Qubit circuit and method for topological protection
A qubit circuit and a method for topological protection of a qubit circuit are described. The circuit comprises a plurality of physical superconducting qubits and a plurality of coupling devices interleaved between pairs of the physical superconducting qubits. The coupling devices are tunable to operate the qubit circuit either in a topological regime or as a series of individual physical qubits. At least two superconducting loops, each one threadable by an external flux, are part of the qubit circuit.
1. A topological superconducting qubit circuit comprising:
a plurality of physical superconducting qubits;
a plurality of coupling devices interleaved between pairs of the physical superconducting qubits, the coupling devices tunable to operate the qubit circuit in a topological regime and as a series of individual physical qubits, wherein energy of the coupling devices is greater than energy of the physical superconducting qubits when the qubit circuit operates in the topological regime; and
at least two superconducting loops per coupling device, each one of the at least two loops threadable by an external flux.
2. The circuit of claim 1 , further comprising at least one component for generating a magnetic field for inducing the external flux in the at least two superconducting loops.
3. The circuit of claim 2 , wherein the at least one component comprises two transmission lines, each one coupled to one of the at least two superconducting loops through a mutual inductance.
4. The circuit of claim 1 , wherein each one of the physical superconducting qubits is composed of at least one capacitor and at least one Josephson junction connected together.
5. The circuit of claim 4 , wherein the at least one Josephson junction is part of a SQUID.
6. The circuit of claim 4 , wherein the at least one capacitor and the at least one Josephson junction are connected together at a first node, and the coupling devices are connected to the physical qubits at the first node.
7. The circuit of claim 6 , wherein one of the at least two superconducting loops comprises a second node having a same superconducting phase as the first node.
8. The circuit of claim 4 , wherein the at least one capacitor and the at least one Josephson junction are connected together at a first node, and the coupling devices are connected to the physical qubits at a second node different from the first node.
9. The circuit of claim 8 , wherein one of the at least two superconducting loops comprises a third node having a same superconducting phase as the second node.
10. The circuit of claim 1 , wherein one of the at least two superconducting loops is a loop of superconducting material interrupted by a SQUID.
11. The circuit of claim 10 , wherein a second one of the at least two superconducting loops is interrupted by a Josephson junction of the SQUID.
12. A method for topological protection of quantum information in a qubit circuit, the method comprising:
coupling a plurality of physical qubits with a plurality of interleaved coupling devices, each one of the coupling devices comprising at least one superconducting loop threadable by an external flux ϕ ext ;
selecting parameters for the external flux ϕ ext such that |J/h|>1, where J is a coupling device energy and h is a physical qubit energy; and
applying the external flux ϕ ext to the at least one superconducting loop to induce a phase shift in the coupling devices and operate the qubit circuit in a topological regime.
13. The method of claim 12 , wherein selecting parameters for the external flux ϕ ext comprises selecting ϕ ext to induce a phase shift with a value between π/2 and 3π/2 (mod 2π) in at least one Josephson junction of the qubit circuit.
14. The method of claim 12 , wherein selecting parameters for the external flux ϕ ext comprises selecting ϕ ext to induce a phase shift of π (mod 2π) in at least one Josephson junction of the qubit circuit.
15. The method of claim 12 , further comprising applying an external flux ϕ SQUID to a second superconducting loop of at least one of the plurality of coupling devices.
16. The method of claim 15 , wherein applying the external flux ϕ SQUID comprises applying the external flux ϕ SQUID to the second superconducting loop of all of the plurality of coupling devices.
17. The method of claim 15 , further comprising selecting parameters for ϕ SQUID =(2n+1)/2*ϕ o , where n is an integer and ϕ o is a flux quantum.
18. The method of claim 15 , further comprising modulating ϕ SQUID for at least one of the plurality of coupling devices.
19. The method of claim 18 , wherein modulating ϕ SQUID comprises changing ϕ SQUID adiabatically.
20. The method of claim 18 , wherein modulating ϕ SQUID comprises changing ϕ SQUID from (2n+1)/2*ϕ o to another value, where n is an integer and ϕ o is a flux quantum.