40,971 research outputs found
Solution to Time-energy Costs of Quantum Channels
We derive a formula for the time-energy costs of general quantum channels proposed in [Phys. Rev. A 88, 012307 (2013)]. This formula allows us to numerically find the time-energy cost of any quantum channel using positive semidefinite programming. We also derive a lower bound to the time-energy cost for any channels and the exact the time-energy cost for a class of channels which includes the qudit depolarizing channels and projector channels as special cases.postprin
Time-Energy Costs of Quantum Measurements
Time and energy of quantum processes are a tradeoff against each other. We
propose to ascribe to any given quantum process a time-energy cost to quantify
how much computation it performs. Here, we analyze the time-energy costs for
general quantum measurements, along a similar line as our previous work for
quantum channels, and prove exact and lower bound formulae for the costs. We
use these formulae to evaluate the efficiencies of actual measurement
implementations. We find that one implementation for a Bell measurement is
optimal in time-energy. We also analyze the time-energy cost for unambiguous
state discrimination and find evidence that only a finite time-energy cost is
needed to distinguish any number of states.Comment: 10 pages, 6 figure
Climbing the Entropy Barrier: Driving the Single- towards the Multichannel Kondo Effect by a Weak Coulomb Blockade of the Leads
We study a model proposed recently in which a small quantum dot is coupled
symmetrically to several large quantum dots characterized by a charging energy
E_c. Even if E_c is much smaller than the Kondo temperature T_K, the
long-ranged interactions destabilize the single-channel Kondo effect and induce
a flow towards a multi-channel Kondo fixed point associated with a rise of the
impurity entropy with decreasing temperature. Such an ``uphill flow'' implies a
negative impurity specific heat, in contrast to all systems with local
interactions. An exact solution found for a large number of channels allows us
to capture this physics and to predict transport properties.Comment: 4 pages, 3 figures. Recent references and new title added in
published versio
Quantum Channel Capacities Per Unit Cost
Communication over a noisy channel is often conducted in a setting in which
different input symbols to the channel incur a certain cost. For example, for
bosonic quantum channels, the cost associated with an input state is the number
of photons, which is proportional to the energy consumed. In such a setting, it
is often useful to know the maximum amount of information that can be reliably
transmitted per cost incurred. This is known as the capacity per unit cost. In
this paper, we generalize the capacity per unit cost to various communication
tasks involving a quantum channel such as classical communication,
entanglement-assisted classical communication, private communication, and
quantum communication. For each task, we define the corresponding capacity per
unit cost and derive a formula for it analogous to that of the usual capacity.
Furthermore, for the special and natural case in which there is a zero-cost
state, we obtain expressions in terms of an optimized relative entropy
involving the zero-cost state. For each communication task, we construct an
explicit pulse-position-modulation coding scheme that achieves the capacity per
unit cost. Finally, we compute capacities per unit cost for various bosonic
Gaussian channels and introduce the notion of a blocklength constraint as a
proposed solution to the long-standing issue of infinite capacities per unit
cost. This motivates the idea of a blocklength-cost duality, on which we
elaborate in depth.Comment: v3: 18 pages, 2 figure
Quantum mechanical calculations of rotational-vibrational scattering in homonuclear diatom-atom collisions
Most calculations of the vibrational scattering of diatom-atom collisions use the breathing sphere approximation (BSA) of orientation averaging the intermolecular potential. The resulting angularly symmetric
potential can not cause rotational scattering. We determine the error introduced by the BSA into observables of the vibrational scattering of low-energy homonuclear diatom-atom collisions by comparing two quantum mechanical calculations, one with the BSA and the other with the full angularly asymmetric intermolecular potential. For ·reasons of economy the rotational scattering of the second calculation is restricted by the use of special incomplete channel sets in the expansion of the scattering wavefunction.
Three representative collision systems are studied: H_2-Ar, O_2-He, and I_2-He. From our calculations, we
reach two conclusions. First, the BSA can be used to analyze accurately experimental measurements of
vibrational scattering. Second, measurements most sensitive to the symmetric part of the intermolecular
potential are, in order, elastic cross sections, inelastic cross sections, and inelastic differential cross sections.
Elastic differential cross sections are sensitive to the potential only if the collision is "sticky," with scattering
over a wide range of angles; I_2-He is such a collision. Otherwise the potential sensitivity of elastic differential
cross sections is concentrated in the experimentally difficult region of very small angle scattering
Entanglement Distribution in Optical Networks
The ability to generate entangled photon-pairs over a broad wavelength range
opens the door to the simultaneous distribution of entanglement to multiple
users in a network by using centralized sources and flexible
wavelength-division multiplexing schemes. Here we show the design of a
metropolitan optical network consisting of tree-type access networks whereby
entangled photon-pairs are distributed to any pair of users, independent of
their location. The network is constructed employing commercial off-the-shelf
components and uses the existing infrastructure, which allows for moderate
deployment costs. We further develop a channel plan and a network-architecture
design to provide a direct optical path between any pair of users, thus
allowing classical and one-way quantum communication as well as entanglement
distribution. This allows the simultaneous operation of multiple quantum
information technologies. Finally, we present a more flexible backbone
architecture that pushes away the load limitations of the original network
design by extending its reach, number of users and capabilities.Comment: 26 pages, 12 figure
Quantum key distribution and 1 Gbit/s data encryption over a single fibre
We perform quantum key distribution (QKD) in the presence of 4 classical
channels in a C-band dense wavelength division multiplexing (DWDM)
configuration using a commercial QKD system. The classical channels are used
for key distillation and 1 Gbps encrypted communication, rendering the entire
system independent from any other communication channel than a single dedicated
fibre. We successfully distil secret keys over fibre spans of up to 50 km. The
separation between quantum channel and nearest classical channel is only 200
GHz, while the classical channels are all separated by 100 GHz. In addition to
that we discuss possible improvements and alternative configurations, for
instance whether it is advantageous to choose the quantum channel at 1310 nm or
to opt for a pure C-band configuration.Comment: 9 pages, 7 figure
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