We present schemes for preparing quantum states, utilizing theorems on ergodicity of quantum channels. By repetition of measurements and feedbacks, quantum system is driven from an arbitrary state to the target state with probability 1.
Quantum phase estimation (QPE) is a crucial subroutine in many quantum algorithms. It is used to estimate the eigenvalues of a unitary operator acting on a target quantum system. Conventional QPE is often expensive to implement because it requires a large number of ancilla qubits and the ability to perform quantum Fourier transform. Recent developments in iterative QPE reduce the implementation cost by repetitive uses of a single ancilla and classical postprocessing. However, both conventional and iterative schemes often require preparation of the target system in an eigenstate of the unitary operator, while it remains ambiguous to achieve QPE of multiple eigenvalues with no need of initial-state preparation. Here we clarify this issue by developing a theoretical framework based on sequential quantum channels for iterative QPE. We find that QPE of multiple eigenvalues can be efficiently realized for arbitrary initial target system state by actively utilizing the measurement backaction of iterative QPE on the target system with a long coherence time. Specifically, we investigate two iterative QPE schemes based on sequential Ramsey interferometry measurements (RIMs) of an ancilla qubit: (a) the repetitive scheme, which conducts repetitive RIMs to achieve the standard quantum limit in estimating the eigenvalues; (b) the adaptive scheme, which adjusts the parameters of each RIM based on prior measurement outcomes to attain the Heisenberg limit. In both schemes, sequential ancilla measurements generate sequential quantum channels on the target system, gradually steering it to the eigenstates of the estimated unitary operator, while the measurement statistics of the ancilla can reveal the embedded information about its eigenvalues with proper postprocessing. We demonstrate the analysis by simulating a central spin model and evaluate the performance and noise resilience of both schemes.
In this paper, we propose a novel scheme for deterministic hierarchical joint remote state preparation (HJRSP). Two senders and three receivers are involved. One sender has the amplitude information of the target state to be prepared at the receiver's port, while the other one has the phase information. There is a hierarchy among the receivers concerning their powers to reconstruct the target state. 6-particle partially entangled state is used as the quantum channel. Participants perform projective measurements under elaborate measurement bases. According to their measurement results, the receiver reconstructs the target state by means of appropriate unitary operation. The higher-power receiver reconstructs the target state independently, and the lower-power receiver reconstructs the target state if and only if all the other receivers help him (her). Different from the previous HJRSP scheme utilizing partially entangled channels, auxiliary qubit is not required in our scheme, and unit success probability can be achieved. It is shown that the success probability is independent of the parameters of the partially entangled quantum channel.