2,696 research outputs found
Bidirectional imperfect quantum teleportation with a single Bell state
We present a bidirectional modification of the standard one-qubit
teleportation protocol, where both Alice and Bob transfer noisy versions of
their qubit states to each other by using single Bell state and auxiliary
(trigger) qubits. Three schemes are considered: the first where the actions of
parties are governed by two independent quantum random triggers, the second
with single random trigger, and the third as a mixture of the first two. We
calculate the fidelities of teleportation for all schemes and find a condition
on correlation between trigger qubits in the mixed scheme which allows us to
overcome the classical fidelity boundary of 2/3. We apply the Choi-Jamiolkowski
isomorphism to the quantum channels obtained in order to investigate an
interplay between their ability to transfer the information,
entanglement-breaking property, and auxiliary classical communication needed to
form correlations between trigger qubits. The suggested scheme for
bidirectional teleportation can be realized by using current experimental
tools.Comment: 8 pages, 4 figures; published versio
Entanglement and secret-key-agreement capacities of bipartite quantum interactions and read-only memory devices
A bipartite quantum interaction corresponds to the most general quantum
interaction that can occur between two quantum systems in the presence of a
bath. In this work, we determine bounds on the capacities of bipartite
interactions for entanglement generation and secret key agreement between two
quantum systems. Our upper bound on the entanglement generation capacity of a
bipartite quantum interaction is given by a quantity called the bidirectional
max-Rains information. Our upper bound on the secret-key-agreement capacity of
a bipartite quantum interaction is given by a related quantity called the
bidirectional max-relative entropy of entanglement. We also derive tighter
upper bounds on the capacities of bipartite interactions obeying certain
symmetries. Observing that reading of a memory device is a particular kind of
bipartite quantum interaction, we leverage our bounds from the bidirectional
setting to deliver bounds on the capacity of a task that we introduce, called
private reading of a wiretap memory cell. Given a set of point-to-point quantum
wiretap channels, the goal of private reading is for an encoder to form
codewords from these channels, in order to establish secret key with a party
who controls one input and one output of the channels, while a passive
eavesdropper has access to one output of the channels. We derive both lower and
upper bounds on the private reading capacities of a wiretap memory cell. We
then extend these results to determine achievable rates for the generation of
entanglement between two distant parties who have coherent access to a
controlled point-to-point channel, which is a particular kind of bipartite
interaction.Comment: v3: 34 pages, 3 figures, accepted for publication in Physical Review
A photon-photon quantum gate based on a single atom in an optical resonator
Two photons in free space pass each other undisturbed. This is ideal for the
faithful transmission of information, but prohibits an interaction between the
photons as required for a plethora of applications in optical quantum
information processing. The long-standing challenge here is to realise a
deterministic photon-photon gate. This requires an interaction so strong that
the two photons can shift each others phase by pi. For polarisation qubits,
this amounts to the conditional flipping of one photon's polarisation to an
orthogonal state. So far, only probabilistic gates based on linear optics and
photon detectors could be realised, as "no known or foreseen material has an
optical nonlinearity strong enough to implement this conditional phase
shift..." [Science 318, 1567]. Meanwhile, tremendous progress in the
development of quantum-nonlinear systems has opened up new possibilities for
single-photon experiments. Platforms range from Rydberg blockade in atomic
ensembles to single-atom cavity quantum electrodynamics. Applications like
single-photon switches and transistors, two-photon gateways, nondestructive
photon detectors, photon routers and nonlinear phase shifters have been
demonstrated, but none of them with the ultimate information carriers, optical
qubits. Here we employ the strong light-matter coupling provided by a single
atom in a high-finesse optical resonator to realise the Duan-Kimble protocol of
a universal controlled phase flip (CPF, pi phase shift) photon-photon quantum
gate. We achieve an average gate fidelity of F=(76.2+/-3.6)% and specifically
demonstrate the capability of conditional polarisation flipping as well as
entanglement generation between independent input photons. Our gate could
readily perform most of the hitherto existing two-photon operations. It also
discloses avenues towards new quantum information processing applications where
photons are essential.Comment: 7 pages, 5 figure
Constructive simulation and topological design of protocols
We give a topological simulation for tensor networks that we call the
two-string model. In this approach we give a new way to design protocols, and
we discover a new multipartite quantum communication protocol. We introduce the
notion of topologically-compressed transformations. Our new protocol can
implement multiple, non-local compressed transformations among multi-parties
using one multipartite resource state.Comment: 16 page
- …