A new gate encodes qubit states into superpositions of squeezed light

Encoding Qubits into Squeezed‑Light Superpositions

A recent study introduces a conditional‑squeezing gate that can transfer the quantum information of a superconducting qubit into non‑Gaussian superpositions of single‑ and two‑mode squeezed states of a resonator. By applying a refocusing protocol that cancels slowly varying Hamiltonian errors, the authors achieve encoding fidelities above 0.99 for any input qubit state, bringing bosonic‑code‑based error detection a step closer to practical use.

Refocusing Suppresses Time‑Dependent Errors

The gate relies on a bichromatic drive that creates a qubit‑state‑dependent parametric resonance in a SQUID‑terminated resonator. Unwanted phase errors accumulate from slowly varying terms in the Hamiltonian; the new protocol inserts a π‑pulse at the midpoint of a 250 ns evolution, effectively inverting the Hamiltonian and retracing the system’s trajectory. Simulations show the induced phase error drops to 1.8 × 10⁻⁸ rad, a dramatic improvement over an uncompensated gate.

Conditional‑Squeezing Extends Controlled‑Squeezing

Building on the authors’ earlier controlled‑squeezing operation (Phys. Rev. A 111, 042606 2025), the conditional gate squeezes the resonator along a direction set by the qubit’s logical state. This creates a superposition of squeezed‑vacuum components that can be used as logical basis states for rotation‑symmetric bosonic codes. Because photon loss flips the parity of the resonator state, a simple parity measurement instantly flags an error, enabling error‑detectable encoding without additional ancilla qubits.

High‑Fidelity Encoding Across the Bloch Sphere

Numerical analysis across cardinal points on the Bloch sphere reveals a uniform fidelity loss of roughly 0.17 % (minimum fidelity 0.9977 at the equator). The dominant source of infidelity is resonator photon loss, yet the two‑mode scheme outperforms the single‑mode version in both fidelity and gate duration. The authors also outline extensions to multi‑mode rotation‑symmetric codes using a control qudit and more elaborate refocusing sequences.

Outlook

The demonstrated gate provides a practical pathway to bosonic‑code quantum memories and error‑detectable processors. Future work will explore experimental validation, integration with surface‑code architectures, and scaling to larger bosonic‑code families. If realized in hardware, such high‑fidelity encodings could reduce the overhead required for fault‑tolerant quantum computation.

--- For the full paper, see “High‑fidelity conditional‑squeezing gate in superconducting circuits via refocusing” (Roman Schiaffino, Fernando C. Lombardo, Juan Pablo Paz).