ZZZ coupler for superconducting qubits

Patent metadata

Field	Value
Publication number	US-10074792-B1
Application number	US-201715455466-A
Country	US
Kind code	B1
Filing date	Mar 10, 2017
Priority date	Mar 10, 2017
Publication date	Sep 11, 2018
Grant date	Sep 11, 2018

Abstract

Systems and methods are provided for a ZZZ coupler. A first tunable coupler is coupled to the first qubit and tunable via a first control signal. A second tunable coupler is coupled to the first tunable coupler to direct a flux of the first qubit into a tuning loop of the second tunable coupler, such that when a first coupling strength associated with the first tunable coupler is non-zero, a second coupling strength, associated with the second tunable coupler, is a function of a second control signal applied to the second tunable coupler and a state of the first qubit. The second qubit and the third qubit are coupled to one another through the second tunable coupler, such that, when the second coupling strength is non-zero it is energetically favorable for the states of the first and second qubits to assume a specific relationship with respect to the Z-axis.

First claim

What is claimed is:

1. A ZZZ coupler assembly for coupling first, second, and third qubits comprising:
a first tunable coupler coupled to the first qubit and tunable via a first control signal; and
a second tunable coupler coupled to the first tunable coupler to direct a flux of the first qubit into a tuning loop of the second tunable coupler such that, when a first coupling strength associated with the first tunable coupler is non-zero, a second coupling strength, associated with the second tunable coupler, is a function of a second control signal applied to the second tunable coupler and a state of the first qubit;
wherein the second qubit and the third qubit are coupled to one another through the second tunable coupler, such that, when the second coupling strength is non-zero, it is energetically favorable for the states of the first and second qubits to assume a specific relationship with respect to the Z-axis of the Bloch sphere.

2. The ZZZ coupler assembly of claim 1 , wherein the second qubit and the third qubit are coupled through the second tunable coupler via galvanic Josephson mutual inductance.

3. The ZZZ coupler assembly of claim 1 , wherein the second control signal can be varied such that the second coupler can provide any of a negative coupling, such that it is energetically favorable for the states of the second and third qubits to align in a same direction along the Z-axis, a positive coupling, such that it is energetically favorable for the states of the second and third qubits to align in opposite directions along the Z-axis, and a zero coupling between the second qubit and the third qubit.

4. The ZZZ coupler assembly of claim 1 , further comprising;
a third tunable coupler coupled to the second qubit and tunable via a third control signal; and
a fourth tunable coupler coupled to the third tunable coupler to direct a flux of the second qubit into the fourth tunable coupler such that, when a third coupling strength associated with the third tunable coupler is non-zero, a fourth coupling strength, associated with the fourth tunable coupler, is a function of a fourth control signal applied to the fourth tunable coupler and a state of the second qubit;
wherein the first qubit and the third qubit are coupled through the fourth tunable coupler, such that, when a coupling strength of the fourth tunable coupler is non-zero, it is energetically favorable for the states of the first and third qubits to assume a specific relationship with respect to the Z-axis.

5. The ZZZ coupler assembly of claim 4 , further comprising;
a fifth tunable coupler coupled to the third qubit and tunable via a fifth control signal; and
a sixth tunable coupler coupled to the fifth tunable coupler to direct a flux of the third qubit into the sixth tunable coupler such that, when a fifth coupling strength associated with the fifth tunable coupler is non-zero, a sixth coupling strength, associated with the sixth tunable coupler, is a function of a sixth control signal applied to the sixth tunable coupler and a state of the third qubit;
wherein the first qubit and the second qubit are coupled through the sixth tunable coupler, such that, when a coupling strength of the sixth tunable coupler is non-zero, it is energetically favorable for the states of the first and second qubits to assume a specific relationship with respect to the Z-axis.

6. The ZZZ coupler assembly of claim 1 , wherein the first control signal can be tuned to reduce the first coupling strength to zero, such that the second coupling strength becomes independent of the state of the first qubit.

7. The ZZZ coupler assembly of claim 6 , wherein each of the first tunable coupler and the second tunable coupler comprises a compound Josephson junction, and each of the first and second control signals comprising a first applied flux and a second applied flux.

8. The ZZZ coupler assembly of claim 7 , further comprising a first classical control, the first classical control being configured to provide the first applied flux as one half of a flux quantum to tune the first coupling strength to zero such that the second coupling strength becomes independent of the state of the first qubit and a Hamiltonian of a system formed by the first qubit, the second qubit, the third qubit, and the ZZZ coupler assembly does not contain a three qubit term.

9. The ZZZ coupler assembly of claim 8 , further comprising a second classical control, the second classical control being configured to provide the second applied flux as one half of a flux quantum to tune the second coupling strength to zero.

10. A quantum circuit assembly comprising:
the first qubit;
the second qubit;
the third qubit; and
the ZZZ coupler assembly of claim 1 .

11. The quantum circuit assembly of claim 10 , wherein each of the first qubit, the second qubit, and the third qubit are flux qubits comprising superconducting loops interrupted by at least one Josephson junction.

12. The quantum circuit assembly of claim 11 , wherein the at least one Josephson junction comprises respective first and second Josephson junctions for each of the first qubit, the second qubit, and the third qubit, and each of the second qubit and the third qubit further comprise respective third and fourth Josephson junctions, each of the second and third qubit sharing respective third and fourth Josephson junctions with the second tunable coupler.

13. The quantum circuit assembly of claim 12 , the first tunable coupler comprising a first set of Josephson junctions, and the second tunable coupler comprising a second set of Josephson junctions, wherein a critical current associated with the third and fourth Josephson junctions on each of the second and third qubits is at least twice a critical current associated with any Josephson junction of the first set of Josephson junctions, the second set of Josephson junctions, and the at least one Josephson junction for each of the first qubit, the second qubit, and the third qubit.

14. The quantum circuit assembly of claim 10 , wherein each of the first qubit, the second qubit, and the third qubit are Transmon qubits.

Assignees

• Ferguson David George
• Przybysz Anthony Joseph
• Strand Joel D
• Northrop Grumman Systems Corp

Inventors

• Ferguson David George
• Przybysz Anthony Joseph
• Strand Joel D

Classifications

• H03K19/1952

  with electro-magnetic coupling of the control current · CPC title

• H01L39/025 Primary

  Electricity · mapped topic

• G06N99/002

  Physics · mapped topic

• G06N10/40 Primary

  Physical realisations or architectures of quantum processors or components for manipulating qubits, e.g. qubit coupling or qubit control · CPC title

• H10N60/805

  for Josephson-effect devices · CPC title