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We propose a scheme to generate entanglement between two distant qubits (two-level atom) which are separately trapped in their own (in general) non-Markovian dissipative cavities by utilizing entangling swapping, considering the case in which the qubits can move along their cavity axes rather than a static state of motion. We first examine the role of movement of the qubit by studying the entropy evolution for each subsystem. The average entropy over the initial states of the qubit is calculated. Then by performing a Bell state measurement on the fields leaving the cavities, we swap the entanglement between qubit-field in each cavity into qubit-qubit and field-field subsystems. The entangling power is used to measure the average amount of swapped entanglement over all possible pure initial states. Our results are presented in two weak and strong coupling regimes, illustrating the positive role of movement of the qubits on the swapped entanglement. It is revealed that by considering certain conditions for the initial state of qubits, it is possible to achieve a maximally long-leaving stationary entanglement (Bell state) which is entirely independent of the environmental variables as well as the velocity of qubits. This happens when the two qubits have the same velocities.
In recent decades, there is a great deal of evidence that quantum phenomena play a central role in the development of information theory. Coherent superposition is one of these features which is usually referred as quantum coherence. The non-local quantum correlations among composite subsystems is called entanglement.[1] The importance of quantum entanglement arises from its various exciting applications such as quantum teleportation,[2,3] quantum cryptography,[4] sensitive measurements,[5] quantum telecloning,[6,7] and superdense coding.[8] Due to the rapid growth of the applications of these kinds of quantum states in quantum information processing implementations, a great deal of attention has been paid to the generation and detection of entangled states. Most of these proposals rely on the interaction of atoms (real or artificial) with optical cavities.[9] Other proposals include quantum dots,[10] atomic ensembles,[11] superconducting quantum interference devices,[12,13] photon pairs,[14] superconducting qubits,[15] and trapped ions.[16–18]
Approximately, all of the introduced schemes depend on the interactions (direct or indirect) between subsystems. For instance, it has been shown that the Jaynes–Cummings model (JCM), which describes the interaction of atoms (two- or multi-level) with cavity field,[19] could generate entanglement between an atom and a quantized field. This model has been extended to include the interaction of multiple atoms with a multi-mode electromagnetic field.[20] Thanks to the nonlocality of quantum correlations, it is possible to entangle two or more particles which are distributed over distances without any interactions and, or common history. This phenomenon is called entanglement swapping.[21,22] In this protocol, the basic recipe is to make a more general system. Then by projection of the quantum state of the whole system onto a maximally entangled Bell state, it is possible to swap the entanglement between these subsystems. There have been many works on this interesting topic. For instance, it has been generated for continuous-variable systems in Ref. [23]. Multiparticle entanglement swapping has been studied in Ref. [24]. The unconditional entanglement swapping has been experimentally demonstrated in Ref. [25]. In Ref. [26], the authors have discussed this phenomenon using a quantum-dot spin system. Researchers have already shown that entanglement swapping could be used for the optimization of entanglement purification.[27] One-cavity scheme enabling to implement delayed choice for entanglement swapping in cavity QED has been investigated in Ref. [28]. The effect of detuning and Kerr medium on the entanglement swapping has been studied in Ref. [29]. Recently, a high-fidelity unconditional entanglement swapping experiment in a superconducting circuit has been performed in Ref. [30]. Very recently, the swapping of entangled states between two pairs of photons emitted by a single quantum dot has been performed experimentally.[31]
On the other hand, contrary to the closed systems which are ideal, the real physical systems are open. This means that dissipation is always present in those systems. Actually, the inescapable interaction between the aim system and its surrounding environment makes the entanglement fragile.[32,33] Because a long-lasting entangled state is an essential resource for the quantum information theory, many strategies have been devoted to fight against the destructive environmental effects: the theory of open quantum systems.[34–42] However, it should be noted that the idea of interaction of a quantum system with the surrounding environment is not always bad. For instance, it has been shown that there exists a long-living entangled state due to the interaction of a two-qubit system with a common environment.[43–47] This idea has been generalized to an arbitrary number of qubits inside an environment.[48–50] Moreover, quantum reservoir engineering has been proven to be useful in stabilizing open quantum systems[51] and remote entanglement and concentration,[52] etc. Recently, it has been shown that an external classical field is a practical scheme to preserve the entanglement between two dissipative systems.[53] In this regard, many studies such as non-Markovianity,[54] quantum speedup,[55] quantum Fisher information[56] have been presented.
