Practical Limits to Single-Mode Vacuum Squeezing in a SNAIL Parametric Amplifier

  1. Theodore Shaw,
  2. Debsuvra Mukhopadhyay,
  3. Zhuoqun Hao,
  4. Josiah Cochran,
  5. Haley Cole,
  6. Archana Kamal,
  7. and Shyam Shankar
We characterize single-mode vacuum squeezing generated by a SNAIL Parametric Amplifier (SPA) operated under conditions representative of practical sensing and qubit-readout experiments.
Motivated by prior expectations that Kerr-induced distortion limits squeezing in degenerate parametric amplifiers, we varied external flux and pump power to explore operating points where Kerr nonlinearity is theoretically minimized. We find that for practical applications where the squeezing frequency is fixed, the Kerr was variable by about a factor of two and the achievable squeezing showed no significant dependence on Kerr. Theoretical modeling supports this observation and indicates that baseline Kerr values in state-of-the-art SPAs are already too small to impose a practical limitation. Instead, squeezing was dominated by internal resonator loss and insertion loss in the microwave chain. These results indicate that, in practical SPAs, reducing loss, rather than suppressing Kerr, is the primary route to improved squeezing performance.

Experimental signatures of a σzσx beam-splitter interaction between a Kerr-cat and transmon qubit

  1. Josiah Cochran,
  2. Haley M. Cole,
  3. Hebah Goderya,
  4. Zhuoqun Hao,
  5. Yao-Chun Chang,
  6. Theo Shaw,
  7. Aikaterini Kargioti,
  8. and Shyam Shankar
Quantum error correction (QEC) requires ancilla qubits to extract error syndromes from data qubits which store quantum information. However, ancilla errors can propagate back to the
data qubits, introducing additional errors and limiting fault-tolerance. In superconducting quantum circuits, Kerr-cat qubits (KCQs), which exhibit strongly biased noise, have been proposed as ancillas to suppress this back-action and enhance QEC performance. Here, we experimentally demonstrate a beamsplitter interaction between a KCQ and a transmon, realizing an effective σzσx coupling that can be employed for parity measurements in QEC protocols. We characterize the interaction across a range of cat sizes and drive amplitudes, confirming the expected scaling of the interaction rate. These results establish a step towards hybrid architectures that combine transmons as data qubits with noise-biased bosonic ancillas, enabling hardware-efficient syndrome extraction and advancing the development of fault-tolerant quantum processors.

Dual on-chip SQUID measurement protocol for flux detection in large magnetic fields

  1. Josiah Cochran,
  2. Giovanni Franco-Rivera,
  3. Denghui Zhang,
  4. Lei Chen,
  5. Zhen Wang,
  6. and Irinel Chiorescu
Sensitive magnetometers that can operate in high magnetic fields are essential for detecting magnetic resonance signals originating from small ensembles of quantum spins. Such devices
have potential applications in quantum technologies, in particular quantum computing. We present a novel experimental setup implementing a differential flux measurement using two DC-SQUID magnetometers. The differential measurement allows for cancellation of background flux signals while enhancing sample signal. The developed protocol uses pulsed readout which minimizes on-chip heating since sub-Kelvin temperatures are needed to preserve quantum spin coherence. Results of a proof of concept experiment are shown as well.