Abstract Entanglement can be distributed using a carrier which is always separable from the rest of the systems involved. Up to now, this effect has predominantly been analyzed in the case where the carrier-system interactions take the form of ideal unitary operations, thus leaving untested its robustness against either non-unitary or unitary errors. We address this issue by considering the effect of incoherent dynamics acting alongside imperfect unitary interactions. In particular, we determine the restrictions that need to be placed on the interaction time, as well as the strength of the incoherent dynamics. We find that with non-unitary errors, we can still successfully distribute entanglement, provided we measure the carrier in a suitable basis. Introducing imperfections in the unitary dynamics, we show that entanglement gain is possible even with substantial unitary errors. Moreover, certain variations in the strength of the unitary dynamics can allow for greater robustness against non-unitary errors. Therefore, even in experimental settings where unitary operations cannot be carried out without imperfections, it is still possible to generate entanglement between two systems using a separable carrier.
Abstract Entanglement can be distributed using a carrier which is always separable from the rest of the systems involved. Up to now, this effect has predominantly been analyzed in the case where the carrier-system interactions take the form of ideal unitary operations, thus leaving untested its robustness against either non-unitary or unitary errors. We address this issue by considering the effect of incoherent dynamics acting alongside imperfect unitary interactions. In particular, we determine the restrictions that need to be placed on the interaction time, as well as the strength of the incoherent dynamics. We find that with non-unitary errors, we can still successfully distribute entanglement, provided we measure the carrier in a suitable basis. Introducing imperfections in the unitary dynamics, we show that entanglement gain is possible even with substantial unitary errors. Moreover, certain variations in the strength of the unitary dynamics can allow for greater robustness against non-unitary errors. Therefore, even in experimental settings where unitary operations cannot be carried out without imperfections, it is still possible to generate entanglement between two systems using a separable carrier.
Distributing entanglement between two or more systems enables a variety of quantum information applications. Surprisingly, entanglement can be distributed using a carrier which is always separable from the rest of the systems involved. Up to now, this effect has predominantly been analyzed in the case where the carrier-system interactions take the form of ideal unitary operations, thus leaving untested its robustness against either non-unitary or unitary errors. We address this issue by considering the effect of incoherent dynamics acting alongside imperfect unitary interactions. In particular, we determine the restrictions that need to be placed on the interaction time, as well as the strength of the incoherent dynamics. We have shown that entanglement distribution is still possible, even though the amount of entanglement gained between the systems is not maximal. Therefore, even in experimental settings where unitary operations cannot be carried out without imperfections, it is still possible to generate entanglement between two systems using a separable carrier.
We recently proposed an entanglement distillation protocol based on local operations and classical communication that probabilistically and universally distills a maximally entangled Bell pair from multiple copies of an unknown two-qubit state [Phys. Rev. Lett. 135, 260202 (2025)]. Here, we investigate the robustness of this protocol against imperfections in its implementation. We consider several incoherent and coherent gate-error models, as well as measurement readout errors, and determine the regimes in which distillation enhances the fidelity with the target Bell state. We find that the protocol remains effective over substantial error ranges, although its robustness depends strongly on the type of error. We further analyze asymmetric errors between the two parties and identify conditions under which complementary postselection outcomes yield identical distillation dynamics, allowing the success probability to be enhanced.
The main focus of this thesis is quantum entanglement and, more specifically, pro- tocols that allow its distribution between distant places. In the last decades, with the rise of attention on quantum technologies, entangle- ment is mostly seen as a resource, that could be exchanged, measured, and ideally stored and used when convenient. However, entanglement is not as stable and cheap as the classical resources. One reason for this is that entanglement between two quantum systems is easily disrupted by the interaction with the environment that surrounds them. The presence of noise is even more significant when we wish to entangle two systems that are very far away, a task that is essential for quantum communication technologies. In this case, what can disrupt the entanglement is not only the noise that the particles experience locally, but also the noise in the quantum channel during the communication process, which can be significant over long distances. For this reason, it is important to design distribution protocols that work well in such conditions. In 2003, Cubitt et al. showed the existence of a very curious class of protocols, often called entanglement distribution with separable states (EDSS). They proved that a non-entangled (separable) carrier can be used to increase, or even gener- ate entanglement between two already distant parties. From this, some questions arise. First, what limits or allows an entanglement distribution protocol, if not the entanglement that is sent. Quantum discord was shown to be a necessary, but not sufficient, resource for entanglement distribution. Another natural question is whether these protocols can beat others, for technological purposes, especially in noisy conditions. I will describe some work I have done in an attempt to partially 9 address both of these questions. After introducing various tools and concepts that are useful to understand this topic (chapter 1) and motivate the necessity of entanglement distribution in quantum communication (chapter 2), I will describe a proof-of-principle experiment demon- strating EDSS schemes (chapter 3). Later (chapter 4), I will introduce a more general classification of distribution protocols (excessive or non-excessive protocols), based on whether the entanglement gain is larger than the one that is communicated, describing specific examples and general properties of such protocols. I will then partially address the problem regarding the robustness to the noise of communica- tion protocols (chapter 5). Finally, in chapter 6, I will describe how entanglement of a bipartite state can grow with a quantum measurement with unknown results. I discuss the properties of the measurement and of the initial state that make this process effective.
Abstract The ability to reliably distribute entanglement among the nodes of a network is an essential requirement for the development of effective quantum communication protocols and the realization of useful quantum networks. It has been demonstrated, in different contexts, that two remote systems can be entangled via local interactions with a carrier system that always remains in a separable state with respect to such distant particles. We develop a strategy for entanglement distribution via separable carriers that can be applied to any number of network nodes to achieve various entanglement distribution patterns. We show that our protocol results in multipartite entanglement, while the carrier mediating the process is always in a separable state with respect to the network. We provide examples showcasing the flexibility of our approach and propose a scheme of principle for the experimental demonstration of the protocol.
Distributing entanglement over long distances remains a challenge due to its fragility when exposed to environmental effects. In this work, we compare various entanglement distribution protocols in a realistic noisy fiber network. We focus specifically on two schemes that only require the sending of a nonentangled carrier photon to remote nodes of the network. These protocols rely on optical / gates, and we vary the probability with which they can be successfully performed. Encoding our entangled states in photon polarization, we analyze the effect of depolarizing noise on the photonic states as the carrier passes through the fibers. Building a robust model of photon loss and calculating the distillable entanglement of the noisy states, we find the entanglement distribution rate. We discover that methods involving a separable carrier can reach a higher rate than the standard entanglement distribution protocol, provided that the success probability of the optical / gates is sufficiently high.
We consider what is the effective amount of entanglement when the measurement operators one has at their disposal are restricted. Such a scenario occurs when superselection rules are in effect or when there are imperfections in our measurement setup. Given a quantum state and the restrictions on measurements, we consider the following scenario. Imagine we have an ideal, or non-restricted, measurement setup and a state ρ1 and a non-ideal, or restricted, measurement setup and a state ρ2. Then the minimum amount of entanglement in ρ1 so that all quantum communication protocols still perform with the same fidelity as with ρ2 is effectively the entanglement with restricted measurement setup. For indistinguishable particles, we find that any quantum communication protocol that can be performed with indistinguishable particles, can thus be performed with a ``normal'' state of no more than EP of entanglement. For the imperfect measurement apparatus we find an upper bound for the effective entanglement, which is a linear function of the usual entanglement, with the proportionality factor dependent only on the imperfections in the measurement device.