Recent experimental schemes in quantum information processing rely on the control of single qubits inside (optical) cavities. However, in practical implementations, achieving a static state of qubits in a cavity is a difficult task, although it is not impossible. In a pioneering work, the effect of the movement of two qubits inside non-Markovian environments on the protection of the initial entanglement has been studied in Ref. [57]. Moreover, other studies illustrated the effect of movement of qubits (both uniform and accelerated) on the interaction between such qubits and electromagnetic radiation.[58,59] This includes the relativistic velocities for qubits.[60,61] In a very recent paper, the authors showed the positive role of movement of qubits on the entanglement dynamics of an arbitrary number of qubits in a Markovian and/or non-Markovian environment.[62] It has also been shown that when all of the qubits have the same velocity, the stationary state of entanglement is independent of the velocity of qubits.[62]
All of the statements mentioned above motivate us to examine the effect of movement of qubits on the entanglement swapping between two separate subsystems. To end this, we consider two independent cavities, each contains a moving two-level system (qubit) in the presence of dissipation. We model the environment as a set of infinite quantized harmonic oscillators. We take the situation in which each qubit is allowed to move along the cavity axis. We also consider the non-relativistic velocities for qubits. In this situation, the exact dynamics for each subsystem is obtained for both weak and strong coupling regimes corresponding to bad and good cavity limits. We first explore the role of movement of the qubit on the entropy evolution for each subsystem. After that, we perform a Bell state measurement (BSM) on the cavity fields leaving the cavities. This swaps the entanglement between the qubit and the field in each cavity into qubit-qubit and field-field entanglements. We use the concurrence measure[63] to quantify the amount of swapped entanglement. Naturally, this depends on the initial state of the qubits. Our parametrization for the initial states of qubits allows us to establish an input-independent dynamics of entanglement by taking a statistical average over the initial states of two qubits. This is called entangling power originally introduced for unitary maps[64] and then generalized for dissipative channels.[65]
The rest of the paper is organized as follows. In Section
We consider two similar, but separate dissipative cavities, each containing a moving two-level atom (qubit) with an excited (ground) |e⟩ (|g⟩) state. These states are separated by transition frequency ωqb. Each qubit is taken to move along the z-axis of the corresponding cavity with constant velocity v (see Fig.
![](cpb_29_5_050304/cpb_29_5_050304_ieqn1.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn2.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn3.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn4.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn5.gif)
![]() | Fig. 1. Pictorial representation of a setup in which a moving qubit is interacting with a dissipative cavity. |
Formally, it is more convenient to work in the interaction picture. The Hamiltonian (
We suppose that there is no excitation in the cavities before the occurrence of interaction and each atom is in the coherent superposition of the exited |e⟩ and ground state |g⟩ as
Using the time-dependent Schrödinger equation in the interaction picture, we are ready to have the integro-differential equation for the amplitude
As is seen,
Substituting (
![](cpb_29_5_050304/cpb_29_5_050304_ieqn8.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn9.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn10.gif)
In the above section, we have solved the Schrödinger equation for the case of a moving qubit inside a cavity in both non-dissipative and dissipative regimes. Before we consider the entanglement swapping phenomena, we intend to illustrate the effect of the movement of the qubits on the entanglement dynamics. Among the various measures for computing the degree of entanglement between bipartite systems, we use the linear entropy defined as[69]
![](cpb_29_5_050304/cpb_29_5_050304_ieqn11.gif)
Using Eqs. (
Figure
In Fig.
