8,338 research outputs found
Optimal control of the propagation of a graph in inhomogeneous media
We study an optimal control problem for viscosity solutions of a HamiltonâJacobi equation describing the propagation of a one-dimensional graph with the control being the speed function. The existence of an optimal control is proved together with an approximate controllability result in the -norm. We prove convergence of a discrete optimal control problem based on a monotone finite difference scheme and describe some numerical results
Single photon quantum non-demolition in the presence of inhomogeneous broadening
Electromagnetically induced transparency (EIT) has been often proposed for
generating nonlinear optical effects at the single photon level; in particular,
as a means to effect a quantum non-demolition measurement of a single photon
field. Previous treatments have usually considered homogeneously broadened
samples, but realisations in any medium will have to contend with inhomogeneous
broadening. Here we reappraise an earlier scheme [Munro \textit{et al.} Phys.
Rev. A \textbf{71}, 033819 (2005)] with respect to inhomogeneities and show an
alternative mode of operation that is preferred in an inhomogeneous
environment. We further show the implications of these results on a potential
implementation in diamond containing nitrogen-vacancy colour centres. Our
modelling shows that single mode waveguide structures of length in single-crystal diamond containing a dilute ensemble of NV
of only 200 centres are sufficient for quantum non-demolition measurements
using EIT-based weak nonlinear interactions.Comment: 21 pages, 9 figures (some in colour) at low resolution for arXiv
purpose
Applications of atomic ensembles in distributed quantum computing
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes - stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium - an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is signifcantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer
Thermal response of jointed rock masses inferred from infrared thermographic surveying (Acuto test-site, Italy)
The Mediterranean region is affected by considerable daily and seasonal temperature variations due to intense solar radiation. In mid-seasons, thermal excursions can exceed tens of degrees thus influencing the long-term behaviour of jointed rock masses acting as a preparatory factor for rock slope instabilities. In order to evaluate the thermal response of a densely jointed rock-block, monitoring has been in operation since 2016 by direct and remote sensing techniques in an abandoned quarry in Acuto (central Italy). Monthly InfraRed Thermographic (IRT) surveys were carried out on its exposed faces and along sections of interest across monitored main joints. The results highlight the daily and seasonal cyclical behaviour, constraining amplitudes and rates of heating and cooling phases. The temperature time-series revealed the effect of sun radiation and exposure on thermal response of the rock-block, which mainly depends on the seasonal conditions. The influence of opened joints in the heat propagation is revealed by the differential heating experienced across it, which was verified under 1D and 2D analysis. IRT has proved to be a valid monitoring technique in supporting traditional approaches, for the definition of the surficial temperature distribution on rock masses or stone building materials
Non-Euclidean geometry in nature
I describe the manifestation of the non-Euclidean geometry in the behavior of
collective observables of some complex physical systems. Specifically, I
consider the formation of equilibrium shapes of plants and statistics of sparse
random graphs. For these systems I discuss the following interlinked questions:
(i) the optimal embedding of plants leaves in the three-dimensional space, (ii)
the spectral statistics of sparse random matrix ensembles.Comment: 52 pages, 21 figures, last section is rewritten, a reference to
chaotic Hamiltonian systems is adde
Creating diamond color centers for quantum optical applications
Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state
qubits. This is because of their large dipole moment, convenient level
structure and very long room-temperature coherence times. In general, a
combination of ion irradiation and subsequent annealing is used to create the
centers, however for the rigorous demands of quantum computing all processes
need to be optimized, and decoherence due to the residual damage caused by the
implantation process itself must be mitigated. To that end we have studied
photoluminescence (PL) from NV, NV and GR1 centers formed by ion
implantation of 2MeV He ions over a wide range of fluences. The sample was
annealed at C to minimize residual vacancy diffusion, allowing for
the concurrent analysis of PL from NV centers and irradiation induced vacancies
(GR1). We find non-monotic PL intensities with increasing ion fluence,
monotonic increasing PL in NV/NV and GR1/(NV + NV) ratios, and
increasing inhomogeneous broadening of the zero-phonon lines with increasing
ion fluence. All these results shed important light on the optimal formation
conditions for NV qubits. We apply our findings to an off-resonant photonic
quantum memory scheme using vibronic sidebands
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