1,720,987 research outputs found
Open Quantum Systems Initially Correlated with their Environment
Full text linkMàster Oficial de Ciència i Tecnologia Quàntiques / Quantum Science and Technology, Facultat de Física, Universitat de Barcelona. Curs: 2022-2023. Tutors: Philipp Strasberg, Javier Cerrillo MorenoMost descriptions of open quantum system dynamics assume a product initial state between the system and its environment, as the presence of correlations causes both practical and conceptual difficulties in characterizing the evolution of the open system. In this thesis we study initially correlated states by employing an operational approach to open quantum dynamics, which resolves these problems by mapping initial preparations to future states. We apply this framework to study the dynamics of systems initially in global thermal equilibrium and we distinguish between classical and quantum correlations as classified by the notion of quantum discord. Regarding the former, we solve for the exact dynamics of the spin-boson pure dephasing model and study the effect that initial correlations have on the geometry of the decoherence in the
Bloch-sphere, while also presenting an analytical treatment of a general class of pure dephasing models. We then move into the quantum domain and derive a family of weak-coupling master equations using second-order perturbation theory in the system-environment coupling. The obtained equation contains a non-negligible inhomogeneous correction term to the conventional Born-Markov master equation and is a general theoretical tool that can be applied to many physical models in the weak-coupling regime
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Laser cooling of quantum systems
Full text linkIn this thesis novel methods for the laser cooling of quantum systems are presented.
The use of quantum interference allows for the tailored cancelation of
heating processes, so that an approximation to a cooling operator is possible that
does not rely on the rotating wave approximation. This makes these schemes considerably
faster and more efficient than existing ground state cooling methods,
and allow for a significant relaxation of current experimental constraints. Several
approaches are investigated in different systems.
On the one hand, a special laser configuration, applicable to trapped ions,
atoms or cantilevers, generates a double dark state that eliminates both the blue
sideband and the carrier transition. As a consequence, vanishing phonon occupation
up to first order in the perturbative expansion is achieved. Underlying this
scheme is a combined action of two cooling schemes which makes the proposal
very stable under parameter fluctuations. Its suitability as a cooling scheme for
several ions in a trap or for a cloud of atoms in a dipole trap is shown.
On the other hand, a pulsed cooling scheme for optomechanical systems is
presented. It can be implemented for both strongly and weakly coupled optomechanical
systems in both weakly and highly dissipative cavities. Its underlying
mechanism is based on interferometric control of optomechanical interactions,
and its efficiency is demonstrated with pulse sequences that are obtained by using
methods from optimal control.
Finally, it is shown how this pulsed method can be combined with continuous
measurement to drive mechanical oscillators to highly squeezed steady states. Its
mechanism relies on the modification of the dissipation and measurement terms,
which drive the system towards a specific quadrature eigenstate. The scheme is
robust to measurement inefficiencies and works also with highly dissipative cavities,
which makes it accessible to implementation with state of the art technology
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Quantum computer characterization by means of magnetic resonance sensing methods
Full text link[SPA] En este trabajo de investigación se ha estudiado la detección cuántica, un campo que utiliza propiedades
cuánticas para medir señales o parámetros físicos, incluyendo señales electromagnéticas. Se han utilizado técnicas de resonancia magnética nuclear (NMR) conocidas como secuencia de Ramsey y Hahn Echo para controlar los transmones de los ordenadores de IBM Quantum y detectar las señales electromagnéticas producidas por la excitación de qubits vecinos.
La secuencia de Ramsey es una técnica de pulsos que mide la frecuencia de un qubit. Consiste en aplicar dos pulsos π/2 para preparar y leer el estado cuántico, separados por un tiempo de espera. Esta secuencia se aplica a un qubit sensor. Paralelamente un qubit actuará como señal externa, permitiendo medir la perturbación en la frecuencia del qubit sensor como resultado de las estimulación de los qubits vecinos.
