Quantum Hardware

Superconducting transmon qubits are key to recent quantum computing advances. I will present our recent work on characterizing the noise afflicting these qubits beyond standard stochastic error models, then discuss quantum control methods to suppress this noise, aiming to improve gate fidelities and overall performance.

References:
- V. Tripathi, H. Chen, M. Khezri, K.-W. Yip, E. M. Levenson-Falk, D.A. Lidar, “Suppression of crosstalk in superconducting qubits using dynamical decoupling”. Phys. Rev. Applied 18, 024068 (2022).
- V. Tripathi, H. Chen, E. Levenson-Falk, D.A. Lidar, “Modeling low- and high-frequency noise in transmon qubits with resource-efficient measurement”. Phys. Rev. X Quantum 5, 010320 (2024).
- V. Tripathi, D. Kowsari, K. Saurav, H. Zhang, E.M. Levenson-Falk, D.A. Lidar, “Deterministic Benchmarking of Quantum Gates”. arXiv:2407.09942.
- A. Vezvaee, V. Tripathi, D. Kowsari, E. Levenson-Falk, D.A. Lidar, “Virtual Z gates and symmetric gate compilation”. arXiv:2407.14782.

The concept of dissipation engineering has enriched the methods available for state preparation, dissipative quantum computing and quantum information processing. Combining such engineered dissipative processes with coherent dynamics allows for new effects to emerge. For example, we found that any factorisable (coherent) Hamiltonian interaction can be rendered nonreciprocal if balanced with the corresponding dissipative interaction. This powerful concept can be exploited to engineer nonreciprocal devices for quantum information processing, computation and communication protocols, e.g., to achieve control over the direction of propagation of photonic signals. However, although nonreciprocal concepts are realizable in quantum architectures, nonreciprocity itself holds up to the classical level and is not inherently quantum; the fundamental aspects of nonreciprocity in the quantum regime have yet to be fully investigated. In this talk we will address these aspects and discuss routes of how nonreciprocal concepts can find application beyond quantum information processing. Moreover, we will take a closer look on how to actually operate an embedded nonreciprocal amplifier on chip, and discuss the resulting efficiency and backaction for dispersive qubit-read-out.

Superconducting circuits have emerged as a leading technology for building qubits. Typically, designing and connecting these circuits resembles a game of assembly, with elements like transmission lines (TLs) functioning as interconnecting links and nonreciprocal devices (e.g., circulators) acting as routing hubs. However, the systematic construction of quantum Hamiltonian models for such systems does not follow a similarly additive property. When elements are added to a circuit, prior constraints are modified and new constraints appear, preventing a simple summation of Hamiltonian terms in constructing these phenomenological quantum mechanical models.

In this presentation, I will show how to tackle the general question of constructing exact quantum mechanical models for so-called quasi-lumped electrical networks [1,2]. These networks comprise lumped elements, such as (nonlinear) capacitors and inductors,  nonreciprocal elements (circulators), and transmission lines. A common technique to handle such circuits involves either node-flux or loop-charge quantization methods, along with their continuous limits when dealing with TLs. However, this process can lead to various types of singularities, which may yield physically nonsensical predictions, such as divergent effective models [3]. This issue is partly due to the incorrect identification of the infinite-dimensional Hilbert space of the TLs, which support 1D scalar fields. Using a geometrical description [1], amenable to the Faddeev-Jackiw method, we tackle the full-fledged problem of quantization by adopting a mixed charge-flux first-order description of the circuit. I will introduce the topic chronologically and pedagogically. 

[1] A. Parra-Rodriguez, I. L. Egusquiza, Quantum 8, 1466 (2024).

[2] A. Parra-Rodriguez, I. L. Egusquiza, arXiv preprint: 2401.09120.

[3] A. Parra-Rodriguez, et al., Quantum Science and Technology 3 (2), 024012 (2018)

Today’s superconducting qubit devices are highly sensitive to noise, making them sensitive probes for axion and particle-like dark matter. While useful quantum computing with these devices is still years away, these devices have already been used to set world-leading limits for some types of dark matter. Our group at Fermilab uses an open source toolchain for designing and simulating our devices that includes Qiskit Metal, the SQUADDs library, and also G4CMP, which is a part of the Geant4 toolkit developed and used by particle physicists. In this talk I will survey the types of devices we are interesting in simulating, the particular properties that are of most interest to us, and our near-term strategy to making progress in this design space using open source tools.

Neutral atom quantum processors are emerging as a leading platform for quantum computing. Integrating them with an optical cavity enables new capabilities, such as fast nondestructive readout, all-to-all connectivity, and access to fast atom-photon entanglement for quantum networking. Here, we introduce a platform that transports and couples single atoms in optical tweezers to a Fabry-Perot Fiber Cavity. Leveraging the strong atom-cavity coupling, we demonstrate fast qubit state readout with 99.96(2) % fidelity in 10μs and two methods for cavity-mediated entanglement generation with integrated error detection. First, we use cavity-carving to generate a Bell state with fidelity 91(4)% and with 32(1)% success rate. Second, we perform a cavity-mediated gate with a deterministic entanglement fidelity of 52.5(18)%, increased to 76(2)% with error detection. The new capabilities enabled by this versatile platform pave the way towards modular quantum computing.

In this talk, I will discuss hybrid quantum platforms with optical interfaces designed to interconnect different quantum machines and enable distributed quantum technologies. I will present our recent results on implementing distributed blind quantum gates across a two-node quantum network utilizing solid-state defects in diamond nanophotonic cavities. Finally, I will briefly explore how these techniques can be extended to neutral atoms and superconducting circuit platforms.