev. A 93, 043845 (2016)]. These directionally unbiased multiports allow photons to reverse direction and exit backwards from the input port, providing a realistic linear optical scattering vertex for quantum walks on arbitrary graph structures. Here, it is shown that arrays of these multiports allow the simulation of a range of discrete-time Hamiltonian systems. Examples are described, including a case where both spatial and internal degrees of freedom are simulated. Because input ports also double as output ports, there is substantial savings of resources compared to feed-forward networks carrying out the same functions. The simulation is implemented in a scalable manner using only linear optics, and can be generalized to higher dimensional systems in a straightforward fashion, thus offering a concrete experimentally achievable implementation of graphical models of discrete-time quantum systems.This research was supported by the National Science Foundation EFRI-ACQUIRE Grant No. ECCS-1640968, NSF Grant No. ECCS-1309209, and by the Northrop Grumman NG Next. (ECCS-1640968 - National Science Foundation EFRI-ACQUIRE Grant; ECCS-1309209 - NSF Grant; Northrop Grumman NG Next
Magneto-optical properties of a new group-IV ferromagnetic semiconductor Ge1-xFex grown by low-temperature molecular beam epitaxy
A new group-IV ferromagnetic semiconductor, Ge1-xFex, was successfully grown
by low-temperature molecular beam epitaxy (LT-MBE) without precipitation of
ferromagnetic Ge-Fe intermetallic compounds. The ferromagnetism of Ge1-xFex
films was investigated by magnetic circular dichroism (MCD). In particular, the
influence of the Fe content (FFe/FGe =1 - 10%) and growth temperature (100,
200OC) on the ferromagnetism was carefully studied. The MCD measurements
revealed that the band structure of the Ge1-xFex films was identical with that
of bulk Ge, and that the large spin splitting of the band structure was induced
by the incorporation of Fe atoms into the Ge matrix, indicating the existence
of s,p-d exchange interactions. The Ge1-xFex films showed ferromagnetic
behavior and the ferromagnetic transition temperature linearly increased with
increasing the Fe composition. These results indicate that the epitaxially
grown Ge1-xFex is an intrinsic ferromagnetic semiconductor.Comment: 15 pages, 4 figures. to appear in J. Appl. Phy
Holographic Techni-dilaton
Techni-dilaton, a pseudo-Nambu-Goldstone boson of scale symmetry, was
predicted long ago in the Scale-invariant/Walking/Conformal Technicolor
(SWC-TC) as a remnant of the (approximate) scale symmetry associated with the
conformal fixed point, based on the conformal gauge dynamics of ladder
Schwinger-Dyson (SD) equation with non-running coupling. We study the
techni-dilaton as a flavor-singlet bound state of techni-fermions by including
the techni-gluon condensate (tGC) effect into the previous (bottom-up)
holographic approach to the SWC-TC, a deformation of the holographic QCD with
by large anomalous dimension . With
including a bulk scalar field corresponding to the gluon condensate, we first
improve the Operator Product Expansion of the current correlators so as to
reproduce gluonic term both in QCD and SWC-TC. We find in QCD about
(negative) contribution of gluon condensate to the meson mass. We
also calculate the oblique electroweak -parameter in the presence of the
effect of the tGC and find that for the fixed value of the tGC effects
dramatically reduce the flavor-singlet scalar (techni-dilaton) mass (in the unit of ), while the vector and axial-vector masses
and are rather insensitive to the tGC, where is the
decay constant of the techni-pion. If we use the range of values of tGC implied
by the ladder SD analysis of the non-perturbative scale anomaly in the large
QCD near the conformal window, the phenomenological constraint predicts the techni-dilaton mass GeV which is within
reach of LHC discovery.Comment: 28 pages, 11 eps files, typos corrected, references added, Fig.1
corrected, some discussions added, to be published in PR
A note on the coupling of the techni-dilaton to the weak bosons
In this note, we study the coupling of the techni-dilaton to the weak bosons.
We consider two cases: (1) The dilaton directly couples to the weak bosons
similarly to the SM. (2) The coupling in question is effectively induced only
through the techni-fermion loops. In both cases, we find that the coupling is
essentially determined by the mass-squared of the weak bosons over the dilaton
decay constant.Comment: 3 pages, 2 figures; minor changes, a reference added, to appear in
PR
Techni-dilaton at Conformal Edge
Techni-dilaton (TD) was proposed long ago in the technicolor (TC) near
criticality/conformality. To reveal the critical behavior of TD, we explicitly
compute the nonperturbative contributions to the scale anomaly
, which
are generated by the dynamical mass m of the techni-fermions. Our computation
is based on the (improved) ladder Schwinger-Dyson equation, with the gauge
coupling replaced by the two-loop running one having the
Caswell-Banks-Zaks IR fixed point : for the IR region , where is
the intrinsic scale (analogue of of QCD) relevant to the
perturbative scale anomaly. We find that
and in the
criticality limit () ("conformal edge"). Our result precisely
reproduces the formal identity , where is the nonperturbative beta function
corresponding to the above essential singularity scaling of .
Accordingly, the PCDC implies at criticality limit, where is the mass of TD and
the decay constant of TD. We thus conclude that at criticality limit
the TD could become a "true (massless) Nambu-Goldstone boson" ,
only when , namely getting decoupled, as was the case of
"holographic TD" of Haba-Matsuzaki-Yamawaki. The decoupled TD can be a
candidate of dark matter.Comment: 17 pages, 14 figures; discussions clarified, references added, to
appear in Phys.Rev.
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