180 research outputs found
Remote preparation of quantum states
Remote state preparation is the variant of quantum state teleportation in
which the sender knows the quantum state to be communicated. The original paper
introducing teleportation established minimal requirements for classical
communication and entanglement but the corresponding limits for remote state
preparation have remained unknown until now: previous work has shown, however,
that it not only requires less classical communication but also gives rise to a
trade-off between these two resources in the appropriate setting. We discuss
this problem from first principles, including the various choices one may
follow in the definitions of the actual resources. Our main result is a general
method of remote state preparation for arbitrary states of many qubits, at a
cost of 1 bit of classical communication and 1 bit of entanglement per qubit
sent. In this "universal" formulation, these ebit and cbit requirements are
shown to be simultaneously optimal by exhibiting a dichotomy. Our protocol then
yields the exact trade-off curve for arbitrary ensembles of pure states and
pure entangled states (including the case of incomplete knowledge of the
ensemble probabilities), based on the recently established quantum-classical
trade-off for quantum data compression. The paper includes an extensive
discussion of our results, including the impact of the choice of model on the
resources, the topic of obliviousness, and an application to private quantum
channels and quantum data hiding.Comment: 21 pages plus 2 figures (eps), revtex4. v2 corrects some errors and
adds obliviousness discussion. v3 has section VI C deleted and various minor
oversights correcte
Experiments with Generalized Quantum Measurements and Entangled Photon Pairs
This thesis describes a linear-optical device for performing generalized quantum measurements
on quantum bits (qubits) encoded in photon polarization, the implementation
of said device, and its use in two diff erent but related experiments. The device works by
coupling the polarization degree of freedom of a single photon to a `mode' or `path' degree
of freedom, and performing a projective measurement in this enlarged state space in order
to implement a tunable four-outcome positive operator-valued measure (POVM) on the
initial quantum bit. In both experiments, this POVM is performed on one photon from a
two-photon entangled state created through spontaneous parametric down-conversion.
In the fi rst experiment, this entangled state is viewed as a two-qubit photonic cluster
state, and the POVM as a means of increasing the computational power of a given resource
state in the cluster-state model of quantum computing. This model traditionally
achieves deterministic outputs to quantum computations via successive projective measurements,
along with classical feedforward to choose measurement bases, on qubits in a highly entangled
resource called a cluster state; we show that `virtual qubits' can be appended to a
given cluster by replacing some projective measurements with POVMs. Our experimental
demonstration fully realizes an arbitrary three-qubit cluster computation by implementing
the POVM, as well as fast active feed-forward, on our two-qubit photonic cluster state.
Over 206 diff erent computations, the average output delity is 0.9832 +/- 0.0002; furthermore
the error contribution from our POVM device and feedforward is only of order 10^-3, less
than some recent thresholds for fault-tolerant cluster computing.
In the second experiment, the POVM device is used to implement a deterministic
protocol for remote state preparation (RSP) of arbitrary photon polarization qubits. RSP
is the act of preparing a quantum state at a remote location without actually transmitting
the state itself. We are able to remotely prepare 178 diff erent pure and mixed qubit
states with an average delity of 0.995. Furthermore, we study the the fidelity achievable
by RSP protocols permitting only classical communication, without shared entanglement,
and compare the resulting benchmarks for average fidelity against our experimental results.
Our experimentally-achieved average fi delities surpass the classical thresholds whenever
classical communication alone does not trivially allow for perfect RSP
Deterministic and Efficient Three-Party Quantum Key Distribution
Quantum information processing is based on the laws of quantum physics and guarantees the unconditional security. In this thesis we propose an efficient and deterministic three-party quantum key distribution algorithm to establish a secret key between two users. Using the formal methodological approach, we study and model a quantum algorithm to distribute a secret key to a sender and a receiver when they only share entanglement with a trusted party but not with each other. It distributes a secret key by special pure quantum states using the remote state preparation and controlled gates. In addition, we employ the parity bit of the entangled pairs and ancillary states to help in preparing and measuring the secret states. Distributing a state to two users requires two maximally entangled pairs as the quantum channel and a two-particle von Neumann projective measurement. This protocol is exact and deterministic. It distributes a secret key of d qubits by 2d entangled pairs and on average d bits of classical communication. We show the security of this protocol against the entanglement attack and offer a method for privacy amplification. Moreover, we also study the problem of distributing Einstein-Podolsky-Rosen (EPR) in a metropolitan network. The EPR is the building block of entanglement-based and entanglement-assisted quantum communication protocols. Therefore, prior shared EPR pair and an authenticated classical channel allow two distant users to share a secret key. To build a network architecture where a centralized EPR source creates entangled states by the process of spontaneous parametric down-conversion (SPDC) then routes the states to users in different access networks. We propose and simulate a metropolitan optical network (MON) architecture for entanglement distribution in a typical telecommunication infrastructure. The architecture allows simultaneous transmission of classical and quantum signals in the network and offers a dynamic routing mechanism to serve the entire metropolitan optical network
Quantum Communication-Celebrating the Silver Jubilee of Teleportation
To celebrate the 25th anniversary of the seminal 1993 quantum teleportation paper, we are pleased to present research works, reviews, and stories about quantum communication, quantum entanglement, and quantum teleportation: (1) How was quantum teleportation invented? (2) Which teleportation experiments were performed at the Sapienza University in Rome? (3) Can we use joint measurements to generate nonclassical correlations? (4) How is classical sampling related to quantum entanglement? (5) How is classical communication related to a special quantum ensemble? (6) How can simplifying a quantum key distribution protocol make it insecure? (7) Can we teleport a two-qubit quantum state using a nonsymmetric channel? This book includes submissions by some of the most prominent quantum teleportation contributors, including Gilles Brassard, Francesco De Martini, Nicolas Gisin, and William K. Wootters, as well as additional researchers, all presenting their up-to-date insights regarding quantum communication
The classical-quantum boundary for correlations: discord and related measures
One of the best signatures of nonclassicality in a quantum system is the
existence of correlations that have no classical counterpart. Different methods
for quantifying the quantum and classical parts of correlations are amongst the
more actively-studied topics of quantum information theory over the past
decade. Entanglement is the most prominent of these correlations, but in many
cases unentangled states exhibit nonclassical behavior too. Thus distinguishing
quantum correlations other than entanglement provides a better division between
the quantum and classical worlds, especially when considering mixed states.
Here we review different notions of classical and quantum correlations
quantified by quantum discord and other related measures. In the first half, we
review the mathematical properties of the measures of quantum correlations,
relate them to each other, and discuss the classical-quantum division that is
common among them. In the second half, we show that the measures identify and
quantify the deviation from classicality in various
quantum-information-processing tasks, quantum thermodynamics, open-system
dynamics, and many-body physics. We show that in many cases quantum
correlations indicate an advantage of quantum methods over classical ones.Comment: Close to the published versio
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