Efficient Multiqubit Entanglement via a Spin Bus

We propose an experimentally feasible architecture with controllable long-range couplings built up from local exchange interactions. The scheme consists of a spin bus, with strong, always-on interactions, coupled dynamically to external qubits of the Loss and DiVincenzo type. Long-range correlations are enabled by a spectral gap occurring in a finite-size chain. The bus can also form a hub for multiqubit entangling operations. We show how multiqubit gates may be used to efficiently generate $W$ states (an important entanglement resource). The spin bus therefore provides a route for scalable solid-state quantum computation, using currently available experimental resources.

Figure 1

Spin-bus architectures. (a) The spin bus is a chain of electronic spins (●) with strong, static couplings (heavy lines). External qubits (○) can be coupled to the bus at any node (light lines). Effective long-range interactions allow for communication between sectors dedicated to rotation, readout or memory, which may benefit from isolation. (b) Additional local couplings enable parallel interactions, in addition to bus-mediated interactions. (c) Coded qubits or larger clusters. (d) Within the ground state manifold, the bus acts as a simple spin-$1/2$ qubit, except for its plurality of qubit couplings.

Figure 2

Energy spectra for an $N=7$ bus. The ground state doublet (the bus manifold) lies below the gap (red lines). Excited states lie above the gap (black lines). (a) No coupled qubits, $B=0$: bus manifold is doubly degenerate. (b) No coupled qubits, $B=0.03$: the doublet splits, defining the working states of the bus. (c) One coupled qubit, $B=0$: bus and qubit states hybridize to form a singlet and a triplet, split by $J_q^{*}$. (d) One coupled qubit, $B=0.03$: the triplet states split. In (a)–(d), we use $J_b=1$ and $J_q=0.3$, with the dimensionless Zeeman coupling $H_B=\sum _{i}B{s}_{iz}$, summed over all spins. Inset: log-log plot of the effective coupling $J_i^{*}$ from Eq. (3), averaged over the antiferromagnetic bus nodes (lower, black curve), with bare couplings set to $J_i=1$. Upper (red) curve shows apparent asymptotic behavior, $J_q^{*}\simeq 1.198N^{-1/2}$.