12 research outputs found
Coupling a single Nitrogen-Vacancy center to a superconducting flux qubit in the far off resonance regime
We present a theoretical proposal to couple a single Nitrogen-Vacancy (NV)
center to a superconducting flux qubit (FQ) in the regime where both systems
are off resonance. The coupling between both quantum devices is achieved
through the strong driving of the flux qubit by a classical microwave field
that creates dressed states with an experimentally controlled characteristic
frequency. We discuss several applications such as controlling the NV center's
state by manipulation of the flux qubit, performing the NV center full
tomography and using the NV center as a quantum memory. The effect of
decoherence and its consequences to the proposed applications are also
analyzed. Our results provide a theoretical framework describing a promising
hybrid system for quantum information processing, which combines the advantages
of fast manipulation and long coherence times.Comment: 8 pages, 9 figure
Building Trust for Continuous Variable Quantum States
In this work we develop new methods for the characterisation of continuous
variable quantum states using heterodyne measurement in both the trusted and
untrusted settings. First, building on quantum state tomography with heterodyne
detection, we introduce a reliable method for continuous variable quantum state
certification, which directly yields the elements of the density matrix of the
state considered with analytical confidence intervals. This method neither
needs mathematical reconstruction of the data nor discrete binning of the
sample space, and uses a single Gaussian measurement setting. Second, beyond
quantum state tomography and without its identical copies assumption, we
promote our reliable tomography method to a general efficient protocol for
verifying continuous variable pure quantum states with Gaussian measurements
against fully malicious adversaries, i.e., making no assumptions whatsoever on
the state generated by the adversary. These results are obtained using a new
analytical estimator for the expected value of any operator acting on a
continuous variable quantum state with bounded support over the Fock basis,
computed with samples from heterodyne detection of the state.Comment: This paper is an extended version of a short paper accepted for the
TQC2020 proceedings (15+28 pages, 2 figures
Probabilistic Fault-Tolerant Universal Quantum Computation and Sampling Problems in Continuous Variables
Continuous-Variable (CV) devices are a promising platform for demonstrating
large-scale quantum information protocols. In this framework, we define a
general quantum computational model based on a CV hardware. It consists of
vacuum input states, a finite set of gates - including non-Gaussian elements -
and homodyne detection. We show that this model incorporates encodings
sufficient for probabilistic fault-tolerant universal quantum computing.
Furthermore, we show that this model can be adapted to yield sampling problems
that cannot be simulated efficiently with a classical computer, unless the
polynomial hierarchy collapses. This allows us to provide a simple paradigm for
short-term experiments to probe quantum advantage relying on Gaussian states,
homodyne detection and some form of non-Gaussian evolution. We finally address
the recently introduced model of Instantaneous Quantum Computing in CV, and
prove that the hardness statement is robust with respect to some experimentally
relevant simplifications in the definition of that model.Comment: Corrected a mistake in v1 concerning squeezing amplificatio
Continuous-variable nonlocality and contextuality
Contextuality is a non-classical behaviour that can be exhibited by quantum
systems. It is increasingly studied for its relationship to
quantum-over-classical advantages in informatic tasks. To date, it has largely
been studied in discrete variable scenarios, where observables take values in
discrete and usually finite sets. Practically, on the other hand,
continuous-variable scenarios offer some of the most promising candidates for
implementing quantum computations and informatic protocols. Here we set out a
framework for treating contextuality in continuous-variable scenarios. It is
shown that the Fine--Abramsky--Brandenburger theorem extends to this setting,
an important consequence of which is that nonlocality can be viewed as a
special case of contextuality, as in the discrete case. The contextual
fraction, a quantifiable measure of contextuality that bears a precise
relationship to Bell inequality violations and quantum advantages, can also be
defined in this setting. It is shown to be a non-increasing monotone with
respect to classical operations that include binning to discretise data.
Finally, we consider how the contextual fraction can be formulated as an
infinite linear program, and calculated with increasing accuracy using
semi-definite programming approximations.Comment: 27 pages including 6 pages supplemental material, 2 figure
Quantifying Astrophysical Uncertainties on Dark Matter Direct Detection Results
We attempt to estimate the uncertainty in the constraints on the spin
independent dark matter-nucleon cross section due to our lack of knowledge of
the dark matter phase space in the galaxy. We fit the density of dark matter
before investigating the possible solutions of the Jeans equation compatible
with those fits in order to understand what velocity dispersions we might
expect at the solar radius. We take into account the possibility of
non-Maxwellian velocity distributions and the possible presence of a dark disk.
