23 research outputs found
The measurement postulates of quantum mechanics are operationally redundant
Understanding the core content of quantum mechanics requires us to
disentangle the hidden logical relationships between the postulates of this
theory. Here we show that the mathematical structure of quantum measurements,
the formula for assigning outcome probabilities (Born's rule) and the
post-measurement state-update rule, can be deduced from the other quantum
postulates, often referred to as "unitary quantum mechanics", and the
assumption that ensembles on finite-dimensional Hilbert spaces are
characterised by finitely many parameters. This is achieved by taking an
operational approach to physical theories, and using the fact that the manner
in which a physical system is partitioned into subsystems is a subjective
choice of the observer, and hence should not affect the predictions of the
theory. In contrast to other approaches, our result does not assume that
measurements are related to operators or bases, it does not rely on the
universality of quantum mechanics, and it is independent of the interpretation
of probability.Comment: This is a post-peer-review, pre-copyedit version of an article
published in Nature Communications. The final authenticated version is
available online at: http://dx.doi.org/10.1038/s41467-019-09348-
Universally-composable privacy amplification from causality constraints
We consider schemes for secret key distribution which use as a resource
correlations that violate Bell inequalities. We provide the first security
proof for such schemes, according to the strongest notion of security, the so
called universally-composable security. Our security proof does not rely on the
validity of quantum mechanics, it solely relies on the impossibility of
arbitrarily-fast signaling between separate physical systems. This allows for
secret communication in situations where the participants distrust their
quantum devices.Comment: 4 page
Useful entanglement can be extracted from all nonseparable states
We consider entanglement distillation from a single-copy of a multipartite
state, and instead of rates we analyze the "quality" of the distilled
entanglement. This "quality" is quantified by the fidelity with the GHZ-state.
We show that each not fully-separable state can increase the "quality"
of the entanglement distilled from other states, no matter how weakly entangled
is . We also generalize this to the case where the goal is distilling
states different than the GHZ. These results provide new insights on the
geometry of the set of separable states and its dual (the set of entanglement
witnesses).Comment: 7 page
Interconversion of Nonlocal Correlations
In this paper we study the correlations that arise when two separated parties
perform measurements on systems they hold locally. We restrict ourselves to
those correlations with which arbitrarily fast transmission of information is
impossible. These correlations are called nonsignaling. We allow the
measurements to be chosen from sets of an arbitrary size, but promise that each
measurement has only two possible outcomes. We find the structure of this
convex set of nonsignaling correlations by characterizing its extreme points.
Taking an information-theoretic view, we prove that all of these extreme
correlations are interconvertible. This suggests that the simplest extremal
nonlocal distribution (called a PR box) might be the basic unit of nonlocality.
We also show that this unit of nonlocality is sufficient to simulate all
quantum states when measured with two outcome measurements.Comment: 7 pages + appendix, single colum
Response to "The measurement postulates of quantum mechanics are not redundant"
Adrian Kent has recently presented a critique [arXiv:2307.06191] of our paper
[Nat. Comms. 10, 1361 (2019)] in which he claims to refute our main result: the
measurement postulates of quantum mechanics can be derived from the rest of
postulates, once we assume that the set of mixed states of a finite-dimensional
Hilbert space is finite-dimensional. To construct his argument, Kent considers
theories resulting from supplementing quantum mechanics with hypothetical
"post-quantum" measurement devices. We prove that each of these theories
contains pure states (i.e. states of maximal knowledge) which are not rays of
the Hilbert space, in contradiction with the "pure state postulate" of quantum
mechanics. We also prove that these alternatives violate the
finite-dimensionality of mixed states. Each of these two facts separately
invalidates the refutation. In this note we also clarify the assumptions used
in the above-cited paper and discuss the notions of pure state, physical
system, and the sensitivity of the structure of the state space under
modifications of the measurements or the dynamics.Comment: 7 page
Atmospheric Carbon Dioxide variability at AigĂŒestortes, Central Pyrenees, Spain
