45 research outputs found
Improved quantum metrology using quantum error-correction
We consider quantum metrology in noisy environments, where the effect of
noise and decoherence limits the achievable gain in precision by quantum
entanglement. We show that by using tools from quantum error-correction this
limitation can be overcome. This is demonstrated in two scenarios, including a
many-body Hamiltonian with single-qubit dephasing or depolarizing noise, and a
single-body Hamiltonian with transversal noise. In both cases we show that
Heisenberg scaling, and hence a quadratic improvement over the classical case,
can be retained. Moreover, for the case of frequency estimation we find that
the inclusion of error-correction allows, in certain instances, for a finite
optimal interrogation time even in the asymptotic limit.Comment: Version 2 is the published version. Appendices contain Supplemental
materia
A true concurrent model of smart contracts executions
The development of blockchain technologies has enabled the trustless
execution of so-called smart contracts, i.e. programs that regulate the
exchange of assets (e.g., cryptocurrency) between users. In a decentralized
blockchain, the state of smart contracts is collaboratively maintained by a
peer-to-peer network of mutually untrusted nodes, which collect from users a
set of transactions (representing the required actions on contracts), and
execute them in some order. Once this sequence of transactions is appended to
the blockchain, the other nodes validate it, re-executing the transactions in
the same order. The serial execution of transactions does not take advantage of
the multi-core architecture of modern processors, so contributing to limit the
throughput. In this paper we propose a true concurrent model of smart contract
execution. Based on this, we show how static analysis of smart contracts can be
exploited to parallelize the execution of transactions.Comment: Full version of the paper presented at COORDINATION 202
Designing Secure Ethereum Smart Contracts: A Finite State Machine Based Approach
The adoption of blockchain-based distributed computation platforms is growing
fast. Some of these platforms, such as Ethereum, provide support for
implementing smart contracts, which are envisioned to have novel applications
in a broad range of areas, including finance and Internet-of-Things. However, a
significant number of smart contracts deployed in practice suffer from security
vulnerabilities, which enable malicious users to steal assets from a contract
or to cause damage. Vulnerabilities present a serious issue since contracts may
handle financial assets of considerable value, and contract bugs are
non-fixable by design. To help developers create more secure smart contracts,
we introduce FSolidM, a framework rooted in rigorous semantics for designing
con- tracts as Finite State Machines (FSM). We present a tool for creating FSM
on an easy-to-use graphical interface and for automatically generating Ethereum
contracts. Further, we introduce a set of design patterns, which we implement
as plugins that developers can easily add to their contracts to enhance
security and functionality
A matrix product solution for a nonequilibrium steady state of an XX chain
A one dimensional XX spin chain of finite length coupled to reservoirs at
both ends is solved exactly in terms of a matrix product state ansatz. An
explicit representation of matrices of fixed dimension 4 independent of the
chain length is found. Expectations of all observables are evaluated, showing
that all connected correlations, apart from nearest neighbor z-z, are zero.Comment: 11 page
General measure for macroscopic quantum states beyond "dead and alive"
We consider the characterization of quantum superposition states beyond the pattern "dead and alive". We propose a measure that is applicable to superpositions of multiple macroscopically distinct states, superpositions with different weights as well as mixed states. The measure is based on the mutual information to characterize the distinguishability between the multiple branches of the superposition. This allows us to overcome limitations of previous proposals, and to bridge the gap between general measures for macroscopic quantumness and measures for Schrödinger-cat type superpositions. We discuss a number of relevant examples, provide an alternative definition using basis-dependent quantum discord and reveal connections to other proposals in the literature. Finally, we also show the connection between the size of quantum states as quantified by our measure and their vulnerability to noise