Session T15: Mini-Symposium: Quantum Information and Quantum Sensing

Visualizing a Multi-Qubit State; an Extension of the Bloch Sphere

The conventional visualization of a single qubit is the Bloch sphere, which has proven to be a successful tool for developing intuition. In addition to showing all the information of an arbitrary state, the Bloch sphere clearly depicts the effect of any 1-qubit gate. Unfortunately, the generalization to multiple qubits is nontrivial. Since nearly all of quantum computing is derived from interactions between qubits, a visualization of multi-qubit states which have similar geometric intuitions to the Bloch sphere would be an effective pedagogical tool. Since any n-qubit gate can be decomposed into combinations of 1- and 2-qubit gates, an intuitive 2-qubit state visualization would be incredibly useful in developing understanding of quantum algorithms.

Here I derive and present a visualization of a pure 2-qubit state consisting of 3 spheres which preserves some of the Bloch sphere's geometric intuitions. Previous visualizations of multi-qubit states have largely focused on directly representing entanglement. This visualization prioritizes geometric intuition with regard to the computational basis states, which gives it higher potential for effectively portraying the effect of an arbitrary 2-qubit gate. The probabilities and phases of each of the four computational basis states are readily apparent in the visual representation, just as they are in the Bloch sphere. Additionally, the depiction can easily be extended to a pure n-qubit state while maintaining these intuitions, using 2ⁿ-1 spheres.    

Receiver Noise Characterization for ADMX Run 1C-Extended

The axion was originally proposed as a solution to the strong CP problem, however, it has been gaining popularity in recent years as a cold dark matter candidate. Many find the DFSZ axion model to be more compelling than its counterpart (KSVZ), due to its GUT compatibility. However, the DFSZ axion is significantly more weakly coupled to photons than the KSVZ axion, making it more difficult to detect. Notably, the Axion Dark Matter eXperiment (ADMX) was the first axion haloscope to reach sensitivity to the elusive DFSZ axion. ADMX was able to reach DFSZ sensitivity by using a dilution refrigerator to keep physical temperatures low, as well as employing low-noise quantum electronics to minimize receiver noise. To date, ADMX has excluded axion-photon couplings predicted by the KSVZ (DFSZ) model for the axion between 2.66-4.2 μeV (2.66-3.3 μeV & 3.9-4.1 μeV). Over the last year ADMX has been taking data designed to bridge the gap in DFSZ sensitivity for regions in the 3.3 to 3.9 μeV range. In this talk, I will be discussing the receiver noise calibration procedures used for this dataset. In particular, I will discuss the different types of measurements taken, as well as the noise models and analysis strategies used on said measurements.   

Reducing phonon backgrounds for quantum computing and cryogenic calorimeters

Superconducting qubits suffer from decoherence caused by phonons in the substrate with energies higher than the superconducting bandgap. While radiation backgrounds and cosmic rays produce large bursts of athermal phonons that causes catastrophic whole-chip failures, other sources, such as stress in films and chip packaging, produce lower energy phonon bursts more frequently. Cryogenic calorimetric sensors have achieved similar sensitivities to these phonon events. Specifically, in transition edge sensor (TES) based dark matter detectors, these events are the limiting background for low-mass dark matter searches. 

There is a strong synergy between detector R&D and quantum computing. I will discuss our efforts to reduce the phonon background by reducing stress in our detectors. The same ideas apply to quantum computing; in addition, qubits can be cloaked from phonons with suspending structures and phonon-reflective coatings. We use superconducting resonators to study the effectiveness of these methods.   

Fundamental Physics Tests with Precision Acoustic Sensors

Crystalline bulk acoustic wave (BAW) resonators provide a high precision architecture for fundamental physics experiments that search for signatures of beyond the standard model physics. In this work we present an overview of recent experiments that exploit the advantageous metrological properties of quartz BAW resonators in order to test quantum gravity, as well as search for high frequency gravitational waves. Utilizing these resonators, we have improved upon existing constraints on the scale of a discrete minimum length, as

proposed in several quantum descriptions of gravity, by several orders of magnitude. We have also developed a unique high frequency gravitational wave detector with competitive strain sensitivity to gravitational radiation in the MHz frequency band. When cooled to mK temperature modes in these devices will naturally be in their ground state, so the possibility exists to improve precision using quantum metrology.