Techniques for improving the performance of a quantum processor are described. Some techniques employ reducing intrinsic/control errors by using quantum processor-wide problems specifically crafted to reveal errors so that corrections may be applied. Corrections may be applied to physical qubits, logical qubits, and couplers so that problems may be solved using quantum processors with greater accuracy.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method of operation for a computational system, the computational system including at least one quantum processor which comprises a plurality of qubits and a plurality of couplers, each of the plurality of qubits having a respective controllable local bias term and each of the plurality of couplers having a respective controllable coupling term, the computational system further including at least one processor-based device communicatively coupled to configure the at least one quantum processor, the method comprising: causing, via the at least one processor-based device, the at least one quantum processor to set each of the coupling terms to a first calibrated zero value; causing, via the at least one processor-based device, the at least one quantum processor to set each of the local bias terms to a target value relative to a second calibrated zero value; calibrating the local bias terms for the respective qubits by, for each qubit in the plurality of qubits, obtaining a number of samples via the at least one quantum processor; constructing an estimate of a population for the qubit using the obtained number of samples; determining whether the qubit exhibits a bias toward a basis state; and modifying the local bias term of the qubit to generate an updated local bias term upon determination that the qubit exhibits a bias toward a basis state to remove the bias. 2. The method of claim 1, further comprising: iteratively calibrating the local bias terms for the respective qubits until one or more criteria are met. 3. The method of claim 2 wherein iteratively calibrating the local bias terms for the respective qubits until one or more criteria are met comprises iteratively calibrating the local bias terms for the respective qubits until one or more of the following criteria are met: an elapsed calibration time, a number of calibration iterations, or a bias threshold. 4. The method of claim 1 wherein each of the plurality of qubits is superconducting below a critical temperature, the method further comprising: maintaining the plurality of qubits at or below the critical temperature; prior to calibrating the local bias terms, initializing the at least one quantum processor in a first configuration embodying an initialization Hamiltonian; and evolving the quantum processor until the quantum system is described by a second configuration embodying a problem Hamiltonian. 5. The method of claim 1, further comprising: repeatedly initializing and evolving the at least one quantum processor for at least N iterations, where N>1; and repeatedly calibrating the local bias terms after at least one of the initialization and evolution iterations while the plurality of qubits is maintained at an operating temperature. 6. The method of claim 1 wherein calibrating the local bias terms comprises calibrating the local bias terms after the plurality of qubits have had sufficient time to thermalize and arrive at a base temperature. 7. A method of operation for a computational system, the computational system including at least one quantum processor which comprises a plurality of qubits and a plurality of couplers, each of the plurality of qubits having a respective controllable local bias term and each of the plurality of couplers having a respective controllable coupling term, the computational system further including at least one processor-based device communicatively coupled to configure the at least one quantum processor, the method comprising: calibrating the coupling term of the coupler by, for each of a plurality of target coupling term values for the coupling term of the coupler, causing, via the at least one processor-based device, the at least one quantum processor to set the coupling term of the coupler to the target coupling term value; for each of a plurality of local bias values, causing the at least one quantum processor to set the local bias term for the at least two qubits to the local bias value; obtaining a number of samples via the at least one quantum processor; assessing the obtained number of samples relative to a model to extract an effective coupling term value for the coupler; comparing the extracted effective coupling term value for the coupler with the target coupling term value; adjusting the target coupling term values based at least in part on a result of the comparison of the extracted effective coupling term values with the target coupling term values. 8. The method of claim 7 wherein calibrating the coupling term of the coupler comprises using a number of target coupling term values that are distributed throughout a range of permissible coupling term values, and using a number of local bias values that are distributed throughout a range of permissible local bias values. 9. The method of claim 7 wherein the at least two qubits are superconducting below a critical temperature, the method further comprising: maintaining the plurality of qubits at or below the critical temperature; prior to calibrating the coupling term, initializing the at least one quantum processor in a first configuration embodying an initialization Hamiltonian; and evolving the quantum processor until the quantum system is described by a second configuration embodying a problem Hamiltonian. 