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Physical Review A

Symmetry-protected creation of superposition states and entanglement using circulant Hamiltonians

Razmik G. Unanyan, Bruce W. Shore, Michael Fleischhauer, Nikolay V. Vitanov

Published Feb 6, 2007• 4 citations
Publisher

Abstract

We describe the use of a special interaction symmetry for the robust generation of the totally symmetric superposition state or entangled state of an $N$-state system. The required symmetry of the Hamiltonian is that of a circulant matrix. Such a matrix has the important property that its eigenstates are independent of the matrix elements as long as the circulant symmetry is maintained. One of the eigenvectors is the target superposition. By inducing a slow evolution of the Hamiltonian into the circulant form, adiabatic following will generate the desired superposition out of a convenient initial state such as a product state. The creation process is robust: it is insensitive to details of the interaction as long as the final Hamiltonian has the required symmetry. We illustrate the procedure with a simple example: a ring of quantum wells that permit interwell tunneling, into which a single atom is placed. By carrying out adiabatic evolution the state vector approaches an equal distribution of probability amplitudes in each well.

DOI
10.1103/physreva.75.022305

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We propose a method for the realization of the two-qubit quantum Fourier transform (QFT) using a Hamiltonian which possesses the circulant symmetry. Importantly, the eigenvectors of the circulant matrices are the Fourier modes and do not depend on the magnitude of the Hamiltonian elements as long as the circulant symmetry is preserved. The QFT implementation relies on the adiabatic transition from each of the spin product states to the respective quantum Fourier superposition states. We show that in ion traps one can obtain a Hamiltonian with the circulant symmetry by tuning the spin-spin interaction between the trapped ions. We present numerical results which demonstrate that very high fidelity can be obtained with realistic experimental resources. We also describe how the gate can be accelerated by using a "shortcut-to-adiabaticity" field.
We propose a method for the realization of the two-qubit quantum Fourier transform (QFT) using a Hamiltonian which possesses the circulant symmetry. Importantly, the eigenvectors of the circulant matrices are the Fourier modes and do not depend on the magnitude of the Hamiltonian elements as long as the circulant symmetry is preserved. The QFT implementation relies on the adiabatic transition from each of the spin product states to the respective quantum Fourier superposition states. We show that in ion traps one can obtain a Hamiltonian with the circulant symmetry by tuning the spin-spin interaction between the trapped ions. We present numerical results which demonstrate that very high fidelity can be obtained with realistic experimental resources. We also describe how the gate can be accelerated by using a "shortcut-to-adiabaticity" field.
Entanglement between spatially distant qubits is perhaps the most counterintuitive and vital resource for distributed quantum computing. However, despite a few special cases, there is no known general procedure to maximally entangle two distant parts of an interacting many-body system. Here we present a symmetry-based approach, whereby one applies several timed pulses to drive a system to a particular symmetry sector with maximal bipartite long-range entanglement. As a concrete example, we demonstrate how a simple sequence of on-site pulses on a qubit array can efficiently produce multiple stable nonlocal Bell pairs, realizable in present-day atomic and photonic experimental platforms. More generally, our approach paves a route for exotic state preparation by harnessing symmetry. For instance, we show how it allows the creation of long-sought-after superconducting η pairs in a repulsive Hubbard model. Published by the American Physical Society 2024
We consider systems of two and three qubits, mutually coupled by Heisenberg-type exchange interaction and interacting with external laser fields. We show that these systems allow one to create maximally entangled Bell states, as well as three-qubit Greenberger-Horne-Zeilinger and WWW states. In particular, we point out that some of the target states are the eigenstates of the initial bare system. Due to this, one can create entangled states by means of pulse area and adiabatic techniques, when starting from a separable (nonentangled) ground state. On the other hand, for target states, not present initially in the eigensystem of the model, we apply the robust stimulated Raman adiabatic passage and $\ensuremath{\pi}$ pulse techniques, that create desired coherent superpositions of nonentangled eigenstates.
Preparation of quantum entanglement states is crucial yet challenging to quantum communication and quantum computation. Recent studies have shown that the intermediate states of adiabatic Grover‘s algorithm are entanglement, although its initial and final states are product states. In light of this result, this paper presents a novel approach of preparing entanglement states based on adiabatic quantum evolution. Firstly, the desired entanglement states are encoded as the ground states of a final Hamiltonian, and the quantum system is started with an easily constructed ground state of an initial Hamiltonian. Secondly, the initial Hamiltonian adiabatically evolved to the final Hamiltonian. At the end of the evolution, the entangled states can be obtained with reasonably high probability. As a result, the whole preparation process of entanglement states looks like typical adiabatic quantum computation. The validation of the proposed approach has been made in the context of EPR and three-particle GHZ states, which show that the relevant entanglement states can be successfully prepared by adiabatic quantum evolution.