A compact 3D quantum memory*

Superconducting qubits coupled to rectangular 3D cavity resonators exhibit coherence times on the order of a few tens of microseconds. In practice, one simultaneously couples the qubit to an even longer-lived memory cavity and low-quality readout cavity. The physical dimensions of such an device can be significantly reduced by integrating both functionalities into a single multimode cavity. Here, we engineer the fundamental and the first harmonic mode of a rectangular aluminum cavity in such a way, that the former one couples well to the external feedline, whereas the latter one does not. A transmon qubit couples to both modes with a rate g/2π=60 MHz. We store qubit states in the memory using a second-order protocol, observing a lifetime enhancement by a factor of 7.2. At the same time, the readout of our qubit is approximately 120 times faster than the storage time.