Physical superconducting qubits are controlled according to an “encoded” qubit scheme, where a pair of physical superconducting qubits constitute an encoded qubit that can be controlled without the use of a microwave signal. For example, a quantum computing system has at least one encoded qubit and a controller. Each encoded qubit has a pair of physical superconducting qubits capable of being selectively coupled together. Each physical qubit has a respective tunable frequency. The controller controls a state of each of the pair of physical qubits to perform a quantum computation without using microwave control signals. Rather, the controller uses DC-based voltage or flux pulses.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method comprising: controlling operation of a superconductor-based quantum computer without using a microwave signal. 2. The method of claim 1, wherein the controlling operation of the quantum computer comprises: controlling a state of an encoded qubit of the quantum computer without using a microwave signal, wherein the encoded qubit comprises a pair of selectively-coupled superconducting physical qubits. 3. The method of claim 2, wherein the selective coupling between superconducting physical qubits is provided by a tunable coupler, and the superconducting physical qubits of said pair have respective idle frequencies that are the same or differ from each other by no more than 20%. 4. The method of claim 2, wherein the controlling the state without using a microwave signal comprises applying z-pulses to control respective gates of the pair of superconducting physical qubits, and the z-pulses comprise DC-based voltage or flux pulses. 5. The method of claim 2, wherein the superconducting physical qubits are tunable with respect to frequency, and the controlling includes tuning one of the superconducting physical qubits by at least one of: controlling a magnetic field applied to a loop having a pair of Josephson junctions of the superconducting physical qubit; and controlling a gate voltage applied to a superconductor-semiconductor junction or to a Cooper-pair box of the superconducting physical qubit. 6. The method of claim 1, wherein the controlling operation of the quantum computer comprises: controlling a plurality of encoded qubits of the superconductor-based quantum computer without using a microwave signal, wherein each encoded qubit comprises a pair of selectively-coupled superconducting physical qubits. 7. The method of claim 6, wherein the controlling the plurality of encoded qubits includes microwave-free control of respective frequencies of each pair of superconducting physical qubits, the microwave-free control comprises applying z-pulses to control respective gates of each pair of superconducting physical qubits, the z-pulses comprising DC-based voltage or flux pulses, and the z-pulse control implements entangling two-qubit operation between a pair of the plurality of encoded qubits so as to enable universal quantum computation. 8. The method of claim 6, wherein the selective coupling between superconducting physical qubits is provided by a capacitive coupling that is controlled by tuning the respective frequencies of the corresponding superconducting physical qubits, each superconducting physical qubit has a respective idle frequency, and a difference between the idle frequencies of each pair of superconducting physical qubits is greater than the capacitive coupling between said pair. 9. A method comprising: providing at least one encoded qubit, each encoded qubit comprising a pair of physical qubits capable of being selectively coupled together, each of the physical qubits of said pair having a respective tunable frequency; and controlling the at least one encoded qubit to perform a quantum computation. 10. The method of claim 9, wherein at least one of the pair of physical qubits comprises a superconducting qubit. 11. The method of claim 9, wherein at least one of the pair of physical qubits comprises: a superconducting qubit with a superconducting-quantum-interference-device-like (SQUID) tunable Josephson junction; a superconducting qubit with a superconductor-semiconductor-based voltage tunable Josephson junction; or a superconducting qubit with a Cooper-pair box. 12. The method of claim 9, wherein the controlling the at least one encoded qubit comprises arbitrarily rotating a state of the respective encoded qubit around the Bloch sphere. 13. The method of claim 9, wherein the controlling includes microwave-free control of respective frequencies of the pair of physical qubits. 14. The method of claim 13, wherein the microwave-free control comprises applying z-pulses to control respective gates of the pair of physical qubits, the z-pulses comprising DC-based voltage or flux pulses. 15. The method of claim 14, wherein the at least one encoded qubit is at least two encoded qubits, and the z-pulse control implements entangling two-qubit operation between the two encoded qubits so as to enable universal quantum computation. 16. The method of claim 9, wherein the selective coupling between each pair of physical qubits is a capacitive coupling. 17. The method of claim 16, wherein each of the pair of physical qubits has a respective idle frequency, and a difference between the idle frequencies of the pair of physical qubits is greater than the capacitive coupling between said pair. 18. The method of claim 9, further comprising measuring one of the pair of physical qubits of the at least one encoded qubit. 19. The method of claim 18, wherein the measuring comprises a microwave-based dispersive measurement. 20. The method of claim 9, wherein the controlling to perform the quantum computation comprises: tuning the selective coupling between the pair of physical qubits of the at least one encoded qubit via a coupler between the pair of physical qubits or by tuning the respective frequencies of the pair of physical qubits; and/or tuning a coupling between adjacent encoded qubits. 21. A quantum computing system comprising: at least one encoded qubit, each encoded qubit comprising a pair of physical qubits capable of being selectively coupled together, each of the physical qubits of said pair having respective tunable frequencies; and a controller configured to control a state of each of the pair of physical qubits to perform a quantum computation, wherein the controller is configured to control said state via microwave-free control signals. 22. The quantum computing system of claim 21, wherein at least one of the pair of physical qubits comprises: a superconducting qubit having a superconducting-quantum-interference-device-like (SQUID) tunable Josephson junction; a superconducting qubit having a superconductor-semiconductor-based voltage tunable Josephson junction; or a superconducting qubit having a Cooper-pair box. 23. The quantum computing system of claim 21, wherein the controller is configured to control said state by applying DC-based voltage or flux pulse signals to respective gates of the physical qubits. 24. The quantum computing system of claim 21, wherein the controller is further configured to tune the selective coupling between the pair of physical qubits and/or tune coupling between encoded qubits. 25. The quantum computing system of claim 21, wherein the coupling between the pair of physical qubits is capacitive coupling. 26. The quantum computing system of claim 25, wherein each of the pair of physical qubits has a respective idle frequency, and a difference between the idle frequencies of the pair of physical qubits is greater than the capacitive coupling between said pair. 27. The quantum computing system of claim 21, further comprising a read-out system with a plurality of resonators, each of the physical qubits being selectively coupled to a respective one of the resonators. 28. The quantum computing system of claim 21, wherein the pair of physical qubits is constructed to allow arbitrary rotations of a state of the respective encoded qubit around the Bloch sphere.