Figure 1
(a) Schematic diagram of an artificial atom (AA) produced by a superconducting quantum circuit. A symmetric SQUID and two identical Josephson junctions with coupling energy
and capacitance
are placed in a superconducting loop pierced by a magnetic flux
(green or gray). The two junctions in the SQUID have coupling energy
and capacitance
, and the flux (yellow or light gray) threading through the SQUID loop is
. Here
,
, and
is the single-particle charging energy of the junction. (b),(c) Energy levels of the superconducting AA as a function of the reduced magnetic flux
, for
and 0.27, where only the four lowest levels are shown and the energy is in units of
. (d),(e) Moduli of the transition matrix elements
(in units of
) as a function of
, for
and 0.27. Note that each figure in (b)–(e) is symmetric about
and half of it is plotted. The vertical dashed lines at
are just a guide to the eye. (f) Transition diagram of the AA. At nonzero temperatures, the flux qubit is thermally activated from the ground state
to the first excited state
. A resonant transition from
to the second excited state
is driven by a microwave field, so as to eliminate the unwanted thermal population of
, and followed by a fast decay to
. While the qubit is cooled to its ground state
, the AA is then switched on, to resonantly interact with a neighboring quantum system for a period of time. Repeating these processes, both the qubit and the neighboring quantum system can be simultaneously cooled.
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