10. The method of claim 7, further comprising: repeatedly initializing and evolving the at least one quantum processor for at least N iterations, where N>1; and repeatedly calibrating the coupling term after at least one of the at least N initialization and evolution iterations while the plurality of qubits is maintained at an operating temperature. 11. The method of claim 7 wherein calibrating the coupling term comprises calibrating the coupling term after the plurality of qubits have had sufficient time to thermalize and arrive at a base temperature. 12. A method of operation for a computational system, the computational system including at least one quantum processor which comprises a plurality of physical qubits, each of the plurality of physical qubits having a respective controllable local bias term, and a plurality of physical qubit couplers, each of the physical qubit couplers coupling a respective set or group of the physical qubits, each of the plurality of physical qubit couplers having a respective controllable coupling term, wherein a first number of the physical qubit couplers are operated as intra-logical qubit couplers where each of the first number of the physical qubit couplers have a respective coupling strength that couples a respective set of the physical qubits as a logical qubit, where each logical qubit represents a variable from a problem, and a second number of the physical qubit couplers are operated as inter-logical qubit couplers, wherein each of the second number of the physical qubit couplers have a respective coupling strength that controllably couples a respective group of the physical qubits, where the physical qubits in the respective group are part of different ones of the logical qubits and wherein at least two variables from the problem are assigned to two respective logical qubits, the computational system further including at least one processor-based device communicatively coupled to configure the at least one quantum processor, the method comprising: causing the at least one quantum processor to set each of the coupling terms of respective intra-logical qubit couplers to a value that provides a coupling strength that couples a respective set of the physical qubits as a logical qubit; causing the at least one quantum processor to set each of the coupling terms of the inter-logical qubit couplers to a target value relative to a first calibrated zero value; causing the at least one quantum processor to set each of the local bias terms to a second calibrated zero value; and calibrating the local bias terms for the qubits by, for each logical qubit, obtaining a number of samples via the at least one quantum processor; constructing an estimate of a population for the logical qubit using the obtained number of samples; determining whether the logical qubit exhibits a bias toward a basis state; and modifying the local bias term of at least one qubit forming the logical qubit upon determination that the logical qubit exhibits a bias toward a basis state to remove the bias. 13. The method of claim 12 wherein each of the physical qubit couplers coupling a respective set or group of the physical qubits is a physical qubit coupler coupling a respective pair of the physical qubits. 14. The method of claim 13 wherein modifying the local bias term of at least one qubit forming the logical qubit comprises modifying the local bias term of each of the qubits forming the logical qubit by a bias adjustment value upon determination that the logical qubit exhibits a bias toward a basis state to remove the bias. 15. The method of claim 13, further comprising: iteratively calibrating the local bias terms until one or more criteria are met. 16. The method of claim 15 wherein, for successive calibration iterations, the method comprises: modifying the local bias term of each of the qubits forming the logical qubit by a bias adjustment value less than a bias adjustment value used on a previous calibration iteration. 17. The method of claim 15 wherein iteratively calibrating the local bias terms until one or more criteria are met comprises iteratively calibrating the local bias terms until one or more of the following criteria are met: an elapsed calibration time, a number of calibration iterations, or a bias threshold. 18. The method of claim 13 wherein each of the plurality of qubits is superconducting below a critical temperature, the method further comprising: maintaining the plurality of qubits at or below the critical temperature, the at least one processor-based device: initializing the at least one quantum processor in a first configuration embodying an initialization Hamiltonian; and evolving the quantum processor until the quantum system is described by a second configuration embodying a problem Hamiltonian. 19. The method of claim 13, further comprising: repeatedly initializing and evolving the at least one quantum processor for at least N iterations, where N>1; and repeatedly calibrating the local bias terms after at least one of the initialization and evolution iterations while the plurality of qubits are maintained at an operating temperature. 20. The method of claim 13 wherein calibrating the local bias terms comprises calibrating the local bias terms after the plurality of qubits have had sufficient time to thermalize and arrive at a base temperature.