Seminars (Fall 2023)

Abstract: Despite recent strides in quantum information, scalability is still hindered by decoherence, a phenomenon closely related to the thermalization of quantum systems. We will demonstrate that disordered transmon arrays experience a thermal to many-body localized phase transition using established diagnostics of spectral statistics. To reinforce our findings we will also introduce a new tool – the Walsh-Hadamard coefficients. Remarkably, even disorder-free systems can achieve MBL using quasi-periodic qubit frequency patterns. We studied this possibility using a perturbation theory scheme suited for large transmon arrays, bridging the gap between theory and experimentally relevant system sizes.

Abstract: Quantum sensing incorporates a broad range of applications, from single photon emission and detection in a discrete variable system, to squeezed light emission and detection in a continuous variable system. For the first half of my talk, I will discuss single photon detectors, which are essential to fundamental tests of entanglement distribution and to applications in quantum networking, quantum computing, and quantum sensing. I demonstrate that superconducting nanowire single photon detectors (SNSPDs) have a position sensitivity to the signal readout pulse that is consistent with a simple model of microwave propagation along the length of the nanowire, and that SNSPDs can operate robustly under large magnetic fields and have the potential to be used as a multifunctional quantum sensor. In the second half of my talk, I will discuss the development of a continuous variable squeezed light source, which enables entanglement generation and quantum noise reduction, which are key components to continuous variable quantum sensing, networking, and computing. The quantum noise is reduced, or “squeezed”, in this light source because the noise in one variable of the optical field is reduced at the expense of the noise in the conjugate variable, thus enabling detection of previously unresolvable signals, with a current target of magneto-optical materials characterization, and a long-term target of dark matter detection.

Abstract: The rapid development of quantum devices—such as cold atoms, ions, superconducting qubits—are an invitation to explore exotic many-body states with a degree of control which was hitherto inaccessible. In this talk, I will discuss recent theoretical insights and experimental realizations of long-range entangled quantum states, with a focus on topological order. These phases of matter have been studied for decades due to their rich emergent properties, such as quasiparticles with ‘anyonic’ exchange statistics. Although these are challenging to create and verify in conventional quantum materials, we will see how they can be efficiently prepared using controlled quantum measurements. A particular highlight will be our recent collaboration with the cold-ion company Quantinuum, leading to the first realization of non-Abelian topological order—its anyonic braiding properties going beyond what is accessible in Abelian states such as the toric code. More broadly, we argue that the efficiently preparable states are related to Galois’ notion of solvable groups.

Abstract: Quantum computing’s potential is immense, promising super-polynomial reductions in execution time, energy use, and memory requirements compared to classical computers. This technology has the power to revolutionize scientific applications such as simulating many-body quantum systems for molecular structure understanding, factorization of large integers, enhance machine learning, and in the process, disrupt industries like telecommunications, material science, pharmaceuticals and artificial intelligence. However, quantum computing’s potential is curtailed by statistical uncertainties introduced by noise, further complicated by non-stationary noise parameter distributions across time and qubits. I will talk about the persistent issue of noise in quantum computing. In particular, the definitions of computational accuracy, device reliability, outcome stability, and result reproducibility, crucial for assessing noisy quantum outputs. By studying non-stationary noise in current quantum computers, I will stalk about a statistical framework to differentiate and analyze these concepts, delving into their nuanced interrelationships.

Abstract: In the analysis of superconducting qubits, the mathematical description of an electrical network appear to require models that lead to singular Lagrangians, describing constrained systems in which not all variables are independent. A procedure due to Dirac and Bergmann is commonly used to derive the Hamiltonian of such constrained electrical networks. But real electric networks are never singular, but rather have a hierarchy of scales. From this hierarchical point of view, we show that a correct treatment of the low-energy dynamics is obtained from a Born-Oppenheimer approach. We show that the Dirac-Bergmann approach gives completely incorrect answers, explaining this as this approach’s neglect of quantum fluctuations.

Abstract: Quantum error correction with erasure qubits promises significant advantages over standard error correction due to favorable thresholds for erasure errors. To realize this advantage in practice requires a qubit for which erasure errors are the dominant error, and the ability to check for erasure errors without dephasing the qubit. We demonstrate that a “dual-rail qubit” consisting of a pair of resonantly-coupled transmons can form a highly coherent erasure qubit, where transmon T1 errors are converted into erasure errors and residual dephasing is strongly suppressed, leading to millisecond-scale coherence within the qubit subspace. We show that single-qubit gates are limited primarily by erasure errors while the residual error rates are ~ 40 times lower. We further demonstrate mid-circuit erasure detection while introducing < 0.1% dephasing error per check. Finally, we show that the suppression of transmon noise allows this dual-rail qubit to preserve high coherence over a broad tunable operating range, offering an improved capacity to avoid frequency collisions. This work establishes transmon-based dual-rail qubits as an attractive building block for hardware-efficient quantum error correction.