289 research outputs found
Continuous-variable versus hybrid schemes for quantum teleportation of Gaussian states
In this paper, we examine and compare two fundamentally different teleportation schemes: the well-known continuous-variable scheme of Vaidman, Braunstein, and Kimble (VBK) and a recently proposed hybrid scheme by Andersen and Ralph (AR). We analyze the teleportation of ensembles of arbitrary pure single-mode Gaussian states using these schemes and see how they fare against the optimal measure-and-prepare strategies—the benchmarks. In the VBK case, we allow for nonunit gain tuning and additionally consider a class of non-Gaussian resources in order to optimize performance. The results suggest that the AR scheme may likely be a more suitable candidate for beating the benchmarks in the teleportation of squeezing, capable of achieving this for moderate resources in comparison to the VBK scheme. Moreover, our quantification of resources, whereby different protocols are compared at fixed values of the entanglement entropy or the mean energy of the resource states, brings into question any advantage due to non-Gaussianity for quantum teleportation of Gaussian states
Divide and Scale: Formalization of Distributed Ledger Sharding Protocols
Sharding distributed ledgers is the most promising on-chain solution for
scaling blockchain technology. In this work, we define and analyze the
properties a sharded distributed ledger should fulfill. More specifically, we
show that a sharded blockchain cannot be scalable under a fully adaptive
adversary, but it can scale up to under an epoch-adaptive
adversary. This is possible only if the distributed ledger creates succinct
proofs of the valid state updates at the end of each epoch. Our model builds
upon and extends the Bitcoin backbone protocol by defining consistency and
scalability. Consistency encompasses the need for atomic execution of
cross-shard transactions to preserve safety, whereas scalability encapsulates
the speedup a sharded system can gain in comparison to a non-sharded system. We
introduce a protocol abstraction and highlight the sufficient components for
secure and efficient sharding in our model. In order to show the power of our
framework, we analyze the most prominent shared blockchains (Elastico,
Monoxide, OmniLedger, RapidChain) and pinpoint where they fail to meet the
desired properties
Cuttlefish: Expressive Fast Path Blockchains with FastUnlock
Cuttlefish addresses several limitations of existing consensus-less and
consensus-minimized decentralized ledgers, including restricted programmability
and the risk of deadlocked assets. The key insight of Cuttlefish is that
consensus in blockchains is necessary due to contention, rather than multiple
owners of an asset as suggested by prior work. Previous proposals proactively
use consensus to prevent contention from blocking assets, taking a pessimistic
approach. In contrast, Cuttlefish introduces collective objects and multi-owner
transactions that can offer most of the functionality of classic blockchains
when objects transacted on are not under contention. Additionally, in case of
contention, Cuttlefish proposes a novel `Unlock' protocol that significantly
reduces the latency of unblocking contented objects. By leveraging these
features, Cuttlefish implements consensus-less protocols for a broader range of
transactions, including asset swaps and multi-signature transactions, which
were previously believed to require consensus
Mandator and Sporades: Robust Wide-Area Consensus with Efficient Request Dissemination
Consensus algorithms are deployed in the wide area to achieve high
availability for geographically replicated applications. Wide-area consensus is
challenging due to two main reasons: (1) low throughput due to the high latency
overhead of client request dissemination and (2) network asynchrony that causes
consensus protocols to lose liveness. In this paper, we propose Mandator and
Sporades, a modular state machine replication algorithm that enables high
performance and resiliency in the wide-area setting.
To address the high client request dissemination overhead challenge, we
propose Mandator, a novel consensus-agnostic asynchronous dissemination layer.
Mandator separates client request dissemination from the critical path of
consensus to obtain high performance. Composing Mandator with Multi-Paxos
(Mandator-Paxos) delivers significantly high throughput under synchronous
networks. However, under asynchronous network conditions, Mandator-Paxos loses
liveness which results in high latency. To achieve low latency and robustness
under asynchrony, we propose Sporades, a novel omission fault-tolerant
consensus algorithm. Sporades consists of two modes of operations --
synchronous and asynchronous -- that always ensure liveness. The combination of
Mandator and Sporades (Mandator-Sporades) provides a robust and high-performing
state machine replication system.
We implement and evaluate Mandator-Sporades in a wide-area deployment running
on Amazon EC2. Our evaluation shows that in the synchronous execution,
Mandator-Sporades achieves 300k tx/sec throughput in less than 900ms latency,
outperforming Multi-Paxos, EPaxos and Rabia by 650\% in throughput, at a modest
expense of latency. Furthermore, we show that Mandator-Sporades outperforms
Mandator-Paxos, Multi-Paxos, and EPaxos in the face of targeted distributed
denial-of-service attacks
Entanglement, Einstein-Podolsky-Rosen steering and cryptographical applications
This PhD Dissertation collects results of my own work on the topic of continuous variable (CV) quantum teleportation, which is one of the most important applications of quantum entanglement, as well as on the understanding, quantification, detection, and applications of a type of quantum correlations known as Einstein-Podolsky-Rosen (EPR) steering, for both bipartite and multipartite systems and with a main focus on CV systems.
For the first results, we examine and compare two fundamentally different teleportation schemes; the well-known continuous variable scheme of Vaidman, Braunstein and Kimble, and a recently proposed hybrid scheme by Andersen and Ralph. We analyse the teleportation of ensembles of arbitrary pure single-mode Gaussian states using these schemes and compare their performance against classical strategies that utilize no entanglement (benchmarks). Our analysis brings into question any advantage due to non-Gaussianity for quantum teleportation of Gaussian states.
For the second part of the results, we study bipartite EPR-steering. We propose a novel powerful method to detect steering in quantum systems of any dimension in a systematic and hierarchical way. Our method includes previous results of the literature as special cases on one hand, and goes beyond them on the other. We proceed to the quantification of steering-type correlations, and introduce a measure of steering for arbitrary bipartite Gaussian states, prove many useful properties, and provide with an operational interpretation of the proposed measure in terms of the key rate in one-sided device independent quantum key distribution. Finally, we show how the Gaussian steering measure gives a lower bound to a more general quantifier of which Gaussian states are proven to be extremal.
We proceed to the study of multipartite steering, and derive laws for the distribution of Gaussian steering among different parties in multipartite Gaussian states. We define an indicator of collective steering-type correlations, which is interpreted operationally in terms of the guaranteed secret key rate in the multi-party cryptographic task of quantum secret sharing.
The final results look at the cryptographical task of quantum secret sharing, whose security has remained unproven almost two decades after its original conception. By utilizing intuition and ideas from steering, we manage to establish for the first time an unconditional security proof for CV entanglement-based quantum secret sharing schemes, and demonstrate their practical feasibility. Our results establish quantum secret sharing as a viable and practically relevant primitive for quantum communication technologies
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