726 research outputs found
Cancer stem cells modulate patterns and processes of evolution in cancers
The clonal evolution (CE) model and the cancer stem cell (CSC) model are two independent models of cancers, yet recent data shows intersections between the two models. This article explores the impacts of the CSC model on the CE model. I show that CSC restriction, which depends on CSC frequency in cancer cell populations and on the probability of dedifferentiation of cancer non-stem cells (non-CSC) into CSCs, can favor or impede some patterns of evolution (linear or branched evolution) and some processes of evolution (drift, evolution by natural selection, complex adaptations). Taking CSC restriction into account for the CE model thus has implications for the way in which we understand the patterns and processes of evolution, and can also provide new leads for therapeutic interventions
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The risk–reward nexus: a qualitative analysis of public–private partnerships for investments in innovation in Brazil
Governments throughout the world are confronted with new challenges, in particular having to reconcile economic growth, sustainability and social inclusion. Tackling these issues requires new forms of collaboration between public and private actors. As such, how can public–private partnerships be effective?
This research addresses the emergence of the state’s active role as a risk-taker and co-investor in technological innovation. The analysis undertaken considers policy issues that arise from public–private partnerships, by focusing on how public actors may directly appropriate financial rewards. From a perspective that views innovation policy as creating markets, experimentation is crucial for shaping equitable partnerships and strengthening the entrepreneurial role of the state. Furthermore, purposeful state action can influence the institutional frameworks within which these partnerships unfold, and favour the realisation of socially desirable policy goals. The interplay between the two – experimentation with partnerships and changes in the institutional environment – is investigated in three papers that are presented as chapters within the thesis.
In Chapter 1 a framework for studying public–private partnerships for investments in innovation is developed. By highlighting the legal dimension of the role of the state in institutional change, the framework includes essential tools that public actors could use for negotiating more equitable reward distribution with business. In chapters 2 and 3 concrete attempts to build investment partnerships in Brazil are examined, focusing on the recent revival of active and explicit industrial policies in the country. Chapter 2 is a comparative analysis of two R&D programmes, oriented towards biofuels and health, which leads to an appraisal of their preliminary outcomes, viewed through the risk–reward nexus lens. Chapter 3 comprises an in-depth case study in the qualities (attributes and functions) of the contracts that enable public actors to appropriate financial rewards of high-risk investments.
Viewed as a whole, these three chapters contribute to the understanding of the legitimation processes that underlie the role of the state as an investor, by offering a nuanced appreciation of the limits, tensions, possibilities and tools for building effective public–private partnerships. The theoretical and practical insights from this thesis should benefit the design, implementation and assessment of innovation policies geared to tackling contemporary challenges in Brazil and elsewhere
Multi-mode and long-lived quantum correlations between photons and spins in a crystal
The realization of quantum networks and quantum repeaters remains an
outstanding challenge in quantum communication. These rely on entanglement of
remote matter systems, which in turn requires creation of quantum correlations
between a single photon and a matter system. A practical way to establish such
correlations is via spontaneous Raman scattering in atomic ensembles, known as
the DLCZ scheme. However, time multiplexing is inherently difficult using this
method, which leads to low communication rates even in theory. Moreover, it is
desirable to find solid-state ensembles where such matter-photon correlations
could be generated. Here we demonstrate quantum correlations between a single
photon and a spin excitation in up to 12 temporal modes, in a Eu
doped YSiO crystal, using a novel DLCZ approach that is inherently
multimode. After a storage time of 1 ms, the spin excitation is converted into
a second photon. The quantum correlation of the generated photon pair is
verified by violating a Cauchy - Schwarz inequality. Our results show that
solid-state rare-earth crystals could be used to generate remote multi-mode
entanglement, an important resource for future quantum networks
High precision measurement of the Dzyaloshinsky-Moriya interaction between two rare-earth ions in a solid
We report on a direct measurement of the pair-wise anti-symmetric exchange
interaction, known as the Dzyaloshinsky-Moriya interaction (DMI), in a
Nd3+-doped YVO4 crystal. To this end we introduce a broadband electron spin
resonance technique coupled with an optical detection scheme which selectively
detects only one Nd3+-Nd3+ pair. Using this technique we can fully determine
the spin-spin coupling tensor, allowing us to experimentally determine both the
strength and direction of the DMI vector. We believe that this ability to fully
determine the interaction Hamiltonian is of interest for studying the numerous
magnetic phenomena where the DMI interaction is of fundamental importance,
including multiferroics. We also detect a singlet-triplet transition within the
pair, with a highly suppressed magnetic-field dependence, which suggests that
such systems could form singlet-triplet qubits with long coherence times for
quantum information applications
Multiplexed on-demand storage of polarization qubits in a crystal
A long-lived and multimode quantum memory is a key component needed for the
development of quantum communication. Here we present temporally multiplexed
storage of 5 photonic polarization qubits encoded onto weak coherent states in
a rare-earth-ion doped crystal. Using spin refocusing techniques we can
preserve the qubits for more than half a millisecond. The temporal multiplexing
allows us to increase the effective rate of the experiment by a factor of 5,
which emphasizes the importance of multimode storage for quantum communication.
