In this work, we present a technique for quantum state preparation in transmon qubit system that employs optimized electromagnetic pulse sequences rather than discrete quantum gates. By focusing directly on quantum correlations rather than specific target states, we optimize the pulse parameters to generate the desired entanglement structures in two and three qubits systems. The resulting protocols reproduce the characteristic entanglement properties associated with the Bell, GHZ, and W classes, reaching values of the corresponding entanglement measures consistent with these entanglement structures. The approach uses a reduced set of parameters to characterize the control pulses, thereby restricting the space explored during optimization while retaining the entanglement properties of interest.
We consider the problem of engineering the two-excitation Dicke state $|{D}_{2}^{3}\ensuremath{\rangle}$ in a three-qubit system with all-to-all Ising-type qubit-qubit interaction, which is also subject to global transverse (Zeeman-type) control fields. The theoretical underpinning for our envisioned state-preparation scheme, in which $|000\ensuremath{\rangle}$ is adopted as the initial state of the system, is provided by a Lie-algebraic result that guarantees state-to-state controllability of this system for an arbitrary choice of initial and final states that are invariant with respect to permutations of qubits. This scheme is envisaged in the form of a pulse sequence that involves three instantaneous control pulses, which are equivalent to global qubit rotations, and two Ising-interaction pulses of finite durations between consecutive control pulses. The design of this pulse sequence (whose total duration is $T\ensuremath{\approx}0.95\phantom{\rule{0.222222em}{0ex}}\ensuremath{\hbar}/J$, where
J is the Ising-coupling strength) leans heavily on the concept of the symmetric sector, a four-dimensional, permutationally invariant subspace of the three-qubit Hilbert space. We demonstrate the feasibility of the proposed state-preparation scheme by carrying out a detailed numerical analysis of its robustness to systematic errors, i.e., deviations from the optimal values of the eight parameters that characterize the underlying pulse sequence. Finally, we discuss how our proposed scheme can be generalized for engineering Dicke states in systems with $N\ensuremath{\ge}4$ qubits. For the sake of illustration, we describe the preparation of the two-excitation Dicke state $|{D}_{2}^{4}\ensuremath{\rangle}$ in a four-qubit system.
Abstract Accurate and efficient preparation of quantum state is a core issue in building a quantum computer. In this paper, we investigate how to prepare a certain single- or two-qubit target state from arbitrary initial states in semiconductor double quantum dots with only a few discrete control pulses by leveraging the deep reinforcement learning. Our method is based on the training of the network over numerous preparing tasks. The results show that once the network is well trained, it works for any initial states in the continuous Hilbert space. Thus repeated training for new preparation tasks is avoided. Our scheme outperforms the traditional optimization approaches based on gradient with both the higher efficiency and the preparation quality in discrete control space. Moreover, we find that the control trajectories designed by our scheme are robust against stochastic fluctuations within certain thresholds, such as the charge and nuclear noises.