7 research outputs found
Information flow in parameterized quantum circuits
In this work, we introduce a new way to quantify information flow in quantum
systems, especially for parameterized quantum circuits. We use a graph
representation of the circuits and propose a new distance metric using the
mutual information between gate nodes. We then present an optimization
procedure for variational algorithms using paths based on the distance measure.
We explore the features of the algorithm by means of the variational quantum
eigensolver, in which we compute the ground state energies of the Heisenberg
model. In addition, we employ the method to solve a binary classification
problem using variational quantum classification. From numerical simulations,
we show that our method can be successfully used for optimizing the
parameterized quantum circuits primarily used in near-term algorithms. We
further note that information-flow based paths can be used to improve
convergence of existing stochastic gradient based methods
Quantum computation of eigenvalues within target intervals
There is widespread interest in calculating the energy spectrum of a Hamiltonian, for example to analyze optical spectra and energy deposition by ions in materials. In this study, we propose a quantum algorithm that samples the set of energies within a target energy-interval without requiring good approximations of the target energy-eigenstates. We discuss the implementation of direct and iterative amplification protocols and give resource and runtime estimates. We illustrate initial applications by amplifying excited states on molecular hydrogen
Quantum interference device for controlled two-qubit operations
© 2020, The Author(s). Universal quantum computing relies on high-fidelity entangling operations. Here, we demonstrate that four coupled qubits can operate as a quantum gate, where two qubits control the operation on two target qubits (a four-qubit gate). This configuration can implement four different controlled two-qubit gates: two different entangling swap and phase operations, a phase operation distinguishing states of different parity, and the identity operation (idle quantum gate), where the choice of gate is set by the state of the control qubits. The device exploits quantum interference to control the operation on the target qubits by coupling them to each other via the control qubits. By connecting several four-qubit devices in a two-dimensional lattice, one can achieve a highly connected quantum computer. We consider an implementation of the four-qubit gate with superconducting qubits, using capacitively coupled qubits arranged in a diamond-shaped architecture