Microwave-free control of a superconductor-based quantum computer
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.
1. A method comprising:
controlling operation of a superconductor-based quantum computer without using a microwave signal,
wherein the controlling operation of the quantum computer comprises controlling a state of one or more encoded qubits of the quantum computer without using a microwave signal, each encoded qubit comprising a pair of selectively-coupled superconducting physical qubits,
wherein the controlling the state of the one or more encoded qubits comprises applying z-pulses to control respective gates of the pair of selectively-coupled superconducting physical qubits, each z-pulse comprising a DC-based voltage or flux pulse, and
wherein the applied z-pulses change respective frequencies of the selectively-coupled superconducting physical qubits.
2. The method of claim 1 , 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%.
3. The method of claim 1 , wherein
the respective frequencies of the superconducting physical qubits are changed 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.
4. The method of claim 1 , wherein
respective states of a plurality of the encoded qubits are controlled without using a microwave signal, 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.
5. The method of claim 1 , 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.
6. The method of claim 1 , wherein the controlling the state of the one or more encoded qubits is such that any single qubit gate operation can be implemented by the quantum computer as a three-step Euler angle rotation around two orthogonal rotation axes.
7. The method of claim 1 , wherein the z-pulses are baseband DC voltage or flux signals.
8. 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,
wherein the controlling includes microwave-free control of a state of the at least one encoded qubit by applying z-pulses to control respective gates of the pair of physical qubits, the z-pulses comprising DC-based voltage or flux pulses, and
wherein the applied z-pulses change respective frequencies of the physical qubits.
9. The method of claim 8 , wherein at least one of the pair of physical qubits comprises a superconducting qubit.
10. The method of claim 8 , 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.
11. The method of claim 8 , wherein the controlling the at least one encoded qubit comprises arbitrarily rotating a state of the respective encoded qubit around the Bloch sphere.
12. The method of claim 8 , 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.
13. The method of claim 8 , wherein the selective coupling between each pair of physical qubits is a capacitive coupling that is controlled by tuning the respective frequencies of the corresponding physical qubits.
14. The method of claim 13 , wherein
each physical qubit has a respective idle frequency, and
a difference between the respective idle frequencies of each pair of physical qubits is greater than the capacitive coupling between said pair.
15. The method of claim 8 , further comprising measuring one of the pair of physical qubits of the at least one encoded qubit.
16. The method of claim 15 , wherein the measuring comprises a microwave-based dispersive measurement.
17. The method of claim 8 , 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.
18. The method of claim 8 , wherein the quantum computation includes a single qubit gate operation that is implemented as a three-step Euler angle rotation around two orthogonal axes.
19. 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 by applying DC-based voltage or flux pulses to respective gates of the pair of physical qubits,
wherein the applied pulses change respective frequencies of the physical qubits.
20. The quantum computing system of claim 19 , 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.
21. The quantum computing system of claim 19 , wherein the controller is further configured to tune the selective coupling between the pair of physical qubits and/or tune coupling between encoded qubits.
22. The quantum computing system of claim 19 , wherein the coupling between the pair of physical qubits is capacitive coupling.
23. The quantum computing system of claim 22 , 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.
24. The quantum computing system of claim 19 , 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.
25. The quantum computing system of claim 19 , wherein the pair of physical qubits is constructed to allow arbitrary rotations of a state of the respective encoded qubit around the Bloch sphere.