2,521 research outputs found
Quantum cryptography based on qutrit Bell inequalities
We present a cryptographic protocol based upon entangled qutrit pairs. We analyze the scheme under a symmetric incoherent attack and plot the region for which the protocol is secure and compare this with the region of violations of certain Bell inequalities
Quantum tomographic cryptography with Bell diagonal states: non-equivalence of classical and quantum distillation protocols
We present a generalized tomographic quantum key distribution protocol in
which the two parties share a Bell diagonal mixed state of two qubits. We show
that if an eavesdropper performs a coherent measurement on many quantum ancilla
states simultaneously, classical methods of secure key distillation are less
effective than quantum entanglement distillation protocols. We also show that
certain Bell diagonal states are resistant to any attempt of incoherent
eavesdropping.Comment: 9 pages. 2 figures There was an error in the formula 4
(transformation of Bell states). This error does not change the main result
of the paper, namely, that quantum distillation is more powerful than
classical advantage distillatio
Quantum System Identification by Bayesian Analysis of Noisy Data: Beyond Hamiltonian Tomography
We consider how to characterize the dynamics of a quantum system from a
restricted set of initial states and measurements using Bayesian analysis.
Previous work has shown that Hamiltonian systems can be well estimated from
analysis of noisy data. Here we show how to generalize this approach to systems
with moderate dephasing in the eigenbasis of the Hamiltonian. We illustrate the
process for a range of three-level quantum systems. The results suggest that
the Bayesian estimation of the frequencies and dephasing rates is generally
highly accurate and the main source of errors are errors in the reconstructed
Hamiltonian basis.Comment: 6 pages, 3 figure
Identifying a Two-State Hamiltonian in the Presence of Decoherence
Mapping the system evolution of a two-state system allows the determination
of the effective system Hamiltonian directly. We show how this can be achieved
even if the system is decohering appreciably over the observation time. A
method to include various decoherence models is given and the limits of this
technique are explored. This technique is applicable both to the problem of
calibrating a control Hamiltonian for quantum computing applications and for
precision experiments in two-state quantum systems. For simple models of
decoherence, this method can be applied even when the decoherence time is
comparable to the oscillation period of the system.Comment: 8 pages, 6 figures. Minor corrections, published versio
A universal quantum estimator
Almost all computational tasks in the modem computer can be designed from basic building blocks. These building blocks provide a powerful and efficient language for describing algorithms. In quantum computers, the basic building blocks are the quantum gates. In this tutorial, we will look at quantum gates that act on one and two qubits and briefly discuss how these gates can be used in quantum networks
An experimental observation of geometric phases for mixed states using NMR interferometry
Examples of geometric phases abound in many areas of physics. They offer both
fundamental insights into many physical phenomena and lead to interesting
practical implementations. One of them, as indicated recently, might be an
inherently fault-tolerant quantum computation. This, however, requires to deal
with geometric phases in the presence of noise and interactions between
different physical subsystems. Despite the wealth of literature on the subject
of geometric phases very little is known about this very important case. Here
we report the first experimental study of geometric phases for mixed quantum
states. We show how different they are from the well understood, noiseless,
pure-state case.Comment: 4 pages, 3 figure
Identifying an Experimental Two-State Hamiltonian to Arbitrary Accuracy
Precision control of a quantum system requires accurate determination of the
effective system Hamiltonian. We develop a method for estimating the
Hamiltonian parameters for some unknown two-state system and providing
uncertainty bounds on these parameters. This method requires only one
measurement basis and the ability to initialise the system in some arbitrary
state which is not an eigenstate of the Hamiltonian in question. The scaling of
the uncertainty is studied for large numbers of measurements and found to be
proportional to one on the square-root of the number of measurements.Comment: Minor corrections, Accepted for publication in Physical Review
Direct estimations of linear and non-linear functionals of a quantum state
We present a simple quantum network, based on the controlled-SWAP gate, that
can extract certain properties of quantum states without recourse to quantum
tomography. It can be used used as a basic building block for direct quantum
estimations of both linear and non-linear functionals of any density operator.
The network has many potential applications ranging from purity tests and
eigenvalue estimations to direct characterization of some properties of quantum
channels. Experimental realizations of the proposed network are within the
reach of quantum technology that is currently being developed.Comment: This paper supersedes the paper quant-ph/0112073, titled "Universal
Quantum Estimator". We emphasise the estimation of linear and non-linear
functionals of a quantum stat
Prediction of the Caspian Sea level using ECMWF seasonal forecasts and reanalysis
This article is made available through the Brunel Open Access Publishing Fund. This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and
reproduction in any medium, provided the original author(s) and the
source are credited.The hydrological budget of the Caspian Sea (CS) is investigated using the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERAi) and seasonal forecast (FCST) data with the aim of predicting the Caspian Sea Level (CSL) some months ahead. Precipitation and evaporation are used. After precipitation events over the Volga River, the discharge (Volga River discharge (VRD)) follows with delays, which are parameterized. The components of the water budget from ERAi and FCSTs are integrated to obtain time series of the CSL. Observations of the CSL and the VRD are used for comparison and tuning. The quality of ERAi data is sufficiently good to calculate the time variability of the CSL with a satisfactory accuracy. Already the storage of water within the Volga Basin allows forecasts of the CSL a few months ahead, and using the FCSTs of precipitation improves the CSL forecasts. The evaporation in the seasonal forecasts is deficient due to unrealistic sea surface temperatures over the CS. Impacts of different water budget terms on the CSL variability are shown by a variety of validation tools. The importance of precipitation anomalies over the catchment of the Volga River is confirmed, but also impacts from the two southern rivers (Sefidrud and Kura River) and the evaporation over the CS become obvious for some periods. When pushing the FCSTs beyond the limits of the seasonal FCSTs to 1 year, considerable forecast skill can still be found. Validating only FCSTs by the present approach, which show the same trend as one based on a statistical method, significantly enhances the skill scores
Involvement of Mhc Loci in immune responses that are not Ir-gene-controlled
Twenty-nine randomly chosen, soluble antigens, many of them highly complex, were used to immunize mice of two strains, C3H and B10.RIII. Lymphnode cells from the immunized mice were restimulated in vitro with the priming antigens and the proliferative response of the cells was determined. Both strains were responders to 28 of 29 antigens. Eight antigens were then used to immunize 11 congenic strains carrying different H-2 haplotypes, and the T-cell proliferative responses of these strains were determined. Again, all the strains responded to seven of the eight antigens. These experiments were then repeated, but this time -antibodies specific for the A (AA) or E (EE) molecules were added to the culture to block the in vitro responsiveness. In all but one of the responses, inhibition with both A-specific and E-specific antibodies was observed. The response to one antigen (Blastoinyces) was exceptional in that some strains were nonresponders to this antigen. Furthermore, the response in the responder strains was blocked with A-specific, but not with E-specific, antibodies. The study demonstrates that responses to antigens not controlled by Irr genes nevertheless require participation of class II Mhc molecules. In contrast to Ir gene-controlled responses involving either the A- or the E-molecule controlling loci (but never both), the responses not Ir-controlled involve participation of both A- and E-controlling loci. The lack of Ir-gene control is probably the result of complexity of the responses to multiple determinants. There is thus no principal difference between responses controlled and those not controlled by Ir genes: both types involve the recognition of the antigen, in the context of Mhc molecules
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