We study the entanglement dynamics of two cavities interacting with independent reservoirs. Expectedly, as the cavity entanglement is depleted, it is transferred to the reservoir degrees of freedom. We find also that when the cavity entanglement suddenly disappear, the reservoir entanglement suddenly and necessarily appears. Surprisingly, we show that this entanglement sudden birth can manifest before, simultaneously, or even after entanglement sudden death. Finally, we present an explanatory study of other entanglement partitions and of higher dimensional systems. Dynamical behavior of entanglement under the action of the environment is a central issue in quantum information [1,2,3,4]. Recently, it has been observed that two qubits affected by uncorrelated reservoirs can experience disentanglement in a finite time despite coherence is lost asymptotically [2,3,4,5,6]. This phenomenon, called entanglement sudden death (ESD), has recently deserved a great attention [7,8,9,10,11,12,13], and has been observed in the lab for entangled photon pairs [14], and atomic ensembles [15].To our knowledge ESD has been studied mainly in relation to bipartite systems, while a deeper understanding is associated to the question of where does the lost entanglement finally go. This question would be properly answered by enlarging the system to include reservoir degrees of freedom. Intuitively, we may think that the lost entanglement has to be transferred to the reservoir degrees of freedom. However, is this entanglement swapped continuously? If the bipartite entanglement suffers ESD, what can we say about the transferred entanglement? Should there be a simultaneous entanglement sudden birth (ESB) on reservoir states, or when would this entanglement be created? In this work, we thoroughly study the entanglement transfer from the bipartite system to their independent reservoirs. We show that ESD of a bipartite system state is intimately linked to ESB of entanglement between the reservoirs, though their apparition times follow counterintuitive rules.To illustrate the problem we have chosen the case of entangled cavity photons being affected by dissipation, as in the case of two modes inside the same dissipative cavity or single modes in two different ones. The present study could certainly be extended to other physical systems like matter qubits. First we study the case of qubits for two uncoupled (cavity) modes having up to one photon. Then, we extend our treatment to investigate wether or not the effect is present in higher dimensions (qudits).Since each mode evolves independently, we can learn how to characterize the evolution of the overall system from the mode-reservoir dynamics. The interaction between a single cavity mode and an N -mode reservoir is described through the HamiltonianLet us consider the case when a cavity mode is containing a single photon and its corresponding reservoir is in the vacuum state,where, |0 r = N k=1 |0 k r . It is not difficult to realize that the evolution given by (1) leads to the statewhere the st...
The roles of quantum correlations, entanglement, discord, and dissonance needed for performing unambiguous quantum state discrimination assisted by an auxiliary system are studied. In general, this procedure for conclusive recognition between two nonorthogonal states relies on the availability of entanglement and discord. However, we find that there exist special cases for which the procedure can be successfully achieved without entanglement. In particular, we show that the optimal case for discriminating between two nonorthogonal states prepared with equal a priori probabilities does not require entanglement but quantum dissonance only.
We study the physical implementation of a qutrit quantum computer in the context of trapped ions. Qutrits are defined in terms of electronic levels of trapped ions. We concentrate our attention on a universal two-qutrit gate, which corresponds to a controlled-NOT gate between qutrits. Using this gate and a general gate of an individual qutrit, any gate can be decomposed into a sequence of these gates. In particular, we show how this works for performing the quantum Fourier transform for n qutrits.
We propose a method of generating unitarily single and two-mode field squeezing in an optical cavity with an atomic cloud. Through a suitable laser system, we are able to engineer a squeeze field operator decoupled from the atomic degrees of freedom, yielding a large squeeze parameter that is scaled up by the number of atoms, and realizing degenerate and nondegenerate parametric amplification. By means of the input-output theory we show that ideal squeezed states and perfect squeezing could be approached at the output. The scheme is robust to decoherence processes.
Machine learning employs dynamical algorithms that mimic the human capacity to learn, where the reinforcement learning ones are among the most similar to humans in this respect. On the other hand, adaptability is an essential aspect to perform any task efficiently in a changing environment, and it is fundamental for many purposes, such as natural selection. Here, we propose an algorithm based on successive measurements to adapt one quantum state to a reference unknown state, in the sense of achieving maximum overlap. The protocol naturally provides many identical copies of the reference state, such that in each measurement iteration more information about it is obtained. In our protocol, we consider a system composed of three parts, the "environment" system, which provides the reference state copies; the register, which is an auxiliary subsystem that interacts with the environment to acquire information from it; and the agent, which corresponds to the quantum state that is adapted by digital feedback with input corresponding to the outcome of the measurements on the register. With this proposal we can achieve an average fidelity between the environment and the agent of more than 90% with less than 30 iterations of the protocol. In addition, we extend the formalism to d-dimensional states, reaching an average fidelity of around 80% in less than 400 iterations for d = 11, for a variety of genuinely quantum and semiclassical states. This work paves the way for the development of quantum reinforcement learning protocols using quantum data and for the future deployment of semi-autonomous quantum systems. * F. Albarrán-Arriagada francisco.albarran@usach.cl
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