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arXiv:2206.10686 · Quantum

Remotely Controlled Entanglement Generation

Ferran Riera-Sàbat, Pavel Sekatski, Wolfgang Dür

Published Jan 24, 2023• 9 citations
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Abstract

We consider a system of multiple qubits without any quantum control. We show that one can mediate entanglement between different subsystems in a controlled way by adding a (locally) controlled auxiliary system of the same size that couples via an always-on, distant dependent interaction to the system qubits. Solely by changing the internal state of the control system, one can selectively couple it to selected qubits, and ultimately generate different kinds of entanglement within the system. This provides an alternative way for quantum control and quantum gates that does not rely on the ability to switch interactions on and off at will, and can serve as a locally controlled quantum switch where all entanglement patterns can be created. We demonstrate that such an approach also offers an increased error tolerance w.r.t. position fluctuations.

DOI
10.22331/q-2023-01-24-904

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We consider a system of multiple qubits without any quantum control. We show that one can mediate entanglement between different subsystems in a controlled way by adding a (locally) controlled auxiliary system of the same size that couples via an always-on, distant dependent interaction to the system qubits. Solely by changing the internal state of the control system, one can selectively couple it to selected qubits, and ultimately generate different kinds of entanglement within the system. This provides an alternative way for quantum control and quantum gates that does not rely on the ability to switch interactions on and off at will, and can serve as a locally controlled quantum switch where all entanglement patterns can be created. We demonstrate that such an approach also offers an increased error tolerance w.r.t. position fluctuations.
Quantum information science relies on the feature of distant quantum entities (mostly "qubits") to form non-local states. A main challenge consists of generating such non-local entangled states between qubits. We exploit the fact that for coupled qubits, the eigenstates of the coupled system are usually highly entangled, and of different excitation energies. This allows to address the different entangled eigenstates by frequency-dependent control schemes. In our proposal, we present such a control mechanism, and demonstrate how it can be used to create entanglement from a fully separable initial state. The mechanism of our choice is time-delayed quantum-coherent feedback. If a qubit occupation decays via the emission of a photon, one can store this photon for a delay time τ and couple the radiation back into the qubit afterwards. Through the choice of τ, one can set the phase of the feedback, which will then lead to either an increased or decreased qubit decay. Since this phase depends on sin(ωτ), this effect strongly depends on the qubit frequency ω. In particular, it can be used to separate different entangled states in a quantum network by enhancing the decay of all entangled eigenstates except one. We discuss this protocol on the example of two coupled qubits, and analyze in detail its effectiveness depending on the feedback delay time τ.
Several recent experiments have shown that long-range exchange interactions can determine collective magnetic ground states of nanostructures in bulk and on surfaces. The ability to generate and control entanglement in a system with long-range interaction will be of great importance for future quantum technology. An important step forward to reach this goal is the creation of entangled states for spins of distant magnetic atoms. Herein, the generation of long-distance entanglement between remote spins at large separations in bulk and on surface is studied theoretically, based on a quantum spin Hamiltonian and time-dependent Schrödinger equation for experimentally realized conditions. We demonstrate that long-distance entanglement can be generated between remote spins by using an appropriate quantum spin chain (a quantum mediator), composed by sets of antiferromagnetically coupled spin dimers. Ground state properties and quantum spin dynamics of entangled atoms are studied. We demonstrate that one can increase or suppress entanglement by adding a single spin in the mediator. The obtained result is explained by monogamy property of entanglement distribution inside a quantum spin system. We present a novel approach for non-local sensing of remote magnetic adatoms via spin entanglement.
An alternative method is proposed for generating efficiently the entangled states in semiconductor quantum dots by using the spatial phase. The method does not rely on the frequently used dipole–dipole interaction among the qubits at all. It was found that as a result of breaking the selection rule by the spatial phase it enables one to create the entanglement that can never be realized when closely located identical quantum dots are used as qubit gates. In this paper generation of entanglements in two- and three-qubit systems is investigated. The experimental setup based on our approach is much clearer and simpler than that based on the dipole-dipole interaction.
Quantum information science (QIS) is an interdisciplinary field that explores the principles and applications of quantum mechanics in information processing, communication and computation. QIS leverages the unique properties of quantum systems such as entanglement and superposition to develop new ways of encoding, transmitting and manipulating information. Eventhough quantum technologies hold enormous promises for quantum processing or quantum communication, their potential is still limited by the ability to transmit quantum information and control quantum systems with high precision. Distributing entanglement over large distances holds vast potential for communication applications. However, the inherent fragility of entangled states presents a significant challenge in their distribution. In Part I of this thesis, we investigate the distribution of entanglement over noisy channels. Concerning bipartite entanglement, we introduce novel hardware-agnostic protocols and tools for various tasks that are relevant for reliable quantum communication in the context of quantum computing, including entanglement purification, entanglement verification and fidelity witnessing. Our protocols rely on auxiliary high-dimensional entangled systems to gather non-local information and on new approaches that use but do not consume all available resources. These new tools enhance the performance of our protocols compared to previous strategies, working particularly well for the verification of entangled states where without using auxiliary resources an exponential improvement even with ``optimal'' strategies is obtained. We also investigate the distribution of tripartite entanglement. On the one hand, we extend our protocols to operate with GHZ states, and on the other hand, we develop a new entanglement swapping protocol and enhance the only known entanglement purification protocol for W states. These two strategies collectively enable us to introduce the first quantum repeater for W states. In Part II of the thesis, we explore quantum processing architectures based on logically encoded systems. We investigate many-body systems subject to non-tunable distant-dependent interactions. We demonstrate how using logical encodings we can execute multiqubit non-local gates in a controlled manner solely using local control. This method presents different functionalities which can be exploited for various applications. (1) Efficient way to control the interaction topology of the system, which we use for quantum control where we show how a logical system can be used to control another system by mediating non-local interactions. Based on this functionality, we also develop a quantum simulator with enhanced performance compared to prior methodologies. (2) Interactions resilient against position fluctuations of the system. We demonstrate a significant enhancement in entangling gate fidelities in different systems subjected to thermal noise. (3) Amplification of interactions. We exploit this capability to design a modular quantum computing architecture, which uses multipartite entanglement to implement different intermodular gates.
We demonstrate that two remote qubits can be entangled through an optically active intermediary even if the coupling strengths between mediator and qubits are different. This is true for a broad class of interactions. We consider two contrasting scenarios. Firstly, we extend the analysis of a previously studied gate operation which relies on pulsed, dynamical control of the optical state and which may be performed quickly. We show that remote spins can be entangled in this case even when the intermediary coupling strengths are unequal. Secondly, we propose an alternative adiabatic control procedure, and find that the system requirements become even less restrictive in this case. The scheme could be tested immediately in a range of systems including molecules, quantum dots, or defects in crystals.