La secuencia Hahn Echo, por su parte, mide el tiempo de coherencia T2 de un qubit. Consiste en aplicar
un primer pulso π/2 para preparar el estado cuántico del qubit, seguido de un tiempo de espera libre y
luego un segundo pulso π. Después de otro tiempo de espera libre, se aplica un tercer pulso π/2 para
leer el estado del qubit. Esta secuencia se aplica a un qubit sensor. Paralelamente un qubit actuará como
señal externa, permitiendo medir la reducción en el parámetro T2 del qubit sensor como resultado de la
estimulación de los qubits vecinos.
En este estudio se llevaron a cabo cuatro experimentos para evaluar la sensibilidad de los sensores de
detección cuántica a las perturbaciones en la frecuencia del qubit y en el parámetro T2 del qubit sensor. El primer experimento midió la sensibilidad del sensor a las perturbaciones en la frecuencia del qubit mediante la secuencia de Ramsey, mientras que el segundo experimento evaluó la sensibilidad del sensor a las perturbaciones en el parámetro T2 mediante la secuencia Hahn Echo. Para revelar nuevas perturbaciones, se realizó un tercer experimento mediante una modificación del anterior, mostrando oscilaciones en la exponencial esperada de T2. Finalmente, en el cuarto experimento se realizó una modificación respecto al anterior, añadiendo un pulso de rotación variable inicial en el qubit que actúa como señal externa, con el objetivo de observar diferencias dependiendo de su valor. Se presentan dos experimentos adicionales para respaldar la intensidad de la señal. El primer experimento investiga la rotación necesaria para producir la máxima señal, concluyendo que se trata de un pulso π. El segundo experimento analiza la posición que tiene mayor impacto en los resultados, concluyendo que las posiciones inicial e intermedia del qubit que actúa como señal externa son las más relevantes.
Los resultados obtenidos demuestran que las perturbaciones en la frecuencia y el parámetro T2 son indicadores precisos y confiables de la estimulación de los qubits vecinos más próximos al sensor. Para hallar la naturaleza de estas perturbaciones se han realizado diversas técnicas, comenzando por descartar el ruido electrónico como la hipótesis más evidente a priori. Y realizando otra técnica conocida como “Quantum State Tomography”, que nos lleva a verificar que el fenómeno responsable de estas perturbaciones es el entrelazamiento cuántico.
En conclusión, este trabajo de investigación proporciona una metodología para medir señales electromagnéticas con mayor precisión y sensibilidad a través de la detección cuántica. Los resultados obtenidos son prometedores y sugieren que la detección cuántica tiene un gran potencial para ser utilizada en una variedad de aplicaciones en el campo de la medición y detección de señales.
[ENG] This research work has studied quantum sensing, a field that uses quantum properties to measure physical signals or parameters, including electromagnetic signals. Nuclear magnetic resonance (NMR) techniques known as Ramsey sequence and Hahn Echo have been used to monitor the transmons of IBM Quantum computers and detect electromagnetic signals produced by the excitation of neighboring qubits.
The Ramsey sequence is a pulse technique that measures the frequency of a qubit. It consists of applying two /2 pulses to prepare and read the quantum state, separated by a time delay. This sequence is applied to a sensing qubit. In parallel a qubit will act as an external signal, allowing to measure the perturbation in the frequency of the sensor qubit as a result of the stimulation of neighboring qubits.
The Hahn Echo sequence, on the other hand, measures the coherence time T2 of a qubit. It consists
of applying a first /2 pulse to prepare the quantum state of the qubit, followed by a free timeout and
then a second /2 pulse. After another free timeout, a third /2 pulse is applied to read the state of the
qubit. This sequence is applied to a sensing qubit. In parallel one qubit will act as an external signal,
allowing to measure the reduction in the T2 parameter of the sensing qubit as a result of the stimulation
of neighboring qubits.
