69 research outputs found
Observer-based quantum state estimation by continuous weak measurement
We propose to apply the Back and Forth Nudging (BFN) method used for
geophysical data assimilations to estimate the initial state of a quantum
system. We consider a cloud of atoms interacting with a magnetic field while a
single observable is being continuously measured over time using homodyne
detection. The BFN method relies on designing an observer forward and backwards
in time. The state of the BFN observer is continuously updated by the measured
data and tends to converge to the systems state. The proposed estimator seems
to be globally asymptotically convergent when the system is observable. A
detailed convergence proof and simulations are given in the 2-level case. A
discussion on the extension of the algorithm to the multilevel case is also
presented
Closed loop observer-based parameter estimation of quantum systems with a single population measurement
11 pagesAn observer-based Hamiltonian identification algorithm for quantum systems has been proposed by Bonnabel-et-al [2]. In this paper we propose another observer enabling the identification of the dipole moments of a multi-level case, and having access to the population of the ground state only
Préparation et stabilisation de systèmes quantiques
This thesis tackles the problem of preparing and stabilizing highly non classical states of quantum systems. We consider specific models based on current experiments in cavity quantum electrodynamics, Josephson circuits and ultra-fast coherent quantum control. The problem is posed in the framework of control theory where we search for a control law which prepares or stabilizes a desired target state.Of particular interest to us are target states with no classical analog: superposition and entangled states. More generally, we propose a scheme for the stabilization of a manifold of quantum states, thus introducing some new ideas for autonomous quantum error correction in a cavity. Close collaborations with experimentalists helped us in the design of control protocols which are readily employable in the laboratory. Experimental demonstrations are currently being implemented and preliminary measurements are in good agreement with the theory introduced in this thesis.Cette thèse s'intéresse au problème de préparation et de stabilisation de systèmes quantiques. Nous considérons des modèles correspondant à des expériences actuelles en électrodynamique quantique en cavité, circuits Josephson, et de contrôle quantique cohérent par laser femtoseconde. Nous posons les problèmes dans le contexte de la théorie du contrôle et nous proposons des lois de commande qui préparent ou stabilisent des états cibles. En particulier, nous nous intéressons à des états cibles qui n'ont pas d'analogue classique: des états superpositions et intriqués. De plus, nous proposons une commande pour la stabilisation d'un sous-espace de l'espace des états, contribuant ainsi au domaine de la correction d'erreur quantique. Ces résultats ont été obtenu en étroite collaboration avec des expérimentateurs. Des mesures expérimentales préliminaires sont en bon accord avec certaines prédictions théoriques de cette thèse
Back and forth nudging for quantum state estimation by continuous weak measurement
International audienceWe propose to apply the Back and Forth Nudging (BFN) method used for geophysical data assimilations [1] to estimate the initial state of a quantum system. We consider a cloud of atoms interacting with a magnetic field while a single observable is being continuously measured over time using homodyne detection. The BFN method relies on designing an observer forward and backwards in time. The state of the BFN observer is continuously updated by the measured data and tends to converge to the system's state. The proposed estimator seems to be globally asymptotically convergent when the system is observable. A detailed convergence proof and simulations are given in the 2-level case. An extension of the algorithm to the multilevel case is also presented
Parameter estimation of a 3-level quantum system with a single population measurement
International audienceAn observer-based Hamiltonian identification algorithm for quantum systems has been proposed in [2]. The later paper provided a method to estimate the dipole moment matrix of a quantum system requiring the measurement of the populations on all states, which could be experimentally difficult to achieve. We propose here an extension to a 3-level quantum system, having access to the population of the ground state only. By a more adapted choice of the control field, we will show that a continuous measurement of this observable, alone, is enough to identify the field coupling parameters (dipole moment)
Continuous Generation and Stabilization of Mesoscopic Field Superposition States in a Quantum Circuit
While dissipation is widely considered as being harmful for quantum
coherence, it can, when properly engineered, lead to the stabilization of
non-trivial pure quantum states. We propose a scheme for continuous generation
and stabilization of Schr\"{o}dinger cat states in a cavity using dissipation
engineering. We first generate non-classical photon states with definite parity
by means of a two-photon drive and dissipation, and then stabilize these
transient states against single-photon decay. The single-photon stabilization
is autonomous, and is implemented through a second engineered bath, which
exploits the photon number dependent frequency-splitting due to Kerr
interactions in the strongly dispersive regime of circuit QED. Starting with
the Hamiltonian of the baths plus cavity, we derive an effective model of only
the cavity photon states along with analytic expressions for relevant physical
quantities, such as the stabilization rate. The deterministic generation of
such cat states is one of the key ingredients in performing universal quantum
computation.Comment: 9 pages, 6 figure
Hamiltonian identification through enhanced observability utilizing quantum control
This paper considers Hamiltonian identification for a controllable quantum
system with non-degenerate transitions and a known initial state. We assume to
have at our disposal a single scalar control input and the population measure
of only one state at an (arbitrarily large) final time T. We prove that the
quantum dipole moment matrix is locally observable in the following sense: for
any two close but distinct dipole moment matrices, we construct discriminating
controls giving two different measurements. Such discriminating controls are
constructed to have three well defined temporal components, as inspired by
Ramsey interferometry. This result suggests that what may appear at first to be
very restrictive measurements are actually rich for identification, when
combined with well designed discriminating controls, to uniquely identify the
complete dipole moment of such systems. The assessment supports the employment
of quantum control as a promising means to achieve high quality identification
of a Hamiltonian.Comment: Submitted to IEEE TAC special issue on quantum contro
Hardware-efficient autonomous quantum error correction
We propose a new method to autonomously correct for errors of a logical qubit
induced by energy relaxation. This scheme encodes the logical qubit as a
multi-component superposition of coherent states in a harmonic oscillator, more
specifically a cavity mode. The sequences of encoding, decoding and correction
operations employ the non-linearity provided by a single physical qubit coupled
to the cavity. We layout in detail how to implement these operations in a
practical system. This proposal directly addresses the task of building a
hardware-efficient and technically realizable quantum memory.Comment: 12 pages,6 figure
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