In the above section, we examined the positive role of the movement of a single qubit on the entropy evolution of the qubit and its surrounding environment. In this section, we consider two similar but separable systems introduced in Section
Experimentally, there are many qubits inside each cavity. In this way, the qubits inside each dissipative cavity can be considered as a Bose–Einstein condensate in an optical cavity[70] in the presence of thermal noise which plays the role of the dissipation. Then, the output of the optical cavity of each node is sent to an intermediate site where a Bell-like detection is performed on these optical pairs.[70]
According to the wave function (
In the continuation, we consider the following projection operator:
![](cpb_29_5_050304/cpb_29_5_050304_ieqn12.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn13.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn14.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn15.gif)
The above relation is fulfilled with the following set of solutions:
θ1 = θ2 = 2nπ and arbitrary values of ϕ1 and ϕ2 with n = 0,1,2,…;
θ1 = θ2 and ϕ1 − ϕ2 = 2mπ, with m = 0, ± 1.
These conditions lead to the maximally entangled Bell state (up to an irrelevant global phase)
Again, similar to the above section, we can establish an input-independent dynamics for the swapped entanglement, which is called entangling power. This is carried out by taking a statistical average over all initial states[65]
![](cpb_29_5_050304/cpb_29_5_050304_ieqn16.gif)
![](cpb_29_5_050304/cpb_29_5_050304_ieqn17.gif)
Figure
In order to explore a deep insight into the role of movement of the qubits on the swapped entanglement, we have shown the density matrix of two qubits at the scaled time τ = 1 for two values of β (see Fig.
In Fig.
Finally, we take into account the effect of initial qubit state on the evolution of the swapped entanglement. In Fig.
In this work, we have considered a model to study the possibility of entanglement swapping between two subsystems each contains a moving qubit inside an environment. Our model allows us to treat the environment in both weak and strong coupling regimes corresponding to bad and good cavity limits. By good cavity limit, we have meant an oscillation behavior of entanglement which is due to the memory depth of the environment. We treat the movement of qubits to be classical. Under certain conditions, we have solved the time-dependent Schrödinger equation for each subsystem.
Before considering the entanglement swapping phenomenon, we have examined the influence of the movement of qubits on the entropy evolution of subsystems. This gives us an insight into the possible role of the movement of qubits on the entanglement dynamics. Our parametrization for the initial state of the qubits in each subsystem allows us to construct an initial state independently for the entropy (and later on for the swapped entanglement). This has been carried out by taking a statistical average over all of the initial states of the qubits. The results show the entanglement between qubits and its surrounding environment due to the interaction among them. However, due to the environmental effects, the entropy has decaying behavior. This deterioration of entanglement is suppressed in the presence of movement of the qubit.
Then we turn to the problem of entanglement swapping between such two subsystems. Our goal is to swap the stored qubit-field entanglement in each subsystem into qubit-qubit and field-filed entanglements. Since the cavities are not perfect, the photons can leak out them. This allows us to perform a BSM on the photons leaving the cavities. We have obtained the analytical expression for the normalized state of qubit-qubit after BSM. We have used the concurrence parameter to quantify the amount of swapped entanglement. Several interesting and noticeable points are found. First of all, by considering a suitable Bell state for field modes, the concurrence would be independent of the shape of the incoming photons. Second, there is a set of the initial states which lead to a long-lived maximally entangled state corresponding to stationary state
The present work can be considered as an extension of Ref. [72] where it has been shown that entanglement swapping in the presence of dissipation can be performed. First of all, by considering a static state of motion for qubits (β = 0), we recover the results presented in the mentioned work. Therefore, this guarantees the validity of our model. Second, it is interesting to notice that the long-lived maximally swapped entanglement (corresponding to a Bell state) (see Eq. (
We should emphasize that our results could be used in quantum communication applications. The main idea behind these protocols is to transmit and exchange quantum information (entangled states) over long distances.[74] For instance, generating and swapping entanglement are at the heart of quantum repeater protocols. In this regard, since the environmental effects are always present, our results could boost the efficiency of such protocols. We also note that our results could be useful in the preparation of quantum states. For instance, in order to prepare an entangled state between two qubits that are located in their own distinct cavities, it is enough to detect a photon from a cavity. If we do not know that from which cavity the detected photon is coming, the qubits will be in an entangled state.
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