In this study, four experiments were conducted to evaluate the sensitivity of quantum detection sensors to
perturbations in the qubit frequency and in the T2 parameter of the sensing qubit. The first experiment
measured the sensitivity of the sensor to perturbations in the qubit frequency using the Ramsey sequence, while the second experiment evaluated the sensitivity of the sensor to perturbations in the T2 parameter using the Hahn Echo sequence. To reveal further perturbations, a third experiment was performed by a modification of the previous one, showing oscillations in the expected exponential of T2. Finally, in the fourth experiment a modification of the previous one was performed by adding an initial variable rotation pulse in the qubit acting as an external signal, with the aim of observing differences depending on its value. Two additional experiments are presented to support the signal strength. The first experiment investigates the rotation required to produce the maximum signal, concluding that it is a pulse. The second experiment analyzes the position that has the greatest impact on the results, concluding that the initial and intermediate positions of the qubit acting as the external signal are the most relevant.
The results obtained show that the perturbations in the frequency and the T2 parameter are accurate and
reliable indicators of the stimulation of the neighboring qubits closest to the sensor. To find the nature of
these perturbations, several techniques have been used, starting by discarding electronic noise as the most obvious hypothesis initially. And performing another technique known as “Quantum State Tomography”, which leads us to verify that the phenomenon responsible for these perturbations is quantum entanglement.
In conclusion, this research provides a methodology to measure electromagnetic signals with greater
precision and sensitivity through quantum detection. The results obtained are promising and suggest
that the quantum sensing has great potential to be used in a variety of applications in the field of signal
measurement and detection.Escuela Técnica Superior de Ingeniería de TelecomunicaciónUniversidad Politécnica de Cartagen
Quantum Simulation of Experiments in Quantum Sensing
Full text link[SPA] La investigación presente se ha enfocado en el análisis de la detección cuántica, un campo que aprovecha los fenómenos cuánticos para la medición de diversas señales o variables físicas, incluidas las de naturaleza electromagnética. Se han aplicado métodos de resonancia magnética nuclear (NMR), con énfasis en la utilización de la secuencia de Hahn Echo, para manejar los transmones en entornos cuánticos suministrados por IBM Quantum. Apoyándose en descubrimientos anteriores, este trabajo ha contribuido a profundizar el conocimiento sobre el entrelazamiento cuántico de qubits a través de un riguroso análisis tanto teórico como experimental.
La investigación se ha enfocado en la inicialización en el estado |+, −⟩, utilizando la secuencia de Hahn Echo en el qubit sensor y profundizando en el modelo de Hamiltoniano aplicado. Se ha realizado un análisis detallado de los datos obtenidos para entender los fenómenos observados a través de indicadores como la pureza, el criterio de Peres Horodecki, la matriz de densidad, y las desigualdades CHSH.
Sin embargo, este estudio ha revelado una discrepancia significativa entre las matrices densidad teóricas y experimentales, sugiriendo que el estado inicial deseado |+, −⟩ no se está generando adecuadamente en la práctica. Aunque se detectaron indicios de entrelazamiento, las conclusiones indican que no se logra violar las desigualdades CHSH en estos experimentos,
señalando que el entrelazamiento presente no alcanza la intensidad teóricamente esperada.
Este descubrimiento enfatiza la necesidad de revisar y optimizar las técnicas de inicialización de estados y control de qubits para garantizar la generación fiable del estado deseado y el desarrollo de un entrelazamiento más robusto. Las futuras líneas de investigación deberán concentrarse en superar estas limitaciones, explorando nuevas configuraciones experimentales que permitan la violación de las desigualdades CHSH y confirmen la existencia de entrelazamientos cuánticos fuertes, abriendo así nuevas posibilidades para aplicaciones prácticas en campos como la medicina y la detección ambiental. [ENG] The present research has focused on the analysis of quantum sensing, a field that exploits quantum phenomena for the measurement of various physical signals or variables, including those of an electromagnetic nature. Nuclear magnetic resonance (NMR) methods, with emphasis on the use of the Hahn Echo sequence, have been applied to handle transmons in quantum environments provided by IBM Quantum. Building on previous discoveries, this work has contributed to deepen the understanding of quantum qubit entanglement through a rigorous theoretical and experimental analysis.
