2,816 research outputs found
An efficient quantum memory based on two-level atoms
We propose a method to implement a quantum memory for light based on
ensembles of two-level atoms. Our protocol is based on controlled reversible
inhomogeneous broadening (CRIB), where an external field first dephases the
atomic polarization and thereby stores an incoming light pulse into collective
states of the atomic ensemble, and later a reversal of the applied field leads
to a rephasing of the atomic polarization and a reemission of the light. As
opposed to previous proposals for CRIB based quantum memories we propose to
only apply the broadening for a short period after most of the pulse has
already been absorbed by the ensemble. We show that with this procedure there
exist certain modes of the incoming light field which can be stored with an
efficiency approaching 100% in the limit of high optical depth and long
coherence time of the atoms. These results demonstrate that it is possible to
operate an efficient quantum memory without any optical control fields
Floquet quantum simulation with superconducting qubits
We propose a quantum algorithm for simulating spin models based on periodic
modulation of transmon qubits. Using Floquet theory we derive an effective
time-averaged Hamiltonian, which is of the general XYZ class, different from
the isotropic XY Hamiltonian typically realised by the physical setup. As an
example, we provide a simple recipe to construct a transverse Ising Hamiltonian
in the Floquet basis. For a 1D system we demonstrate numerically the dynamical
simulation of the transverse Ising Hamiltonian and quantum annealing to its
ground state. We benchmark the Floquet approach with a digital simulation
procedure, and demonstrate that it is advantageous for limited resources and
finite anharmonicity of the transmons. The described protocol can serve as a
simple yet reliable path towards configurable quantum simulators with currently
existing superconducting chips.Comment: 6+12 pages, 4+5 figure
Efficient atomic clocks operated with several atomic ensembles
Atomic clocks are typically operated by locking a local oscillator (LO) to a
single atomic ensemble. In this article we propose a scheme where the LO is
locked to several atomic ensembles instead of one. This results in an
exponential improvement compared to the conventional method and provides a
stability of the clock scaling as with being the number
of atoms in each of the ensembles and is a constant depending on
the protocol being used to lock the LOComment: 10 pages, 8 figure
Quantum networks with chiral light--matter interaction in waveguides
We propose a scalable architecture for a quantum network based on a simple
on-chip photonic circuit that performs loss-tolerant two-qubit measurements.
The circuit consists of two quantum emitters positioned in the arms of an
on-chip Mach-Zehnder interferometer composed of waveguides with chiral
light--matter interfaces. The efficient chiral light--matter interaction allows
the emitters to perform high-fidelity intranode two-qubit parity measurements
within a single chip, and to emit photons to generate internode entanglement,
without any need for reconfiguration. We show that by connecting multiple
circuits of this kind into a quantum network, it is possible to perform
universal quantum computation with heralded two-qubit gate fidelities achievable in state-of-the-art quantum dot systems.Comment: 5 pages plus supplementary materia
High dimensional measurement device independent quantum key distribution on two dimensional subspaces
Quantum key distribution (QKD) provides ultimate cryptographic security based
on the laws of quantum mechanics. For point-to-point QKD protocols, the
security of the generated key is compromised by detector side channel attacks.
This problem can be solved with measurement device independent QKD (mdi-QKD).
However, mdi-QKD has shown limited performances in terms of the secret key
generation rate, due to post-selection in the Bell measurements. We show that
high dimensional (Hi-D) encoding (qudits) improves the performance of current
mdi-QKD implementations. The scheme is proven to be unconditionally secure even
for weak coherent pulses with decoy states, while the secret key rate is
derived in the single photon case. Our analysis includes phase errors,
imperfect sources and dark counts to mimic real systems. Compared to the
standard bidimensional case, we show an improvement in the key generation rate.Comment: 6 pages, 3 figure
A Model for Freight Transport crossing Storebælt
In this note a model for growth in total freight transport crossing Storebælt is put forward. First, a model forecasts redistribution of freight generated by reduced transportation costs caused by the Fixed Link. Redistributed freight originates from present internal traffic within Jylland/Fyn or Sjælland/Lolland-Falster that will cross Storebælt in the future after establishment of the Fixed Link. This is the gravity model. Second, the results from the gravity model is compared with the existing literature.
