Figure 1
Description of the hybrid quantum circuit demonstrated in this work. (a),(b) Three-dimensional sketch of the device and corresponding electrical scheme. The ensemble
of electronic spins (magenta) consists of
NV centers in a diamond crystal glued on the chip surface. The transmon qubit
(in red) is capacitively coupled to a resonator
(in blue) made nonlinear with a Josephson junction and used to readout its state. The bus
(in yellow) is electrostatically coupled to
and magnetically coupled to
. Bus
contains a SQUID that makes its frequency
tunable by applying in its loop a flux
via a fast on-chip current line
(in green). A magnetic field
is applied parallel to the [1,1,1] crystallographic axis. (c), (lower left inset) Energy level structure of NV centers. Transitions between
and
at frequency
are further split in three resonance lines due to the hyperfine interaction with the
nuclear spin [
20]. (main panel) Two-dimensional plot of the transmission
through
in dB units, with
expressed in units of the superconducting flux quantum
, for a field
applied to the spins. Color scale goes from
(background, green) to
(magenta). Four vacuum Rabi splittings are observed whenever
matches one NV center resonance frequency. The four transition frequencies
correspond to the two distinct families
and
of NV centers, aligned along the [1,1,1] crystal direction parallel to
or along one of the three other possible
axes, respectively (see upper right inset). (main panel, bottom right) Qubit excited state probability
as a function of the frequency of the exciting microwave and
. Color scale goes from 0.1 (background, purple) to 0.3 (yellow). When
matches the qubit frequency
, the qubit spectrum shows an anticrossing demonstrating its coupling to
with constant
.
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