The research has focused on initialisation in the |+, −⟩ state, using the Hahn Echo sequence in the sensor qubit and delving into the applied Hamiltonian model. A detailed analysis of the obtained data has been performed to understand the observed phenomena through indicators such as purity, Peres Horodecki criterion, density matrix, and CHSH inequalities.
However, this study has revealed a significant discrepancy between the theoretical and experimental density matrices, suggesting that the desired initial state ket+, − is not being properly generated in practice. Although hints of entanglement were detected, the findings indicate that the CHSH inequalities are not violated in these experiments, indicating that the entanglement present does not reach the theoretically expected intensity.
This discovery emphasises the need to review and optimise state initialisation and qubit control techniques to ensure reliable generation of the desired state and the development of more robust entanglement.
Future lines of research should concentrate on overcoming these limitations by exploring new experimental setups that allow the violation of CHSH inequalities and confirm the existence of strong quantum entanglement, thus opening up new possibilities for practical applications in fields such as medicine and environmental sensing.Escuela Técnica Superior de Ingeniería de TelecomunicaciónUniversidad Politécnica de Cartagen
Quantum Simulation of Experiments in Quantum Sensing
No full text[SPA] La investigación presente se ha enfocado en el análisis de la detección cuántica, un campo que aprovecha los fenómenos cuánticos para la medición de diversas señales o variables físicas, incluidas las de naturaleza electromagnética. Se han aplicado métodos de resonancia magnética nuclear (NMR), con énfasis en la utilización de la secuencia de Hahn Echo, para manejar los transmones en entornos cuánticos suministrados por IBM Quantum. Apoyándose en descubrimientos anteriores, este trabajo ha contribuido a profundizar el conocimiento sobre el entrelazamiento cuántico de qubits a través de un riguroso análisis tanto teórico como experimental.
La investigación se ha enfocado en la inicialización en el estado |+, −⟩, utilizando la secuencia de Hahn Echo en el qubit sensor y profundizando en el modelo de Hamiltoniano aplicado. Se ha realizado un análisis detallado de los datos obtenidos para entender los fenómenos observados a través de indicadores como la pureza, el criterio de Peres Horodecki, la matriz de densidad, y las desigualdades CHSH.
Sin embargo, este estudio ha revelado una discrepancia significativa entre las matrices densidad teóricas y experimentales, sugiriendo que el estado inicial deseado |+, −⟩ no se está generando adecuadamente en la práctica. Aunque se detectaron indicios de entrelazamiento, las conclusiones indican que no se logra violar las desigualdades CHSH en estos experimentos,
señalando que el entrelazamiento presente no alcanza la intensidad teóricamente esperada.
Este descubrimiento enfatiza la necesidad de revisar y optimizar las técnicas de inicialización de estados y control de qubits para garantizar la generación fiable del estado deseado y el desarrollo de un entrelazamiento más robusto. Las futuras líneas de investigación deberán concentrarse en superar estas limitaciones, explorando nuevas configuraciones experimentales que permitan la violación de las desigualdades CHSH y confirmen la existencia de entrelazamientos cuánticos fuertes, abriendo así nuevas posibilidades para aplicaciones prácticas en campos como la medicina y la detección ambiental. [ENG] The present research has focused on the analysis of quantum sensing, a field that exploits quantum phenomena for the measurement of various physical signals or variables, including those of an electromagnetic nature. Nuclear magnetic resonance (NMR) methods, with emphasis on the use of the Hahn Echo sequence, have been applied to handle transmons in quantum environments provided by IBM Quantum. Building on previous discoveries, this work has contributed to deepen the understanding of quantum qubit entanglement through a rigorous theoretical and experimental analysis.
