Flux-tunable qubit device with multiple Josephson junctions
In a general aspect, a qubit device includes two circuit loops. In some aspects, a first circuit loop includes a first Josephson junction, a second circuit loop includes a second Josephson junction, and the first and second loops are configured to receive a magnetic flux that defines a transition frequency of a qubit device. In some aspects, a quantum integrated circuit includes an inductor connected between a first circuit node and a second circuit node; the first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and the second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node.
1. A qubit device comprising:
an inductor connected between a first circuit node and a second circuit node;
a first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and
a second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node,
wherein the qubit device is configured to receive, during operation of the qubit device, a magnetic flux that controls a qubit frequency of the qubit device, and the qubit frequency as a function of the magnetic flux comprises multiple flux sweet spots.
2. The qubit device of claim 1 , comprising:
a first circuit loop that includes the first Josephson junction; and
a second circuit loop that includes the second Josephson junction, wherein the first and second circuit loops are configured to receive, during operation of the qubit device, the magnetic flux that controls a qubit frequency of the qubit device.
3. The qubit device of claim 2 , wherein:
the first circuit loop includes the first Josephson junction and the second Josephson junction; and
the second circuit loop includes the inductor and the second Josephson junction.
4. The qubit device of claim 2 , wherein:
the first circuit loop includes the first Josephson junction and the inductor; and
the second circuit loop includes the second Josephson junction and the inductor.
5. The qubit device of claim 2 , wherein:
the first Josephson junction has a first Josephson energy;
the second Josephson junction has a second Josephson energy; and
a ratio of the first Josephson energy to the second Josephson energy is substantially an integer value.
6. The qubit device of claim 2 , wherein:
the first Josephson junction has a first Josephson energy;
the second Josephson junction has a second Josephson energy; and
the first and second Josephson energies are unequal.
7. The qubit device of claim 2 , wherein:
the first circuit loop defines a first magnetic flux area to receive a first portion of the magnetic flux;
the second circuit loop defines a second magnetic flux area to receive a second portion of the magnetic flux; and
a ratio of the first portion of the magnetic flux to the second portion of the magnetic flux is substantially an integer value.
8. The qubit device of claim 2 , wherein:
the first circuit loop defines a first magnetic flux area to receive a first portion of the magnetic flux;
the second circuit loop defines a second magnetic flux area to receive a second portion of the magnetic flux; and
the first portion and second portions of the magnetic flux are unequal.
9. The qubit device of claim 2 , wherein the qubit frequency as a function of the magnetic flux comprises a minimum, a maximum, and at least one of the flux sweet spots between the minimum and maximum.
10. The qubit device of claim 1 , wherein the first Josephson junction and the second Josephson junction each comprise a respective parasitic capacitance in parallel with the inductor.
11. The qubit device of claim 1 , wherein the inductor comprises a superinductor that includes a serial array of Josephson junctions.
12. The qubit device of claim 1 , wherein the inductor comprises a superinductor that includes a superconducting wire.
13. A quantum computing method comprising operating a qubit device, the qubit device comprising:
an inductor connected between a first circuit node and a second circuit node;
a first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and
a second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node,
wherein operating the qubit device comprises controlling a magnetic flux received by the qubit device, the magnetic flux controls a qubit frequency of the qubit device, and the qubit frequency as a function of the magnetic flux comprises multiple flux sweet spots.
14. A quantum computing system comprising:
a qubit device comprising a first superconducting quantum interference device (SQUID) loop and a second SQUID loop, wherein the first and second SQUID loops are configured to receive a magnetic flux that defines a qubit frequency of the qubit device, and the qubit frequency as a function of the magnetic flux comprises multiple flux sweet spots.
15. The quantum computing system of claim 14 , wherein the first and second SQUID loops are configured such that the qubit frequency as a function of the magnetic flux comprises a maximum, a minimum and at least one of the flux sweet spots between the maximum and the minimum.
16. The quantum computing system of claim 14 , comprising a chip that includes the qubit device and a readout resonator coupled to the qubit device.
17. The quantum computing system of claim 14 , wherein the first SQUID loop include a first Josephson junction, the second SQUID loop includes a second Josephson junction and at least one of the first SQUID loop or the second SQUID loop includes a superinductor.
18. The quantum computing system of claim 17 , wherein:
the first SQUID loop includes the first Josephson junction and the second Josephson junction; and
the second SQUID loop includes the superinductor and the second Josephson junction.
19. The quantum computing system of claim 17 , wherein:
the first SQUID loop includes the first Josephson junction and the superinductor; and
the second SQUID loop includes the second Josephson junction and the superinductor.
20. The quantum computing system of claim 14 , comprising:
a quantum circuit system that includes the qubit device;
a signal delivery system connected to the quantum circuit system and configured to communicate signals between the quantum circuit system and a control system.
21. The qubit device of claim 7 , wherein the number of number n sp of sweet spots is given by
n
s
p
=
{
r
Φ
,
for
even
r
Φ
,
r
Φ
+
1
,
for
odd
r
Φ
.
where r Φ represents the ratio of the first portion of the magnetic flux to the second portion of the magnetic-flux.