2 research outputs found
Multi-Level Variational Spectroscopy using a Programmable Quantum Simulator
Energy spectroscopy is a powerful tool with diverse applications across
various disciplines. The advent of programmable digital quantum simulators
opens new possibilities for conducting spectroscopy on various models using a
single device. Variational quantum-classical algorithms have emerged as a
promising approach for achieving such tasks on near-term quantum simulators,
despite facing significant quantum and classical resource overheads. Here, we
experimentally demonstrate multi-level variational spectroscopy for fundamental
many-body Hamiltonians using a superconducting programmable digital quantum
simulator. By exploiting symmetries, we effectively reduce circuit depth and
optimization parameters allowing us to go beyond the ground state. Combined
with the subspace search method, we achieve full spectroscopy for a 4-qubit
Heisenberg spin chain, yielding an average deviation of 0.13 between
experimental and theoretical energies, assuming unity coupling strength. Our
method, when extended to 8-qubit Heisenberg and transverse-field Ising
Hamiltonians, successfully determines the three lowest energy levels. In
achieving the above, we introduce a circuit-agnostic waveform compilation
method that enhances the robustness of our simulator against signal crosstalk.
Our study highlights symmetry-assisted resource efficiency in variational
quantum algorithms and lays the foundation for practical spectroscopy on
near-term quantum simulators, with potential applications in quantum chemistry
and condensed matter physics
Cancelling microwave crosstalk with fixed-frequency qubits
Scalable quantum information processing requires that modular gate operations
can be executed in parallel. The presence of crosstalk decreases the individual
addressability, causing erroneous results during simultaneous operations. For
superconducting qubits which operate in the microwave regime, electromagnetic
isolation is often limited due to design constraints, leading to signal
crosstalk that can deteriorate the quality of simultaneous gate operations.
Here, we propose and demonstrate a method based on AC Stark effect for
calibrating the microwave signal crosstalk. The method is suitable for
processors based on fixed-frequency qubits which are known for high coherence
and simple control. The optimal compensation parameters can be reliably
identified from a well-defined interference pattern. We implement the method on
an array of 7 superconducting qubits, and show its effectiveness in removing
the majority of crosstalk errors