The research has focused on initialisation in the |+, −⟩ state, using the Hahn Echo sequence in the sensor qubit and delving into the applied Hamiltonian model. A detailed analysis of the obtained data has been performed to understand the observed phenomena through indicators such as purity, Peres Horodecki criterion, density matrix, and CHSH inequalities.
However, this study has revealed a significant discrepancy between the theoretical and experimental density matrices, suggesting that the desired initial state ket+, − is not being properly generated in practice. Although hints of entanglement were detected, the findings indicate that the CHSH inequalities are not violated in these experiments, indicating that the entanglement present does not reach the theoretically expected intensity.
This discovery emphasises the need to review and optimise state initialisation and qubit control techniques to ensure reliable generation of the desired state and the development of more robust entanglement.
Future lines of research should concentrate on overcoming these limitations by exploring new experimental setups that allow the violation of CHSH inequalities and confirm the existence of strong quantum entanglement, thus opening up new possibilities for practical applications in fields such as medicine and environmental sensing.Escuela Técnica Superior de Ingeniería de TelecomunicaciónUniversidad Politécnica de Cartagen
Quantum computer characterization by means of magnetic resonance sensing methods
No full text[SPA] En este trabajo de investigación se ha estudiado la detección cuántica, un campo que utiliza propiedades
cuánticas para medir señales o parámetros físicos, incluyendo señales electromagnéticas. Se han utilizado técnicas de resonancia magnética nuclear (NMR) conocidas como secuencia de Ramsey y Hahn Echo para controlar los transmones de los ordenadores de IBM Quantum y detectar las señales electromagnéticas producidas por la excitación de qubits vecinos.
La secuencia de Ramsey es una técnica de pulsos que mide la frecuencia de un qubit. Consiste en aplicar dos pulsos π/2 para preparar y leer el estado cuántico, separados por un tiempo de espera. Esta secuencia se aplica a un qubit sensor. Paralelamente un qubit actuará como señal externa, permitiendo medir la perturbación en la frecuencia del qubit sensor como resultado de las estimulación de los qubits vecinos.
La secuencia Hahn Echo, por su parte, mide el tiempo de coherencia T2 de un qubit. Consiste en aplicar
un primer pulso π/2 para preparar el estado cuántico del qubit, seguido de un tiempo de espera libre y
luego un segundo pulso π. Después de otro tiempo de espera libre, se aplica un tercer pulso π/2 para
leer el estado del qubit. Esta secuencia se aplica a un qubit sensor. Paralelamente un qubit actuará como
señal externa, permitiendo medir la reducción en el parámetro T2 del qubit sensor como resultado de la
estimulación de los qubits vecinos.
En este estudio se llevaron a cabo cuatro experimentos para evaluar la sensibilidad de los sensores de
detección cuántica a las perturbaciones en la frecuencia del qubit y en el parámetro T2 del qubit sensor. El primer experimento midió la sensibilidad del sensor a las perturbaciones en la frecuencia del qubit mediante la secuencia de Ramsey, mientras que el segundo experimento evaluó la sensibilidad del sensor a las perturbaciones en el parámetro T2 mediante la secuencia Hahn Echo. Para revelar nuevas perturbaciones, se realizó un tercer experimento mediante una modificación del anterior, mostrando oscilaciones en la exponencial esperada de T2. Finalmente, en el cuarto experimento se realizó una modificación respecto al anterior, añadiendo un pulso de rotación variable inicial en el qubit que actúa como señal externa, con el objetivo de observar diferencias dependiendo de su valor. Se presentan dos experimentos adicionales para respaldar la intensidad de la señal. El primer experimento investiga la rotación necesaria para producir la máxima señal, concluyendo que se trata de un pulso π. El segundo experimento analiza la posición que tiene mayor impacto en los resultados, concluyendo que las posiciones inicial e intermedia del qubit que actúa como señal externa son las más relevantes.
