Gaurav Bothara

High-fidelity QND readout and measurement back-action in a Tantalum-based high-coherence fluxonium qubit

  1. Gaurav Bothara,
  2. Srijita Das,
  3. Kishor V. Salunkhe,
  4. Madhavi Chand,
  5. Jay Deshmukh,
  6. Meghan P. Patankar,
  7. and R. Vijay
Implementing a precise measurement of the quantum state of a qubit is very critical for building a practical quantum processor as it plays an important role in state initialization
and quantum error correction. While the transmon qubit has been the most commonly used design in small to medium-scale processors, the fluxonium qubit is emerging as a strong alternative with the potential for high-fidelity gate operation as a result of the high anharmonicity and high coherence achievable due to its unique design. Here, we explore the measurement characteristics of a tantalum-based high-coherence fluxonium qubit and demonstrate single-shot measurement fidelity (assignment fidelity) of 96.2% and 97.8% without and with the use of a Josephson Parametric Amplifier respectively. We study the back-action of the measurement photons on the qubit and measure a QND (repeatability) fidelity of 99.6%. We find that the measurement fidelity and QND nature are limited by state-mixing errors and our results suggest that a careful study of measurement-induced transitions in the fluxonium is needed to further optimize the readout performance.
arxiv pdf

Engineering Cross Resonance Interaction in Multi-modal Quantum Circuits

  1. Sumeru Hazra,
  2. Kishor V. Salunkhe,
  3. Anirban Bhattacharjee,
  4. Gaurav Bothara,
  5. Suman Kundu,
  6. Tanay Roy,
  7. Meghan P. Patankar,
  8. and R. Vijay
Existing scalable superconducting quantum processors have only nearest-neighbor coupling. This leads to reduced circuit depth, requiring large series of gates to perform an arbitrary
unitary operation in such systems. Recently, multi-modal devices have been demonstrated as a promising candidate for small quantum processor units. Always on longitudinal coupling in such circuits leads to implementation of native high fidelity multi-qubit gates. We propose an architecture using such devices as building blocks for a highly connected larger quantum circuit. To demonstrate a quantum operation between such blocks, a standard transmon is coupled to the multi-modal circuit using a 3D bus cavity giving rise to small exchange interaction between the transmon and one of the modes. We study the cross resonance interaction in such systems and characterize the entangling operation as well as the unitary imperfections and cross-talk as a function of device parameters. Finally, we tune up the cross resonance drive to implement multi-qubit gates in this architecture.
arxiv pdf