Parallel Information Transfer in a Multinode Quantum Information Processor

We describe a method for coupling disjoint quantum bits (qubits) in different local processing nodes of a distributed node quantum information processor. An effective channel for information transfer between nodes is obtained by moving the system into an interaction frame where all pairs of cross-node qubits are effectively coupled via an exchange interaction between actuator elements of each node. All control is achieved via actuator-only modulation, leading to fast implementations of a universal set of internode quantum gates. The method is expected to be nearly independent of actuator decoherence and may be made insensitive to experimental variations of system parameters by appropriate design of control sequences. We show, in particular, how the induced cross-node coupling channel may be used to swap the complete quantum states of the local processors in parallel.

Figure 1

$2\times (1e\mathrm{\text{−}}3n)$ node schematic. The nodes are taken to be identical, with resolved anisotropic hyperfine interactions (solid red lines) between electron actuator spins and nuclear processor spins. The local processors are initially disjoint, but may be effectively coupled (dotted lines) by modulating an isotropic actuator exchange interaction (solid blue double line) and moving into an appropriate microwave Hamiltonian interaction frame. The spin labeling is $e_i$ for electron actuator spins and $n_{ij}$ for nuclear processor spins, where $i$ labels the nodes and $j$ labels the qubits.

Figure 2

Energy Structure of $2\times (1e\mathrm{\text{−}}1n)$ system. Quantum information is encoded in the states $|\downarrow \downarrow 00⟩$, $|\downarrow \downarrow 01⟩$, $|\downarrow \downarrow 10⟩$, and $|\downarrow \downarrow 11⟩$ of the actuator ground-state (computational) manifold, where arrows indicate electron actuator spin states and binary digits indicate nuclear processor spin states. The desired transition (bold red arrow) for a swap operation is implemented by applying a selective microwave field that induces transitions between the two manifolds (dotted lines), effectively moving the induced cross-node processor transitions (dashed lines) in the actuator zero-quantum (ZQ) manifold to the computational manifold. Note that the actuator excited state manifold, $|\uparrow \uparrow ⟩$, is not included as it is not involved in the internode transfer process.

Figure 3

Internode coupling network. The induced coupling network of local processor elements consists of interactions between every pair of cross-node spins. For implementation of a parallel swap operation, we wish to keep interactions between identical spins (solid red lines) while refocusing all other interactions (dotted lines).

Figure 4

Channel noise robustness. A plot of the channel fidelity, $F$, as a function of noise strength, $T_1=T_2$, scaled by computing $-\log [1-F]$. The induced channel (solid line) performs significantly better than a serial swap operation (dotted line) [36]. Given a modest Rabi frequency, ${\omega }_1=100\mathrm{MHz}$, the induced channel is nearly independent of actuator decoherence for electron relaxation times above $100\mu \mathrm{s}$.