Los resultados obtenidos demuestran que las perturbaciones en la frecuencia y el parámetro T2 son indicadores precisos y confiables de la estimulación de los qubits vecinos más próximos al sensor. Para hallar la naturaleza de estas perturbaciones se han realizado diversas técnicas, comenzando por descartar el ruido electrónico como la hipótesis más evidente a priori. Y realizando otra técnica conocida como “Quantum State Tomography”, que nos lleva a verificar que el fenómeno responsable de estas perturbaciones es el entrelazamiento cuántico.
En conclusión, este trabajo de investigación proporciona una metodología para medir señales electromagnéticas con mayor precisión y sensibilidad a través de la detección cuántica. Los resultados obtenidos son prometedores y sugieren que la detección cuántica tiene un gran potencial para ser utilizada en una variedad de aplicaciones en el campo de la medición y detección de señales.
[ENG] This research work has studied quantum sensing, a field that uses quantum properties to measure physical signals or parameters, including electromagnetic signals. Nuclear magnetic resonance (NMR) techniques known as Ramsey sequence and Hahn Echo have been used to monitor the transmons of IBM Quantum computers and detect electromagnetic signals produced by the excitation of neighboring qubits.
The Ramsey sequence is a pulse technique that measures the frequency of a qubit. It consists of applying two /2 pulses to prepare and read the quantum state, separated by a time delay. This sequence is applied to a sensing qubit. In parallel a qubit will act as an external signal, allowing to measure the perturbation in the frequency of the sensor qubit as a result of the stimulation of neighboring qubits.
The Hahn Echo sequence, on the other hand, measures the coherence time T2 of a qubit. It consists
of applying a first /2 pulse to prepare the quantum state of the qubit, followed by a free timeout and
then a second /2 pulse. After another free timeout, a third /2 pulse is applied to read the state of the
qubit. This sequence is applied to a sensing qubit. In parallel one qubit will act as an external signal,
allowing to measure the reduction in the T2 parameter of the sensing qubit as a result of the stimulation
of neighboring qubits.
In this study, four experiments were conducted to evaluate the sensitivity of quantum detection sensors to
perturbations in the qubit frequency and in the T2 parameter of the sensing qubit. The first experiment
measured the sensitivity of the sensor to perturbations in the qubit frequency using the Ramsey sequence, while the second experiment evaluated the sensitivity of the sensor to perturbations in the T2 parameter using the Hahn Echo sequence. To reveal further perturbations, a third experiment was performed by a modification of the previous one, showing oscillations in the expected exponential of T2. Finally, in the fourth experiment a modification of the previous one was performed by adding an initial variable rotation pulse in the qubit acting as an external signal, with the aim of observing differences depending on its value. Two additional experiments are presented to support the signal strength. The first experiment investigates the rotation required to produce the maximum signal, concluding that it is a pulse. The second experiment analyzes the position that has the greatest impact on the results, concluding that the initial and intermediate positions of the qubit acting as the external signal are the most relevant.
The results obtained show that the perturbations in the frequency and the T2 parameter are accurate and
reliable indicators of the stimulation of the neighboring qubits closest to the sensor. To find the nature of
these perturbations, several techniques have been used, starting by discarding electronic noise as the most obvious hypothesis initially. And performing another technique known as “Quantum State Tomography”, which leads us to verify that the phenomenon responsible for these perturbations is quantum entanglement.
In conclusion, this research provides a methodology to measure electromagnetic signals with greater
precision and sensitivity through quantum detection. The results obtained are promising and suggest
that the quantum sensing has great potential to be used in a variety of applications in the field of signal
measurement and detection.Escuela Técnica Superior de Ingeniería de TelecomunicaciónUniversidad Politécnica de Cartagen
- …
