Flux-Activated Resonant Control of a Bosonic Quantum Memory

Abstract

Universal control of bosonic degrees of freedom provides a hardware-efficient route for quantum information processing with high-dimensional systems. Bosonic circuit quantum electrodynamics (cQED), which leverages transmon ancillae to coherently control long-lived superconducting cavities, is well suited to this goal. However, the cavity transitions are nearly degenerate in the usual dispersive regime, which limits the direct addressability of individual excitation levels and increases the complexity of engineered gates. Here, we integrate an on-chip flux-control architecture with a long-lived bosonic memory housed in a 3D superconducting cavity to dynamically access resonant Jaynes-Cummings (JC) interactions, and realize efficient arbitrary rotations between any pair of Fock levels in the memory. This on-demand access to JC interactions offers a versatile toolbox for implementing robust Fock-basis qudits and harnessing the rich dynamics of high-dimensional bosonic elements for quantum information processing.

Figures (13)

• Figure 1: Comparison of the dispersive and resonant regimes. (a) The bosonic transitions in the dispersive regime are degenerate within the each qubit manifold and are separated by $\chi\sim$ MHz between the $|g\rangle$ and $|e\rangle$ manifold. (b) This degeneracy is lifted in the resonant JC regime, with level splittings $\propto g~\sim$ tens of MHz, enabling fast selective excitation of individual bosonic levels. c) The frequency spectra of both coupling regimes shown on the same axis.
• Figure 2: Architecture for robust flux-delivery in a high-Q bosonic circuit QED device. (a) High-Q $\lambda/4$ coaxial cavity coupled to an asymmetric SQUID-transmon (shown in bottom-left inset). The flux bias is delivered by a transmission line wirebonded to an SMA port at the opposite end of the chip. The flux line and a readout resonator partially adopt a coplanar waveguide (CPW) architecture. They are equipped with two $3$rd-order Chebyshev low-pass filters in series (schematics shown in top-right inset) and a Purcell filter, respectively, to protect the cavity and the transmon against decay. (b) The cavity $T_1$ is measured while the transmon is parked at a few selected flux points with different values of $\Delta = \omega_{cav} - \omega_T$, showing an increase of memory lifetime $T_1$ as the transmon-induced decay is mitigated at larger detunings.
• Figure 3: State preparation in the resonant regime. Wigner tomography and a 1D cut along $\mathrm{Re}[\beta]$ of states created using the flux-activated resonant interactions, together with corresponding steps activated to excite the JC ladder. (a) state $\tfrac{1}{\sqrt{2}}\left(|0\rangle + |3\rangle\right)$, and (b) state $\tfrac{1}{2}|0\rangle + \tfrac{\sqrt{3}}{\sqrt{8}}\left( i|2\rangle+ |4\rangle \right)$ are created using sequential drives. (c) Fock $|5\rangle$ created from vacuum by simultaneously driving multiple JC transitions.
• Figure 4: Givens rotations between two Fock levels. (a) The $|1+\rangle$ state population is swapped with $|3-\rangle$ state using a $J_x$ pulse, which is then followed by a sideband pulse to $|4+\rangle$ and a second $J_x$ pulse that reverts the first swap. This sequence effectively implements an operation between states $|1+\rangle$ and $|4+\rangle$ that translates into a Givens rotation between Fock states $|1\rangle$ and $|4\rangle$ after adiabatic detuning. (b) Measurement of the normalized final state parity for arbitrary rotation angle $\theta$ from an initial state $|1\rangle$, showing the protocol can create arbitrary superpositions of the two Fock states. (c) Wigner plots and Fock states populations obtained from the reconstructed density matrices of the cavity state for $\theta = 0, \pi/4, \pi/2$ and $\pi$. The measured population distribution agrees well with the simulated results with real experimental parameters.
• Figure 5: Chip schematic. It features ancilla DC-SQUID transmon, low-Q readout resonator with Purcell filter, and on-chip flux line with microwave low-pass filter