Vineesha Srivastava

Entanglement generation and quantum gates with quantum emitters in a cavity: Cette thèse présente de nouveaux protocoles pour les portes quantiques multi-qubits non locales et la génération d’intrication dans des systèmes où plusieurs émetteurs quantiques interagissent avec un mode bosonique partagé. Elle introduit les portes de phase géométrique et adiabatique, avec des expressions analytiques de l’infidélité dépendant du nombre de qubits et de la coopérativité. Pour deux qubits, elles forment un ensemble universel, tandis que dans les systèmes multi-qubits, elles permettent des portes déterministes pour la simulation quantique et la correction d’erreurs. Une contribution majeure est un protocole de détection optimisé par l’intrication, atteignant une haute précision de mesure grâce au contrôle optimal. La thèse explore aussi un mécanisme de blocage polaritonique en cavité pour la génération d’états W non locaux et de portes multi-qubits. Ces opérations déterministes, basées sur des excitations classiques de cavité et parfois des impulsions globales, offrent une base évolutive pour l’informatique quantique, la détection quantique et l'internet quantique de demain, en particulier pour les systèmes à atomes neutres.

We present a deterministic protocol for the preparation of entangled states in the symmetric Dicke subspace of N spins coupled to a common cavity mode that prepares entangled states useful for quantum sensing, achieving a precision significantly better than the standard quantum limit in the presence of photon cavity loss, spontaneous emission and dephasing. The protocol combines a new geometric phase gate which can be utilized for exact unitary synthesis on the Dicke subspace, an analytic solution of the noisy quantum channel dynamics and optimal control methods. This Letter opens the way to entanglement-enhanced sensing with cold trapped atoms in cavities and is extendable to other spin systems coupled to a bosonic mode.

We propose a scheme for realizing multi-qubit entangled W-state and non-local CZ and C 2 Z gates via a cavity polariton blockade mechanism with a system of atomic qubits coupled to a common cavity mode. The polariton blockade is achieved by tuning the system, an N − qubit register, such that no two atoms are simultaneously excited to the qubit excited state, and there is an effective coupling only between the ground state and a singly-excited W state of the qubit register. The control step requires only an external drive of the cavity mode and a global qubit pulse and no individual qubit addressing. We analytically obtain the state preparation error upper bound for an N − qubit W state which scales as ( 1 − 1 / N ) / C where C is the single particle cooperativity. We additionally show the application of the polariton blockade mechanism in realizing a non-local CZ and C 2 Z gate by using a different set of computational qubit states, and characterize the gate infidelity upper bounds which scale as ∼ 1 / C .

We study the population dynamics in a two-atom setup in which each atom is driven independently by different light fields, but coupling the same Rydberg state. In particular, we look at how an offset in the Rabi frequencies between two atoms affects the dynamics. We find novel features such as amplifying the Rabi frequency of one atom, together with strong Rydberg-Rydberg interactions freezes the dynamics in the second atom. We analyze this Rydberg-biased freezing phenomenon in detail, with effective Hamiltonians obtained for various limits of the system parameters. In the absence of Rabi-offset, the doubly excited state population exhibits a Lorentzian profile as a function of interaction, whereas for very small offsets it shows splitting and thus peaks. Using an effective Hamiltonian, as well as the perturbation theory in the weak interaction limit, we show that the peak arises from a competition between Rabi-offset and Rydberg-Rydberg interactions when both are sufficiently small, together with the Rydberg blockade at large interactions. The effective Hamiltonians provide us with analytical results for long time dynamics, which are in an excellent agreement with exact numerical solutions. Also, we analyze the growth and the dynamics of quantum correlations such as entanglement entropy/quantum discord for the coherent dynamics. Finally, we extend our studies to the dissipative case in which the spontaneous emission from the Rydberg state is taken into account and in particular, we look at how the Rabi-offset modifies the purity and quantum discord in the steady states. We conclude that the local manipulation of an atom by introducing a Rabi-offset can be a useful tool to control the quantum correlations and in general, the quantum states of the composite two-qubit systems.