Reducing spontaneous emission in circuit quantum electrodynamics by a combined readout and filter technique
A mechanism relates a superconductor circuit. A Δ circuit includes a first node connecting a Purcell capacitor to a qubit coupling capacitor, a second node connecting the Purcell capacitor to a readout coupling capacitor, and a third node connecting the qubit coupling capacitor to the readout coupling capacitor. A qubit is connected to the first node and has a qubit frequency. A readout resonator connects to the third node combining with the Purcell capacitor to form a filter. Capacitance of the Purcell capacitor is determined as a factor of the qubit frequency of the qubit and blocks emissions of the qubit at the qubit frequency. Capacitance of the Purcell capacitor causes destructive interference, between a first path containing Purcell capacitor and a second path containing both the qubit coupling capacitor and readout coupling capacitor, in order to block emissions of the qubit at the qubit frequency to the external environment.
1. A superconductor circuit comprising:
a circuit comprising a first node connecting a Purcell capacitor (C P ) to a qubit coupling capacitor (C q ), a second node connecting the Purcell capacitor (C P ) to a readout coupling capacitor (C c ), and a third node connecting the qubit coupling capacitor (C q ) to the readout coupling capacitor (C c ), wherein the first node is configured for connection to a qubit having a qubit frequency; and
a readout resonator connected to the third node;
wherein a capacitance of the Purcell capacitor (C P ) is determined as a factor of the qubit frequency of the qubit, such that the capacitance of the Purcell capacitor (C P ) filters emissions of the qubit at the qubit frequency to an external environment.
2. The superconductor circuit of claim 1 , wherein the readout resonator combines with the Purcell capacitor (C P ) to form a filter.
3. The superconductor circuit of claim 1 , wherein the second node is configured for connection to the external environment.
4. The superconductor circuit of claim 3 , wherein the capacitance of the Purcell capacitor (C P ) is set to cause destructive interference, between a first path containing the Purcell capacitor (C P ) and a second path containing both the qubit coupling capacitor (C q ) and the readout coupling capacitor (C c ), at the qubit frequency in order to filter the emissions of the qubit at the qubit frequency to the external environment.
5. The superconductor circuit of claim 1 , wherein the capacitance of the Purcell capacitor (C P ) has a direct relationship to the qubit frequency of the qubit in which the capacitance of the Purcell capacitor (C P ) is to be increased when a value of the qubit frequency is increased in order to filter the qubit frequency to the external environment.
6. The superconductor circuit of claim 1 , wherein the readout resonator further comprises an inductance and a capacitance connected in parallel together.
7. The superconductor circuit of claim 1 , wherein the readout resonator provides dispersive readout of the qubit to the external environment.
8. The superconductor circuit of claim 7 , wherein the readout resonator combined with the Purcell capacitor (C P ) provides filtering, at the qubit frequency, radiation from the qubit to the external environment.
9. The superconductor circuit of claim 1 , wherein the capacitance of the Purcell capacitor (C P ) is determined based on factors which include the qubit frequency of the qubit, an inductance of the readout resonator, and a capacitance of the readout resonator such that the capacitance of the Purcell capacitor (C P ) filters emissions of the qubit at the qubit frequency to the external environment while allowing dispersive readout of a state of the qubit.
10. The superconductor circuit of claim 1 , wherein the qubit includes a Josephson junction.
11. The superconductor circuit of claim 1 , wherein the circuit is a delta (Δ) circuit that has a relationship to a Y circuit; and
wherein the capacitance of the Purcell capacitor (C P ) is determined based on the relationship between the Δ circuit and the Y circuit.
12. A method of providing a superconductor circuit, the method comprising:
providing a circuit comprising a first node connecting a Purcell capacitor (C P ) to a qubit coupling capacitor (C q ), a second node connecting the Purcell capacitor (C P ) to a readout coupling capacitor (C c ), and a third node connecting the qubit coupling capacitor (C q ) to the readout coupling capacitor (C c ), wherein the first node is configured for connection to a qubit having a qubit frequency; and
providing a readout resonator connected to the third node;
wherein a capacitance of the Purcell capacitor (C P ) is determined as a factor of the qubit frequency of the qubit, such that the capacitance of the Purcell capacitor (C P ) filters emissions of the qubit at the qubit frequency to an external environment.
13. The method of claim 12 , wherein the readout resonator combines with the Purcell capacitor (C P ) to form a filter.
14. The method of claim 12 , wherein the second node is configured for connection to the external environment.
15. The method of claim 14 , wherein the capacitance of the Purcell capacitor (C P ) is set to cause destructive interference, between a first path containing the Purcell capacitor (C P ) and a second path containing both the qubit coupling capacitor (C q ) and the readout coupling capacitor (C c ), at the qubit frequency in order to filter the emissions of the qubit at the qubit frequency to the external environment.
16. The method of claim 12 , wherein the capacitance of the Purcell capacitor (C P ) has a direct relationship to the qubit frequency of the qubit in which the capacitance of the Purcell capacitor (C P ) is to be increased when a value of the qubit frequency is increased in order to filter the qubit frequency to the external environment.
17. The method of claim 12 , wherein the readout resonator comprises an inductance and a capacitance connected in parallel together.
18. The method of claim 12 , wherein the readout resonator provides dispersive readout of the qubit to the external environment.
19. The method of claim 18 , wherein the readout resonator combined with the Purcell capacitor (C P ) provides filtering, at the qubit frequency, radiation from the qubit to the external environment.
20. The method of claim 12 , wherein the capacitance of the Purcell capacitor (C P ) is determined based on factors which include the qubit frequency of the qubit, an inductance of the readout resonator, and a capacitance of the readout resonator such that the capacitance of the Purcell capacitor (C P ) filters emissions of the qubit at the qubit frequency to the external environment while allowing dispersive readout of a state of the qubit.