Systems, devices, and methods for controllably coupling qubits
View Patent ↗A transverse coupling system may include a first qubit, a second qubit, a first conductive path capacitively connecting the first qubit and the second qubit, a second conductive path connecting the first qubit and the second qubit, and a dc SQUID connecting the first and the second conductive paths wherein the compound junction loop is threaded by an amount of magnetic flux.
1. A transverse coupling system comprising:
a first qubit;
a second qubit;
a first current path including a first coupling capacitance and a second coupling capacitance connecting the first qubit and the second qubit;
a second current path connecting the first qubit and the second qubit; and
an inductance connecting the second current path to a node of the first current path, the node located between the first coupling capacitance and the second coupling capacitance.
2. The transverse coupling system of claim 1 wherein at least one of the first current path and the second current path are made from a superconducting material.
3. The transverse coupling system of claim 1 wherein the inductance is a controllable inductance.
4. The transverse coupling system of claim 1 wherein the inductance is provided by a dc SQUID threaded by an amount of magnetic flux.
5. The transverse coupling system of claim 4 wherein the dc SQUID comprises:
a loop of superconducting material interrupted by a first Josephson junction and a second Josephson junction;
a first lead connecting the loop of superconducting material to the first conductive path at the node between the first coupling capacitance and the second coupling capacitance; and
a second lead connecting the loop or superconducting material to the second conductive path wherein current traveling from the first lead passes through at least one of the first Josephson junction and the second Josephson junction before reaching the second lead.
6. The transverse coupling system of claim 5 wherein the first qubit and the second qubit are most weakly coupled with about nΦ 0 magnetic flux threading the dc SQUID, where n is an integer.
7. The transverse coupling system of claim 5 wherein the first qubit and the second qubit are most strongly coupled with about
(
n
+
1
2
)
Φ
0
magnetic flux threading the dc SQUID, where n is an integer.
8. The transverse coupling system of claim 1 wherein there exists a coupling strength between the first qubit and the second qubit.
9. The transverse coupling system of claim 8 wherein the coupling strength causes the first qubit and the second qubit to exhibit XX+YY coupling.
10. The transverse coupling system of claim 1 wherein at least one of the first qubit and the second qubit is a superconducting flux qubit.
11. The transverse coupling system of claim 1 wherein the first qubit comprises a first loop of superconducting material interrupted by a first at least one Josephson junction and wherein the second qubit comprises a second loop of superconducting material interrupted by a second at least one Josephson junction.
12. The transverse coupling system of claim 11 wherein the first at least one Josephson junction, the second at least one Josephson junction and the inductance are connected in parallel between the first conductive path and the second conductive path.
13. The transverse coupling system of claim 1 wherein the second conductive path capacitively connects the first qubit and the second qubit.
14. The transverse coupling system of claim 13 wherein the second conductive path is interrupted by a first auxiliary capacitance and a second auxiliary capacitance.
15. The transverse coupling system of claim 14 wherein the inductance connects to the second conductive path at a second node positioned between the first auxiliary capacitance and the second auxiliary capacitance.
16. The transverse coupling system of claim 14 wherein the first auxiliary capacitance takes the form of a first discrete capacitor and the second auxiliary capacitance takes the form of a second discrete capacitor.
17. The transverse coupling system of claim 14 wherein the first auxiliary capacitance takes the form of a first parasitic capacitance associated with the first current path and the second auxiliary capacitance takes the form of a second parasitic capacitance associated with the first current path.
18. The transverse coupling system of claim 1 wherein the first coupling capacitance takes the form of a first discrete capacitor and the second coupling capacitance takes the form of a second discrete capacitor.
19. The transverse coupling system of claim 1 wherein the first coupling capacitance takes the form of a first parasitic capacitance associated with the first current path and the second coupling capacitance takes the form of a second parasitic capacitance associated with the first current path.
20. The transverse coupling system of claim 1 wherein the inductance is provided by an at least two dc SQUID in series, each dc SQUID threaded by an amount of magnetic flux.
21. A method of transversely coupling a first qubit to a second qubit using a first conductive path interrupted by a first coupling capacitance and a second coupling capacitance connecting the first qubit and the second qubit, a second conductive path connecting the first and second qubit, and a controllable inductance connecting the first conductive path, connected between the first coupling capacitance and the second coupling capacitance, to the second conductive path, the method comprising:
adjusting the controllable inductance there by varying a divider for signals going from the first qubit to the second qubit.
22. The method of claim 21 wherein the controllable inductance is provided by a dc SQUID threaded by an amount of flux and adjusting the controllable inductance includes varying the amount of flux threading the dc SQUID.
23. The method of claim 22 wherein the amount of flux is about nΦ 0 magnetic flux and the first qubit and the second qubit are weakly coupled, where n is an integer.
24. The method of claim 22 wherein the amount of flux is about
(
n
+
1
2
)
Φ
0
magnetic flux and the first qubit and the second qubit are strongly coupled, where n is an integer.
25. The method of claim 21 wherein at least one of the first qubit and the second qubit is a superconducting flux qubit.
26. The method of claim 21 wherein there exists an XX+YY coupling between the first qubit and the second qubit.
27. The method of claim 21 wherein adjusting the controllable inductance there by varying a divider for signals going from the first qubit to the second qubit controls a coupling strength between the first qubit and the second qubit.