In Section 2, the gravity model is analysed. Here, the applied data set is discussed and the model is formulated and estimated. Moreover, the model is applied to forecast redistributed traffic in rela- tion to establishment of the Fixed Link crossing Storebælt. In Section 3, the estimated elasticities of freight volumes with respect to the production value are discussed and compared to estimates available from existing literature. Section 4 concludes
Elementary test for non-classicality based on measurements of position and momentum
We generalise a non-classicality test described by Kot et al. [Phys. Rev.
Lett. 108, 233601 (2010)], which can be used to rule out any classical
description of a physical system. The test is based on measurements of
quadrature operators and works by proving a contradiction with the classical
description in terms of a probability distribution in phase space. As opposed
to the previous work, we generalise the test to include states without
rotational symmetry in phase space. Furthermore, we compare the performance of
the non-classicality test with classical tomography methods based on the
inverse Radon transform, which can also be used to establish the quantum nature
of a physical system. In particular, we consider a non-classicality test based
on the so-called filtered back-projection formula. We show that the general
non-classicality test is conceptually simpler, requires less assumptions on the
system and is statistically more reliable than the tests based on the filtered
back-projection formula. As a specific example, we derive the optimal test for
a quadrature squeezed single photon state and show that the efficiency of the
test does not change with the degree of squeezing
Enhancing quantum transduction via long-range waveguide mediated interactions between quantum emitters
Efficient transduction of electromagnetic signals between different frequency
scales is an essential ingredient for modern communication technologies as well
as for the emergent field of quantum information processing. Recent advances in
waveguide photonics have enabled a breakthrough in light-matter coupling, where
individual two-level emitters are strongly coupled to individual photons. Here
we propose a scheme which exploits this coupling to boost the performance of
transducers between low-frequency signals and optical fields operating at the
level of individual photons. Specifically, we demonstrate how to engineer the
interaction between quantum dots in waveguides to enable efficient transduction
of electric fields coupled to quantum dots. Owing to the scalability and
integrability of the solid-state platform, our transducer can potentially
become a key building block of a quantum internet node. To demonstrate this, we
show how it can be used as a coherent quantum interface between optical photons
and a two-level system like a superconducting qubit.Comment: The maintext has 6 pages, two column and 4 figure
Entrepreneurship, Job Creation, and Wage Growth
This paper analyses the importance of entrepreneurs for job creation and wage growth. Relying on unique data that cover all establishments, firms and individuals in the Danish private sector, we are able to distil a number of different subsets from the total set of new establishments – subsets which allow us to more precisely capture the "truly new" or "entrepreneurial" establishments than in previous studies. Using these data, we find that while new establishments in general account for one third of the gross job creation in the economy, entrepreneurial establishments are responsible for around 25% of this, and thus only account for about 8% of total gross job creation in the economy. However, entrepreneurial establishments seem to generate more additional jobs than other new establishments in the years following entry. Finally, the jobs generated by entrepreneurial establishments are to a large extent low-wage jobs, as they are not found to contribute to the growth in average wages.job creation; entrepreneurial establishments; wage growth
Efter BZ: Farvel til ungdomsoprøret
Den danske BZ-bevægelse udgjorde kulminationen på ungdomsoprøret, der på mange måder startede i 1940’erne. I dag er ungdomsoprøret definitivt forbi, og derfor er det interessant at se tilbage på. Tiden er blevet en anden, og tiden er ikke længere til oprør. Så hvordan ser oprøret ud fra 2017? Bogen indeholder en række mindre essays om ungdomsoprøret samt et par tekster om BZ-bevægelsen set fra vores tid
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