System, devices and methods for coupling qubits
View Patent ↗A ladder structure is ferromagnetically coupled to a first qubit where the ladder structure has a monostable energy potential in use, such that the first qubit and the ladder structure effectively operate as a single qubit. The ladder structure and first qubit may be coupled via a superconducting flux coupler. The ladder structure may be a chain of at least two ferromagnetically coupled ladder elements. A value for each ladder element may be less than about 1.
1. A method of coupling qubits, the method comprising:
biasing a first qubit into a first bistable energy potential;
biasing a second qubit into a second bistable energy potential;
biasing each ladder element in a chain of ferromagnetically coupled ladder elements into a respective monostable energy potential, wherein each ladder element includes a respective qubit;
ferromagnetically coupling the first qubit to a first end of the chain of ferromagnetically coupled ladder elements such that the first qubit and the chain of ferromagnetically coupled ladder elements operate as a single effective qubit; and
coupling the second qubit to a second end of the chain of ferromagnetically coupled ladder elements.
2. The method of claim 1 wherein biasing each ladder element in a chain of ferromagnetically coupled ladder elements into a respective monostable energy potential includes biasing each ladder element to have a respective β l value less than about 1.
3. The method of claim 1 wherein biasing a first qubit into a first bistable energy potential includes biasing the first qubit to have a β l value greater than about 1, and biasing a second qubit into a second bistable energy potential includes biasing the second qubit to have a β l value greater than about 1.
4. The method of claim 1 , further comprising:
biasing a third qubit into a third bistable energy potential; and
coupling the third qubit to one of the ladder elements in the chain of ferromagnetically coupled ladder elements.
5. The method of claim 1 wherein at least one of the first qubit and the second qubit includes a loop of superconducting material interrupted by at least one Josephson junction.
6. The method of claim 5 wherein the first qubit includes a loop of superconducting material interrupted by a compound Josephson junction and biasing the first qubit into a first bistable energy potential includes biasing the compound Josephson junction of the first qubit.
7. The method of claim 1 wherein each qubit in each respective ladder element in the chain of ferromagnetically coupled ladder elements includes a respective loop of superconducting material, each loop interrupted by at least one Josephson junction.
8. The method of claim 7 wherein each qubit in each respective ladder element in the chain of ferromagnetically coupled ladder elements includes a respective compound Josephson junction and biasing each ladder element in a chain of ferromagnetically coupled ladder elements into a respective monostable energy potential includes biasing the respective compound Josephson junction in each respective ladder element.
9. The method of claim 1 wherein tunneling dynamics of the first qubit occur at a lower energy than tunneling dynamics of the chain of ferromagnetically coupled ladder elements.
10. The method of claim 1 wherein coupling the second qubit to a second end of the chain of ferromagnetically coupled ladder elements includes anti-ferromagnetically coupling the second qubit to the second end of the chain of ferromagnetically coupled ladder elements.
11. The method of claim 1 wherein ferromagnetically coupling the first qubit to a first end of the chain of ferromagnetically coupled ladder elements includes ferromagnetically coupling the first qubit to the first end of the chain of ferromagnetically coupled ladder elements via a superconducting flux coupler.
12. The method of claim 11 wherein coupling the second qubit to a second end of the chain of ferromagnetically coupled ladder elements includes coupling the second qubit to the second end of the chain of ferromagnetically coupled ladder elements via a superconducting flux coupler.