2,063 research outputs found
Characterizing the performance of continuous-variable Gaussian quantum gates
The required set of operations for universal continuous-variable quantum
computation can be divided into two primary categories: Gaussian and
non-Gaussian operations. Furthermore, any Gaussian operation can be decomposed
as a sequence of phase-space displacements and symplectic transformations.
Although Gaussian operations are ubiquitous in quantum optics, their
experimental realizations generally are approximations of the ideal Gaussian
unitaries. In this work, we study different performance criteria to analyze how
well these experimental approximations simulate the ideal Gaussian unitaries.
In particular, we find that none of these experimental approximations converge
uniformly to the ideal Gaussian unitaries. However, convergence occurs in the
strong sense, or if the discrimination strategy is energy bounded, then the
convergence is uniform in the Shirokov-Winter energy-constrained diamond norm
and we give explicit bounds in this latter case. We indicate how these
energy-constrained bounds can be used for experimental implementations of these
Gaussian unitaries in order to achieve any desired accuracy.Comment: v3: 26 pages, 10 figures, final version accepted for publication in
Physical Review Researc
Information-theoretic aspects of the generalized amplitude damping channel
The generalized amplitude damping channel (GADC) is one of the sources of
noise in superconducting-circuit-based quantum computing. It can be viewed as
the qubit analogue of the bosonic thermal channel, and it thus can be used to
model lossy processes in the presence of background noise for low-temperature
systems. In this work, we provide an information-theoretic study of the GADC.
We first determine the parameter range for which the GADC is entanglement
breaking and the range for which it is anti-degradable. We then establish
several upper bounds on its classical, quantum, and private capacities. These
bounds are based on data-processing inequalities and the uniform continuity of
information-theoretic quantities, as well as other techniques. Our upper bounds
on the quantum capacity of the GADC are tighter than the known upper bound
reported recently in [Rosati et al., Nat. Commun. 9, 4339 (2018)] for the
entire parameter range of the GADC, thus reducing the gap between the lower and
upper bounds. We also establish upper bounds on the two-way assisted quantum
and private capacities of the GADC. These bounds are based on the squashed
entanglement, and they are established by constructing particular squashing
channels. We compare these bounds with the max-Rains information bound, the
mutual information bound, and another bound based on approximate covariance.
For all capacities considered, we find that a large variety of techniques are
useful in establishing bounds.Comment: 33 pages, 9 figures; close to the published versio
Conditional quantum one-time pad
Suppose that Alice and Bob are located in distant laboratories, which are
connected by an ideal quantum channel. Suppose further that they share many
copies of a quantum state , such that Alice possesses the
systems and Bob the systems. In our model, there is an identifiable part
of Bob's laboratory that is insecure: a third party named Eve has infiltrated
Bob's laboratory and gained control of the systems. Alice, knowing this,
would like use their shared state and the ideal quantum channel to communicate
a message in such a way that Bob, who has access to the whole of his laboratory
( systems), can decode it, while Eve, who has access only to a sector of
Bob's laboratory ( systems) and the ideal quantum channel connecting Alice
to Bob, cannot learn anything about Alice's transmitted message. We call this
task the conditional one-time pad, and in this paper, we prove that the optimal
rate of secret communication for this task is equal to the conditional quantum
mutual information of their shared state. We thus give the
conditional quantum mutual information an operational meaning that is different
from those given in prior works, via state redistribution, conditional erasure,
or state deconstruction. We also generalize the model and method in several
ways, one of which demonstrates that the negative tripartite interaction
information of a tripartite state
is an achievable rate for a secret-sharing task, i.e., the case in
which Alice's message should be secure from someone possessing only the or
systems but should be decodable by someone possessing all systems ,
, and .Comment: v2: 16 pages, final version accepted for publication in Physical
Review Letter
Controlled Flow of Spin-Entangled Electrons via Adiabatic Quantum Pumping
We propose a method to dynamically generate and control the flow of
spin-entangled electrons, each belonging to a spin-singlet, by means of
adiabatic quantum pumping. The pumping cycle functions by periodic time
variation of localized two-body interactions. We develop a generalized approach
to adiabatic quantum pumping as traditional methods based on scattering matrix
in one dimension cannot be applied here. We specifically compute the flow of
spin-entangled electrons within a Hubbard-like model of quantum dots, and
discuss possible implementations and identify parameters that can be used to
control the singlet flow.Comment: 4 pages, 3 figure
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