Superconducting 3D microwave cavities offer state-of-the-art coherence times and a well-controlled environment for superconducting qubits. In order to realize at the same time fast readout and long-lived quantum information storage, one can couple the qubit to both a low-quality readout and a high-quality storage cavity. However, such systems are bulky compared to their less coherent 2D counterparts. A more compact and scalable approach is achieved by making use of the multimode structure of a 3D cavity. In our work, we investigate such a device where a transmon qubit is capacitively coupled to two modes of a single 3D cavity. External coupling is engineered so that the memory mode has an about 100 times larger quality factor than the readout mode. Using an all-microwave second-order protocol, we realize a lifetime enhancement of the stored state over the qubit lifetime by a factor of 6 with a fidelity of approximately 80% determined via quantum process tomography. We also find that this enhancement is not limited by fundamental constraints.
Compact 3D quantum memory
Edwar Xie, Frank Deppe, Michael Renger, Daniel Repp, Peter Eder, Michael Fischer, Jan Goetz, Stefan Pogorzalek, Kirill G. Fedorov, Achim Marx, Rudolf Gross; Compact 3D quantum memory. Appl. Phys. Lett. 14 May 2018; 112 (20): 202601. https://doi.org/10.1063/1.5029514
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