Combining all these effects, we still find that the uncertainty in the
interpretation of direct detection experiments for high (>100 GeV) mass dark
matter candidates is less than an order of magnitude in cross section.Comment: 18 pages, 17 figure
Traitement réaliste de l'information quantique : des dispositifs aux modèles de calcul
The theory of quantum computing lies at the very boundary between quantum physics and computer science. As such, both fields bring their own methods and mathematical tools to make quantum computing even richer. The present thesis attempts to reflect this specificity by addressing questions ranging from experimental physics to computational models. The goal is to provide novel ways of demonstrating quantum advantage.After a short introduction to basic notions of quantum mechanics, some computer science aspects are discussed. We describe the powerful formalism of quantum complexity classes and the concept of quantum computations based on continuous variables. We then translate the model of instantaneous quantum computing to continuous variables, which is experimentally appealing. The chapter concludes with a discussion on a hybrid protocol involving Grover’s algorithm in a quantum communication framework.The last part of the thesis is devoted to experimentally driven issues. A fundamental connection between the Hong-Ou-Mandel experiment and the Wigner function of two-photon states is derived and a verification protocol is designed accordingly. We then move to the field of superconducting circuits to discuss proposals for future experiments. We show how to use a flux qubit to manipulate a NV color center. We also describe how to use to probe the Rabi model in the ultra strong coupling regime using an additional weakly coupled qubit.La théorie du calcul quantique se situe à la frontière de la physique quantique et de l’informatique. Par conséquent, les deux domaines contribuent à la rendre d’autant plus riche en apportant leurs propres méthodes et outils mathématiques. La présente thèse tente de mettre en évidence cette particularité en traitant des problématiques qui vont la physique expérimentale aux modèles de calcul. Le but est d’offrir de nouvelles possibilités pour démontrer un avantage quantique.Après une brève introduction aux notions de base de la mécanique quantique, certains aspects liés à l’informatique sont discutés. Le formalisme des classes de complexité quantiques ainsi que le concept du calcul quantique en variables continues sont décrits. Ensuite, le modèle connu comme instantaneous quantum computing est traduit en variables continues, le rendant attrayant d’un point de vue expérimental. Le chapitre conclut sur une discussion concernant un protocole hybride impliquant l’algorithme de Grover dans le cadre des communications quantiques.La dernière partie de la thèse s’intéresse à des problématiques issues de la physique expérimentale. Le lien entre l’effet Hong-Ou-Mandel et la fonction de Wigner d’un état à deux photons est mise en évidence, et un protocole expérimental est décrit en conséquence. La suite traite du domaine des circuits supraconducteurs et envisage de possibles expériences. Il est montré comment utiliser un qubit de flux pour manipuler un centre coloré du diamant. Il est également décrit comment sonder le modèle de Rabi dans le régime de couplage ultra fort en utilisant un qubit supplémentaire faiblement couplé
Traitement réaliste de l'information quantique : des dispositifs aux modèles de calcul
La théorie du calcul quantique se situe à la frontière de la physique quantique et de l’informatique. Par conséquent, les deux domaines contribuent à la rendre d’autant plus riche en apportant leurs propres méthodes et outils mathématiques. La présente thèse tente de mettre en évidence cette particularité en traitant des problématiques qui vont la physique expérimentale aux modèles de calcul. Le but est d’offrir de nouvelles possibilités pour démontrer un avantage quantique. Après une brève introduction aux notions de base de la mécanique quantique, certains aspects liés à l’informatique sont discutés. Le formalisme des classes de complexité quantiques ainsi que le concept du calcul quantique en variables continues sont décrits. Ensuite, le modèle connu comme instantaneous quantum computing est traduit en variables continues, le rendant attrayant d’un point de vue expérimental. Le chapitre conclut sur une discussion concernant un protocole hybride impliquant l’algorithme de Grover dans le cadre des communications quantiques. La dernière partie de la thèse s’intéresse à des problématiques issues de la physique expérimentale. Le lien entre l’effet Hong-Ou-Mandel et la fonction de Wigner d’un état à deux photons est mise en évidence, et un protocole expérimental est décrit en conséquence. La suite traite du domaine des circuits supraconducteurs et envisage de possibles expériences. Il est montré comment utiliser un qubit de flux pour manipuler un centre coloré du diamant. Il est également décrit comment sonder le modèle de Rabi dans le régime de couplage ultra fort en utilisant un qubit supplémentaire faiblement couplé.The theory of quantum computing lies at the very boundary between quantum physics and computer science. As such, both fields bring their own methods and mathematical tools to make quantum computing even richer. The present thesis attempts to reflect this specificity by addressing questions ranging from experimental physics to computational models. The goal is to provide novel ways of demonstrating quantum advantage. After a short introduction to basic notions of quantum mechanics, some computer science aspects are discussed. We describe the powerful formalism of quantum complexity classes and the concept of quantum computations based on continuous variables. We then translate the model of instantaneous quantum computing to continuous variables, which is experimentally appealing. The chapter concludes with a discussion on a hybrid protocol involving Grover’s algorithm in a quantum communication framework. The last part of the thesis is devoted to experimentally driven issues. A fundamental connection between the Hong-Ou-Mandel experiment and the Wigner function of two-photon states is derived and a verification protocol is designed accordingly. We then move to the field of superconducting circuits to discuss proposals for future experiments. We show how to use a flux qubit to manipulate a NV color center. We also describe how to use to probe the Rabi model in the ultra strong coupling regime using an additional weakly coupled qubit
Probabilistic fault-tolerant universal quantum computation and sampling problems in continuous variables
We define a quantum computational model with continuous variables composed of vacuum input states, a finite set of gates and homodyne detection. We prove universality, fault tolerance and, as a sampling problem, quantum advantage