Quantum Motion attends APS Global Physics Summit 2025: Who, What, Where, and When
Five team members are heading to Anaheim, California to attend this year’s American Physical Society’s (APS) Global Physics Summit. The APS Global Physics Summit brings together scientists and students from around the world to connect and collaborate across academia, industry, and major labs.
Quantum Motion team members will be presenting seven talks at this year’s conference. Find out who is speaking, when and where below:
RF-Squad: A Radio-frequency Reflectometry Simulator for Quantum Dot Arrays
Monday 17 March 2025, 5:24PM (GMT-7), 163 (Level 1)
Tara Murphy, Quantum Engineer, University of Cambridge
Spins in semiconductor quantum dots represent a large-scale integration route for quantum computing hardware. Many efforts have been devoted to creating quantum dot array simulators, with an aim to better understand the complexity of these systems as they scale up. Typically based on the well-established Constant Interaction Model (CIM), they often fail to simulate realistic datasets or recreate important experimental details, such as the set up used to measure the device. We present RF-Squad, a physics-based quantum dot array simulator capable of realistically replicating the outputs of radiofrequency reflectometry measurements. Implemented in JAX, an accelerated linear algebra library, RF-Squad facilitates the simulation a double quantum dot in milliseconds at the CIM level. A key feature of RF-Squad is the inclusion of advanced physical phenomena, such as the use of tunnel couplings, the WKB approximation, Fock-Darwin states, different noise models and tunnel rates. Furthermore, the code has been optimized and structured in a layered approach, giving users the flexibility to balance realism and computational speed. In summary, RF-Squad enables efficient generation of large, realistic charge stability diagram datasets, and serves as a practical tool for researchers to gain a deeper understanding of experimental data. Finally, with its user-friendly design, RF-Squad is accessible to a broad range of users, making it easy to integrate across a wide range of experimental contexts.
High-Fidelity Quantum Control for Spin Qubits via Pulse Engineering
Wednesday 19 March, 2025, 9.48AM (GMT-7), 258A (Level 2)
Sebastian Orbell , Senior Quantum Machine Learning Engineer
We present ELQ (Efficiently Learning Quantum systems), an open-source framework for designing optimal quantum control schemes that achieve high-fidelity gate operations. Our method leverages automatic differentiation and GPU acceleration in JAX to efficiently compute gradients through realistic experimental constraints, including bandwidth limitations, non-linear transmission functions, and environmental noise. We demonstrate ELQ’s capabilities by optimizing single-, simultaneous single-, and two-qubit gates for hole spins in silicon quantum dots. By incorporating numerically sampled noise trajectories, we develop control pulses that are robust to both high-frequency noise from ensembles of two-state fluctuators (1/f noise) and quasi-static errors from charge and magnetic field variations. Our results show that pulses optimized for simultaneous single-qubit operations significantly outperform those designed for sequential operation, highlighting the importance of considering cross-talk and unwanted coupling effects. The ELQ codebase enables the broader quantum computing community to design and optimize high-fidelity quantum gates while accounting for device-specific constraints and noise environments.
Semi-automated Device Design and Simulation of SiMOS Quantum Dot Devices
Thursday 20 March 2025, 8.48AM (GMT-7), 202 (Level 2)
James Williams, Senior Quantum Engineer
Silicon quantum dot devices represent a promising platform for quantum computing due to their scalability and integration with classical CMOS process. Simulating and modelling these devices is essential for understanding their electronic properties, optimising their design, and predicting their behaviour in qubit implementations. However, the complexity of quantum dot systems, which involves potential generation, state calculation, and the interaction between different quantum dots, requires an efficient workflow.
In this work we present an automated simulation workflow for quantum dot devices, integrating geometry generation, mesh processing, and wave function analysis. By automating key steps such as mesh generation, surface tagging, and visualization, we accelerate the overall simulation process. This approach not only studies variation on device configurations but also provides a framework for exploration of device performance under different bias conditions. This workflow allows us to automatically output the tunnel coupling, lever arms, coulomb interaction matrix and exchange also with charge stability generation which can be compared and contrasted with real world device data.
The automation of this modelling workflow will become crucial for scaling up the design and simulation of quantum dot devices, ultimately contributing to the realisation of practical quantum computing.