The fidelity upon retrieval is higher than the maximum classical fidelity
achievable with qubits encoded onto single photons and we show that the memory
fidelity is mainly limited by the memory signal-to-noise ratio. These results
show the viability and versatility of long-lived, multimode quantum memories
based on rare-earth-ion doped crystals
Cavity-enhanced storage in an optical spin-wave memory
We report on the experimental demonstration of an optical spin-wave memory,
based on the atomic frequency comb (AFC) scheme, where the storage efficiency
is strongly enhanced by an optical cavity. The cavity is of low finesse, but
operated in an impedance matching regime to achieve high absorption in our
intrinsically low-absorbing Eu3+:Y2SiO5 crystal. For storage of optical pulses
as an optical excitation (AFC echoes), we reach efficiencies of 53% and 28% for
2 and 10 microseconds delays, respectively. For a complete AFC spin-wave memory
we reach an efficiency of 12%, including spin-wave dephasing, which is a
12-fold increase with respect to previous results in this material. This result
is an important step towards the goal of making efficient and long-lived
quantum memories based on spin waves, in the context of quantum repeaters and
quantum networks
Fatores locacionais e o investimento estrangeiro em pesquisa e desenvolvimento: Evidências para o Brasil, Argentina e México
The aim of this paper is to analyze if specific characteristics of host countries contribute to explain foreign investment in research and development. We have estimated two step selection models to explain the decision and the level of R&D investment by multinational companies in Brazil, Argentina and Mexico. The innovation surveys in these three countries were pooled in order to do the estimations. The main results suggest that the size of the market and the interaction between firms and research institutes and universities are important locational factors explaining multinational companies investments in research and development
Coherent spin control at the quantum level in an ensemble-based optical memory
Long-lived quantum memories are essential components of a long-standing goal
of remote distribution of entanglement in quantum networks. These can be
realized by storing the quantum states of light as single-spin excitations in
atomic ensembles. However, spin states are often subjected to different
dephasing processes that limit the storage time, which in principle could be
overcome using spin-echo techniques. Theoretical studies have suggested this to
be challenging due to unavoidable spontaneous emission noise in ensemble-based
quantum memories. Here we demonstrate spin-echo manipulation of a mean spin
excitation of 1 in a large solid-state ensemble, generated through storage of a
weak optical pulse. After a storage time of about 1 ms we optically read out
the spin excitation with a high signal-to-noise ratio. Our results pave the way
for long-duration optical quantum storage using spin-echo techniques for any
ensemble-based memory.Comment: 5 pages, 2 figures, 1 tabl
Towards highly multimode optical quantum memory for quantum repeaters
Long-distance quantum communication through optical fibers is currently
limited to a few hundreds of kilometres due to fiber losses. Quantum repeaters
could extend this limit to continental distances. Most approaches to quantum
repeaters require highly multimode quantum memories in order to reach high
communication rates. The atomic frequency comb memory scheme can in principle
achieve high temporal multimode storage, without sacrificing memory efficiency.
However, previous demonstrations have been hampered by the difficulty of
creating high-resolution atomic combs, which reduces the efficiency for
multimode storage. In this article we present a comb preparation method that
allows one to increase the multimode capacity for a fixed memory bandwidth. We
apply the method to a Eu-doped YSiO crystal, in which we
demonstrate storage of 100 modes for 51 s using the AFC echo scheme (a
delay-line memory), and storage of 50 modes for 0.541 ms using the AFC
spin-wave memory (an on-demand memory). We also briefly discuss the ultimate
multimode limit imposed by the optical decoherence rate, for a fixed memory
bandwidth.Comment: 10 pages, 8 figure
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