Barenco gate implementation using driven two- and three-qubit spin chains

Abstract

We propose a protocol for implementing Barenco-type multi-qubit controlled gates using short driven spin chains. Starting from an Ising interaction with a transverse drive on the last spin, we construct an effective two-qubit Hamiltonian whose time evolution implements the Barenco gate $V_2(\varphi,ω,φ)$ and, in particular, a CNOT gate. We then embed this construction into a three-qubit $XXZ$ chain to realize the three-qubit Barenco gate $V_3(\varphi,ω,φ)$, which includes the Toffoli gate as a special case. The derivation is fully analytical: we perform a sequence of unitary transformations, identify decoupled subspaces, and apply a rotating-wave approximation to obtain simple effective Hamiltonians. We derive explicit conditions on the coupling strengths and driving parameters, provide closed-form expressions for the time-evolution operators in each relevant subspace, and characterize the quality of the implementation using the operator fidelity. Numerical simulations show that the protocol achieves high fidelities over broad parameter ranges, demonstrating its robustness and suitability for quantum information processing in spin-chain platforms.

Figures (2)

• Figure 1: Average fidelity $\braket{F(t)}$ for the two- and three-qubit spin-chain implementations of the Barenco operators $V_2(\varphi,\omega,\phi)$ and $V_3(\varphi,\omega,\phi)$ as a function of time. We average $1100$ realizations by varying $0\le \varphi \le \pi$, $0\le \omega < 2\pi$, and $0\le \phi < 2\pi$ with steps of $10\%$ of each range. Parameters: $\alpha=2A$ for both cases, and $J=\sqrt{l^2-k^2}\,A$ with $l=17$ and $k=8$ for the three-qubit case. The curves reach $\braket{F(\pi)}>0.998$, illustrating efficiency and robustness of the protocol.
• Figure 2: Disorder analysis with variations $m\!\to\! m+\delta m$, $k\!\to\! k+\delta k$, and $l\!\to\! l+\delta l$ about $(m,k,l)=(2,8,17)$. For each $\delta$ value (from $-0.10$ to $0.10$ in steps of $0.01$), we average $1100$ realizations (blue points) over $(\varphi,\omega,\phi)$ as in Fig. \ref{['Fig1']}. Red points and the dashed line indicate per-$\delta$ averages. Panels: (a) $\delta m$ only; (b) $\delta k$ only; (c) $\delta l$ only; (d) joint variation $\delta m=\delta k=\delta l=\delta$. Even with disorder, average fidelities remain above $0.99$ for $|\delta|<0.01$ across all panels, indicating strong robustness.