Unidad de excelencia MarĂa de Maeztu MdM-2015-0552In order to improve the understanding of the carbon cycle in the Pyrenean region, two atmospheric monitoring mountain stations were set up within the Long-Term Ecological Research node of AigĂŒestortes i Estany de Sant Maurici at Central Pyrenees, Spain. The atmospheric concentration of carbon dioxide (CO2) was measured over 2008-2014 and 2010-2014 at Estany Llong (ELL) site and Centre de Recerca d'Alta Muntanya (CRAM), respectively. Measurements were carried out fortnightly off-line with high precision instrumentation at ELL and every minute online with a lower precision sensor at CRAM in conjunction with meteorological variables. The two datasets were analyzed in this study, quantifying whenever possible annual growth rates (AGR), seasonal variability, and diurnal amplitudes. Results were also compared with the NOAA Marine Boundary Layer (MBL) reference product and CO2 data from other background monitoring stations. Four-harmonics adjusted CO2 data from ELL showed a high correlation with the NOAA MBL reference product for the same latitude (Spearman's rho Ï =â0.96). In addition, AGRs of CO2 at ELL correlated well with those observed at Mace Head (MHD) station (Ï =â0.94), suggesting that ELL can be considered a background station. Winter CRAM CO2 data was not statistically different from ELL data, while in summer, it was 5.5 ppm lower on average, suggesting a higher photosynthesis uptake. The amplitude of the CO2 diurnal cycle at CRAM was found to be exponentially related to the local mean daily temperature and dependent on forthcoming wind sector (N-NW or E-SE-S-SW). An increase in CRAM CO2 concentrations was observed under N-NW winds during daytime, which could be related to traffic emissions. This study demonstrates that the use of CO2 sensors with low precision but continuously corrected and periodically calibrated can be used for the study of local and regional CO2 sources and sinks
The complexity of energy eigenstates as a mechanism for equilibration
Understanding the mechanisms responsible for the equilibration of isolated
quantum many-body systems is a long-standing open problem. In this work we
obtain a statistical relationship between the equilibration properties of
Hamiltonians and the complexity of their eigenvectors, provided that a
conjecture about the incompressibility of quantum circuits holds. We quantify
the complexity by the size of the smallest quantum circuit mapping the local
basis onto the energy eigenbasis. Specifically, we consider the set of all
Hamiltonians having complexity C, and show that almost all such Hamiltonians
equilibrate if C is super-quadratic with the system size, which includes the
fully random Hamiltonian case in the limit C to infinity, and do not
equilibrate if C is sub-linear. We also provide a simple formula for the
equilibration time-scale in terms of the Fourier transform of the level
density. Our results are statistical and, therefore, do not apply to specific
Hamiltonians. Yet, they establish a fundamental link between equilibration and
complexity theory.Comment: improved version (6 pages + appendix
Convertibility between two-qubit states using stochastic local quantum operations assisted by classical communication
In this paper we classify the four-qubit states that commute with UUVV, where U and V are arbitrary members of the Pauli group. We characterize the set of separable states for this class, in terms of a finite number of entanglement witnesses. Equivalently, we characterize the two-qubit, Bell-diagonal-preserving, completely positive maps that are separable. These separable completely positive maps correspond to protocols that can be implemented with stochastic local operations assisted by classical communication (SLOCC). This allows us to derive a complete set of SLOCC monotones for Bell-diagonal states, which, in turn, provides the necessary and sufficient conditions for converting one two-qubit state to another by SLOCC
Three-dimensionality of space and the quantum bit: an information-theoretic approach
It is sometimes pointed out as a curiosity that the state space of quantum
two-level systems, i.e. the qubit, and actual physical space are both
three-dimensional and Euclidean. In this paper, we suggest an
information-theoretic analysis of this relationship, by proving a particular
mathematical result: suppose that physics takes place in d spatial dimensions,
and that some events happen probabilistically (not assuming quantum theory in
any way). Furthermore, suppose there are systems that carry "minimal amounts of
direction information", interacting via some continuous reversible time
evolution. We prove that this uniquely determines spatial dimension d=3 and
quantum theory on two qubits (including entanglement and unitary time
evolution), and that it allows observers to infer local spatial geometry from
probability measurements.Comment: 13 + 22 pages, 9 figures. v4: some clarifications, in particular in
Section V / Appendix C (added Example 39