Join us at the QSI Seminars to explore quantum state preparation and analog quantum simulations
UTS Associate Professor Chris Ferrie.
Seminar 1
Title: Quantum state preparation via inequality testing
Speaker: Yuval Sanders, Macquarie University
Date and Time: Monday, 17 June 2019, 11 am - 12 pm.
Location: University of Technology, Sydney
City Campus, Broadway
Building 10, Level 2, Room 470
Abstract: Recently, I co-authored a paper that improved over an algorithm of Grover for preparing a quantum state whose amplitudes are specified by an oracle. We were able to improve over Grover’s approach by replacing a step in which Grover would require the quantum computer to perform a sequence of conditional qubit rotations, where the rotation angles are conditional on the outcome of an expensive trigonometry calculation. This computational cost is paid many times over, as the trigonometry calculation must be done during each round of amplitude amplification. In this talk, I will explain how such expensive trigonometry calculations can be replaced with a much cheaper inequality test. The technique is quite general and so can be applied in many other quantum algorithms involving a state preparation step. I will use this work as a case study in how quantum algorithms research is moving from abstract computational complexity scaling arguments to concrete gate- and depth-cost analysis for the purpose of forecasting useful quantum computation.
Seminar 2
Title: Tuneable transmon interactions for novel many-body quantum simulations
Speaker: Nathan Langford, UTS
Date and Time: Wednesday, 19 June 2019, 11 am - 12 pm
Location: University of Technology, Sydney
City Campus, Broadway
Building 10, Level 2, Room 470
Abstract: Analog quantum simulations offer rich opportunities for exploring complex quantum systems, especially for interacting many-body lattices where the connectivity can be closely mapped onto the physical network structure of a specially engineered, well-controlled simulator system. In this paradigm, the versatility of a simulator platform is determined by its range of accessible complexity and interaction types, and in particular its ability to access and tune into different interaction regimes in situ. Platforms based on superconducting circuit quantum electrodynamics (QED) are very attractive because of site-specific control and readout, and flexible and engineerable system designs. But adding new components to the circuit QED design toolbox increases the range of phenomena that can be simulated: while arrays of transmon qubits with tuneable (linear) hopping interactions are already being studied, incorporating additional nonlinear interactions would enable access to far more complex Hamiltonians.
In this work, we demonstrated tuneable hopping (linear) and cross-Kerr (nonlinear) interactions in a highly coherent two-transmon unit cell [1]. Using a parallel capacitor and tuneable nonlinear inductor as a coupling element gives access to different interaction regimes. We characterised the unit cell's energy landscape spectroscopically and showed excellent agreement with a full theoretical model we developed to describe the underlying circuit Hamiltonian including higher transmon excitation manifolds, as well as showing high qubit coherence at all coupler bias points (relaxation and dephasing times up to 40 microseconds). Our device illustrates a scalable building block for simulators of extended Bose-Hubbard and Heisenberg XXZ models, and may also have applications in quantum computing such as realising fast two-qubit gates and perfect state transfer protocols.
[1] M. Kounalakis, C. Dickel, A. Bruno, N. K. Langford and G. A. Steele, Tuneable hopping and nonlinear cross-Kerr interactions in a high-coherence superconducting circuit, npj Quantum Information 4, 38 (2018)