{"title":"Unavoidable decoherence in the quantum control of an unknown state","authors":"D. Kielpinski, R.A. Briggs, H. Wiseman","doi":"10.2478/qmetro-2013-0001","DOIUrl":"https://doi.org/10.2478/qmetro-2013-0001","url":null,"abstract":"Abstract A common objective for quantum control is to force a quantum system, initially in an unknown state, into a particular target subspace. We show that if the subspace is required to be a decoherence-free subspace of dimension greater than 1, then such control must be decoherent. That is, it will take almost any pure state to a mixed state. We make no assumptions about the control mechanism, but our result implies that for this purpose coherent control offers no advantage, in principle, over the obvious measurement-based feedback protocol.","PeriodicalId":421179,"journal":{"name":"Quantum Measurements and Quantum Metrology","volume":"4 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2013-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126619503","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Quantum continuous measurements: The stochastic Schrödinger equations and the spectrum of the output","authors":"Alberto Barchielli, M. Gregoratti","doi":"10.2478/qmetro-2013-0005","DOIUrl":"https://doi.org/10.2478/qmetro-2013-0005","url":null,"abstract":"Abstract The stochastic Schrödinger equation, of classical or quantum type, allows to describe open quantum systems under measurement in continuous time. In this paper we review the link between these two descriptions and we study the properties of the output of the measurement. For simplicity we deal only with the diffusive case. Firstly, we discuss the quantum stochastic Schrödinger equation, which is based on quantum stochastic calculus, and we show how to transform it into the classical stochastic Schrödinger equation by diagonalization of suitable commuting quantum observables. Then, we give the a posteriori state, the conditional system state at time t given the output up to that time, and we link its evolution to the classical stochastic Schrödinger equation. Moreover, the relation with quantum filtering theory is shortly discussed. Finally, we study the output of the continuous measurement, which is a stochastic process with probability distribution given by the rules of quantum mechanics. When the output process is stationary, at least in the long run, the spectrum of the process can be introduced and its properties studied. In particular we show how the Heisenberg uncertainty relations give rise to characteristic bounds on the possible spectra and we discuss how this is related to the typical quantum phenomenon of squeezing. We use a simple quantum system, a two-level atom stimulated by a laser, to discuss the differences between homodyne and heterodyne detection and to explicitly show squeezing and anti-squeezing of the homodyne spectrum and the Mollow triplet in the fluorescence spectrum.","PeriodicalId":421179,"journal":{"name":"Quantum Measurements and Quantum Metrology","volume":"23 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2013-01-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129863540","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Quantum estimation of a two-phase spin rotation","authors":"C. Vaneph, T. Tufarelli, M. Genoni","doi":"10.2478/qmetro-2013-0003","DOIUrl":"https://doi.org/10.2478/qmetro-2013-0003","url":null,"abstract":"Abstract system, characterised by two unknown phases, and compare the estimation precision achievable with two different strategies. The first is a standard ‘joint estimation’ strategy, in which a single probe state is used to estimate both parameters, while the second is a ‘sequential’ strategy in which the two phases are estimated separately, each on half of the total number of system copies. In the limit of small angles we show that, although the joint estimation approach yields in general a better performance, the two strategies possess the same scaling of the total phase sensitivity with respect to the spin number j, namely ΔΦ≃ 1/j. Finally, we discuss a simple estimation strategy based on spin squeezed states and spin measurements, and compare its performance with the ultimate limits to the estimation precision that we have derived above.","PeriodicalId":421179,"journal":{"name":"Quantum Measurements and Quantum Metrology","volume":"50 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114840663","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Error Free Quantum Reading by Quasi Bell State of Entangled Coherent States","authors":"O. Hirota","doi":"10.1515/qmetro-2017-0009","DOIUrl":"https://doi.org/10.1515/qmetro-2017-0009","url":null,"abstract":"Abstract Nonclassical states of light field have been exploited to provide marvellous results in quantum information science. Usefulness of nonclassical states in quantum information science depends on whether a physical parameter as a signal is continuous or discrete. Here we present an investigation of the potential of quasi Bell states of entangled coherent states in quantum reading of the classical digital memory which was pioneered by Pirandola (Phys.Rev.Lett.,106,090504,2011). This is a typical example of discrimination for discrete quantum parameters. We show that the quasi Bell state gives the error free performance in the quantum reading that cannot be obtained by any classical state.","PeriodicalId":421179,"journal":{"name":"Quantum Measurements and Quantum Metrology","volume":"27 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121614129","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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