Quantum Information Systems (QIS) refers to the use of coupled quantum mechanical systems (qubits) to perform calculations in ways which are either not possible, or prohibitively time consuming, for existing “classical” computing systems. Qubits are typically some form of two-state system (an electron spin up or down, an atom with two energy levels, a superconducting wire with or without current), which can be in either one of the two states, or a superposition of both states at once. If a QIS “device” with multiple qubits can be sufficiently isolated from the environment, those qubits can be entangled, so they have a shared quantum state which evolves in such a way as to perform a desired computation. When the computation finishes, the result can be read out as a measurement of the state of the system.
A central area of research in QIS is the problem of decoherence, where random influences from the environment (often heat or radiation), can affect the qubits in such a way as to break down their entanglement, changing them from one state to another in an incoherent or uncorrelated way. For superconducting qubits, patterned onto a silicon chip, this decoherence takes the form of phonons (packets of heat energy) disrupting the superconductivity.
The Toback Group is involved in collaborative projects to simulate the process of decoherence in superconducting qubits, by tracking the flow of energy (phonons) through a substrate and model absorption on individual qubit elements. We are the main developers and maintainers of a particle-physics simulation library (G4CMP) to model these processes in ultracryogenic devices, both for QIS and as particle-physics sensors for phonon and charge (ionization) detection (see the SuperCDMS experiment for more details on TES sensors).
With PNNL and MIT we are using G4CMP to develop a chip design optimized for producing and tracking controllable bursts of phonons, and observing their effect on individual qubits. With SLAC, we are expanding the features of G4CMP to support a wider variety of qubit and energy sensor designs, inlcuding transmons and MKIDs, and additional materials beyond germanium and silicon crystals.
Projects and Information
Toback Group QIS Documentation:
General QIS Documentation:
G4CMP Specific Papers:
- G4CMP: Condensed Matter Physics Simulation Using the Geant4 Toolkit
Overview of G4CMP:
G4CMP is a framework designed to add to the Geant4 toolkit for use in condensed matter and low-energy physics. Developed for the low-temperature community, G4CMP is capable of modeling several physics processes relevant to phonon and charge collection at cryogenic temperatures. These include anisotropic phonon transport and focusing, phonon isotope scattering, anharmonic downconversion, oblique charge carrier propagation with inter-valley scattering, and emission of Neganov-Trofimov-Luke phonons by accelerated carriers.
The package provides a collection of particle types, physics processes, and supporting utilities for this purpose. It has been used by collaborators at the SuperCDMS project to successfully reproduce theoretical predictions and experimental observations such as phonon caustics, heat pulse propagation times, and mean carrier drift velocities; the G4CMP package, however, is sufficiently general that it is useful for other experiments employing cryogenic phonon and/or ionization detectors.