Compact Quantum Dot Models for Analog Microwave co-Simulation
Thursday 20 March 2025, 10.48AM (GMT-7), 202 (Level 2)
Lorenzo Peri, Quantum Engineer, University of Cambridge
Modelling the quantum-classical electrical interface of quantum systems is of paramount importance to design scalable quantum devices embedded in electrical circuits, and it is at the heart of the simulation, control, and readout of modern quantum computing systems. In this work we present Verilog-A compact models of quantum-dot-based quantum systems, capable of faithfully reproducing coherent quantum behavior within a standard mixed-signal electronic simulator, allowing form compromise-free co-simulation of quantum devices. We present results from simulations performed in Cadence Spectre®, an industry-standard commercial simulator, showcasing quantum phenomena such as Rabi oscillations and Landau-Zener-Stückelberg-Majorana interference, and we demonstrate vacuum Rabi splitting caused by coupling a qubit with an RLC resonator. Moreover, we explore the use of the developed simulation software in the design quantum-dot-based analog circuits and present an implementation of a frequency multiplier leveraging the nonlinear properties of quantum dots. Our work paves the way for a new paradigm of designing quantum devices, which leverages the power of computer-aided design and automation. Our faithful reproduction of quantum effects allows for in silico testing and optimization quantum processing units, as well as control and readout schemes, pulses and circuitry, overall allowing quantum technologies to leverage the many decades of tools and innovations responsible for the unrivaled scaling and integration of classical electronics.
Radio-frequency Electron Cascade in Semiconductor Quantum Dots
Thursday 20 March 2025, 12.30PM (GMT-7), 202 (Level 2)
Tara Murphy, Quantum Engineer, University of Cambridge
Electron cotunneling in semiconductor quantum dots refers to higher-order transport processes in which two or more charge carriers tunnel simultaneously. These phenomena have been typically studied in the context of direct transport measurements and have been utilized for fundamental purposes such as quantum dot energy-level spectroscopy with enhanced resolution [1]. Here, we expand this picture to alternating cotunneling events through quantum dot systems subject to radiofrequency excitation. We theoretically explore and experimentally demonstrate these AC cotunneling events in multi-quantum dot systems and exploit them for charge polarizability measurements with enhanced sensitivity with respect to more established techniques such as in-situ dispersive readout [2]. Particularly, we introduce a new charge polarizability detection method, which we dub the radio-frequency electron cascade, that leverages the synchronised single-electron AC currents in a three-quantum dot system to produce signal enhancement of interdot charge transitions. Our discovery expands the portfolio of rf readout methods in semiconductor nanostructures and provides a route to increased dispersive readout fidelity in semiconductor quantum computing architectures.
[1] De Franceschi, S., et al. “Electron cotunneling in a semiconductor quantum dot.” Physical review letters 86.5 (2001): 878.
[2] Gonzalez-Zalba, M. F., et al. “Probing the limits of gate-based charge sensing.” Nature communications 6.1 (2015): 1-8.
Polarimetry With Spins in the Solid State
Thursday 20 March 2025, 12.54PM (GMT-7), 202 (Level 2)
Lorenzo Peri, Quantum Engineer, University of Cambridge
The ability for optically active media to rotate the polarization of light is the basis of polarimetry, an illustrious technique responsible for many breakthroughs in fields as varied as astronomy, medicine and material science. In our work, we recast the primary mechanism for spin readout in semiconductor-based quantum computers, Pauli spin-blockade (PSB), as the natural extension of polarimetry to the third dimension.
We perform polarimetry with spins through a silicon quantum dot exchanging a hole with a Boron acceptor, illustrating the role of spin-orbit coupling in giving rise to spin misalignment. Akin to the humble polarimeter, the misalignment may be tuned by varying an external degree of freedom, the applied magnetic field direction, and we demonstrate how it can be rotated to a magic angle to recover perfect spin alignment and re-establish PSB.
Finally, we discuss the effect of spin misalignment on spin readout, which sets a fundamental upper limit for the fidelity of PSB-based spin readout. Its dependence on sample-to-sample and device-to-device variability poses serious challenges for the feasibility and scaling of quantum computing architectures in the presence of strong spin-orbit coupling.
Floquet Interferometry of a Dressed Semiconductor Quantum Dot
Friday 21 March 2025, 12.54PM (GMT-7), 202 (Level 2)
Felix-Ekkehard von Horstig, Senior Quantum Engineer
A quantum system interacting with a time-periodic excitation creates a ladder of hybrid eigenstates in which the system is mixed with an increasing number of photons. This mechanism, referred to as dressing, has been observed in the context of light-matter interaction in systems as varied as atoms, molecules and solid-state qubits.
In this work, we demonstrate state dressing in a semiconductor quantum dot tunnel-coupled to a charge reservoir. We observe the emergence of a Floquet ladder of states in the system’s high-frequency electrical response, manifesting as interference fringes at the multiphoton resonances despite the system lacking an avoided crossing and transitions being allowed solely via stochastic charge exchanges with the reservoir that erase the charge’s quantum phase.
We study the dressed quantum dot while changing reservoir temperature, charge lifetime, and excitation amplitude and reveal the fundamental nature of the mechanism by developing a theory based on the quantum dynamics of the Floquet ladder, which is in excellent agreement with the data. Furthermore, we show how the technique finds applications in the accurate and fast electrostatic characterisation of semiconductor quantum dots.
