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arXiv:1912.03171 · Physical Review A

Generation of n -qubit W states using spin torque

Amritesh Sharma, Ashwin A. Tulapurkar

Published Jun 22, 2020• 19 citations
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Abstract

The $W$ state is a symmetrically entangled multipartite state where a single excitation is shared by all parties. It is an important resource for various quantum algorithms and communication systems, and hence, its preparation is of immense interest to the quantum information community. We examine here a deterministic scheme to prepare a $W$ state of an $n$-qubit system with all-to-all pairwise exchange interaction between $n$ qubits. This relies on sharing superposed excitations of a smaller number of $q$ qubits among others. We present a bound on the maximal jumps from $q$ to $n$ and formalize a scheme to generate the ${W}_{n}$ state in $O(logn)$ stages. We demonstrate this scheme in the context of spin-torque-based quantum computing architecture that is characterized by repeated interactions between static and flying qubits.

DOI
10.1103/physreva.101.062330

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The WWW state is a symmetrically entangled multipartite state where a single excitation is shared by all parties. It is an important resource for various quantum algorithms and communication systems, and hence, its preparation is of immense interest to the quantum information community. We examine here a deterministic scheme to prepare a WWW state of an nnn-qubit system with all-to-all pairwise exchange interaction between nnn qubits. This relies on sharing superposed excitations of a smaller number of qqq qubits among others. We present a bound on the maximal jumps from qqq to nnn and formalize a scheme to generate the Wn{W}_{n}Wn​ state in O(logn)O(logn)O(logn) stages. We demonstrate this scheme in the context of spin-torque-based quantum computing architecture that is characterized by repeated interactions between static and flying qubits.
There has been an extensive development in the use of multi-partite entanglement as a resource for various quantum information processing tasks. In this paper we focus on preparing arbitrary spin eigenstates whose subset contain important entangled resources like Dicke states as well as some other sub-radiant states that are difficult to prepare. Leveraging on the symmetry of these states we consider uniform pairwise exchange coupling between every pair of qubits. Starting from a product state of a given spin eigenstate with a single qubit state, another spin eigenstate can be prepared using simple time evolutions. This expansion paves a deterministic approach to prepare arbitrary Dicke states in linear steps. We discuss an improvement in this cost building up on a previous work for W states deterministic preparation in logarithmic circuit depth. The modified algorithm requires several iterations of pumping spin angular momentum into the system and is akin to the amplitude amplification in Grover search. As a use case to demonstrate the proposed scheme, we choose a system of non-interacting static spin qubits connected to a ferromagnetic reservoir. The flying qubits emerging from the reservoir locally interact with static qubits successively, mediating an in-direct exchange interaction between all the pairs.
This paper presents the generation of Greenberger–Horne–Zeilinger (GHZ) states using a spin-torque-based qubit architecture, introducing a hardware-native decomposition of the Hadamard and controlled NOT (CNOT) gates. Unlike optical or superconducting implementations, the proposed approach exploits intrinsic spin-transfer-torque dynamics to realize single-qubit and entangling operations with minimal external control. The method reduces gate overhead and decoherence, enabling high fidelity (> 99%) GHZ formation. An unequal entanglement amplitude naturally arises from spin-torque non-linearities and is analytically characterized as a tunable property advantageous for quantum secret sharing (QSS) and asymmetric quantum communication schemes. Numerical simulations of state evolution and density-matrix fidelity validate the robustness and efficiency of the approach. The results demonstrate that current-driven spin-torque interactions provide a compact, energy efficient platform for scalable multi-qubit entanglement, linking spintronic device physics with quantum information processing.
Entanglement represents “the” key resource for several applications of quantum information processing, ranging from quantum communications to distributed quantum computing. Despite its fundamental importance, deterministic generation of maximally entangled qubits represents an on-going open problem. Here, we design a novel generation scheme exhibiting two attractive features, namely, i) deterministically generating different classes – namely, GHZ-like, W-like and graph states – of genuinely multipartite entangled states, ii) without requiring any direct interaction between the qubits. Indeed, the only necessary condition is the possibility of coherently controlling – according to the indefinite causal order framework – the causal order among the unitaries acting on the qubits. Through the paper, we analyze and derive the conditions on the unitaries for deterministic generation, and we provide examples for unitaries practical implementation. We conclude the paper by discussing the scalability of the proposed scheme to higher dimensional genuine multipartite entanglement (GME) states and by introducing some possible applications of the proposal for quantum networks.
We describe a simple, practical scheme for generating multi-qubit W states in resonator-based architectures, in which N Josephson phase qubits are capacitively coupled to a common resonator bus. The entire control sequence consists of three pulses: a local Rabi pulse that excites a single qubit in the circuit; a coupling pulse that transfers the qubit excitation to the resonator bus; and the main, entangling operation that simultaneously couples the bus to all N qubits. If the qubit-resonator coupling strength g is much smaller than the qubit energy splitting, the system initially excited into the near-degenerate single-excitation subspace stays within that subspace, while smoothly evolving toward the fully uniform W state superposition. The duration of the final entangling operation is found to decrease with the total number of the qubits according to t = pi/[2gN^(1/2)], in agreement with some of the previously proposed cavity QED W state generation schemes.
It is known that maximally entangled Bell state and three-qubit W-type states are very useful in various quantum information processing task. Thus the problem of preparation of these type of states is very important in quantum information theory. But the factor which prohibit the generation of the above mentioned pure states shared between two and three distant partners is decoherence. When we send one qubit, from a two qubit state, through decoherence channel like amplitude damping channel, the purity of the qubit is lost and it ends up with a mixed state. Therefore it is very difficult to keep the pure maximally entangled state in a maximally entangled pure state or in an entangled state with high entanglement. In this work we have provided a method by which one can generate experimentally a maximally entangled Bell states shared between distant parties with a non-zero probability when a qubit, from a two qubit general state, passes through decoherence channel. Therefore, despite of the fact that qubit is interacting with the noisy channel, we are able to generate Bell state shared between two distant partners. Further, we have shown that it is possible to generate pure three-qubit W-type states shared between three distant partners using economical quantum cloning machine and weak-measurement based feed-forward control scheme, even though the second and third qubit is interacting with the noisy channel. Lastly, we have shown that the generated three-qubit W-type states can be used in teleporting one of the two non-orthogonal states.
Quantum entanglement plays an irreplaceable role in various remote quantum information processing tasks. Here we present protocols for generating deterministic and heralded NNN-qubit entangled states across multiple network nodes. By utilizing a pre-shared maximally entangled state and single-qubit operations within an indefinite causal order framework, the multi-qubit entangled state between distant parties can be generated deterministically. The complex entangled state measurements and multiple pre-shared entangled states, are essential in conventional entanglement swapping technique, but are not required in our approach. This greatly reduces the complexity of the quantum circuit and makes it more experimentally feasible. Furthermore, we develop optical architectures to implement these protocols by encoding qubits in polarization degree of freedom. The results indicate that our protocols significantly improve the efficiency of long-distance entanglement generation and provide a practical framework for establishing large-scale quantum networks.