Adiabatic quantum computation with superconducting qubits
View Patent ↗A method for computing using a quantum system comprising a plurality of superconducting qubits is provided. Quantum system can be in any one of at least two configurations including (i) an initialization Hamiltonian H 0 and (ii) a problem Hamiltonian H P . The plurality of superconducting qubits are arranged with respect to one another, with a predetermined number of couplings between respective pairs of superconducting qubits in the plurality of qubits, such that the plurality of superconducting qubits, coupled by the predetermined number of couplings, collectively define a computational problem to be solved. In the method, quantum system is initialized to the initialization Hamiltonian H O . Quantum system is then adiabatically changed until it is described by the ground state of the problem Hamiltonian H P . The quantum state of quantum system is then readout thereby solving the computational problem to be solved.
1. A computation device for adiabatic quantum computing comprising:
a plurality of superconducting qubits wherein each superconducting qubit in the plurality of superconducting qubits comprises two basis states each associated with an eigenstate of the σ Z Pauli matrix that can be biased;
a plurality of couplings, each coupling in the plurality of couplings disposed between a superconducting qubit pair in the plurality of superconducting qubits, wherein each term Hamiltonian for a coupling in the plurality of couplings has a principal component proportional to σ Z {circle around (×)}σ Z , and wherein the sign for at least one principal component proportional to σ Z {circle around (×)}σ Z for a coupling in the plurality of couplings is antiferromagnetic;
wherein said superconducting qubits and said plurality of couplings are collectively capable of being in any one of at least two configurations, the at least two configurations including:
a first configuration characterized by an initialization Hamiltonian H 0 , and
a second Hamiltonian characterized by a problem Hamiltonian H P , the problem Hamiltonian having a ground state, wherein each respective first superconducting qubit in said plurality of superconducting qubits is coupled with a respective second superconducting qubit in the plurality of superconducting qubits such that the first respective superconducting qubit and the corresponding second respective superconducting qubit define a predetermined coupling strength and wherein the predetermined coupling strength between each said first respective superconducting qubit and corresponding second respective superconducting qubit collectively define a computational problem to be solved; and
a read out circuit coupled to at least one superconducting qubit in the plurality of superconducting qubits.
2. An apparatus comprising:
(A) a plurality of superconducting charge qubits, wherein
(i) each respective first superconducting charge qubit in said plurality of superconducting charge qubits is coupled with a respective second superconducting charge qubit in the plurality of superconducting charge qubits such that the first respective superconducting charge qubit and the second respective superconducting charge qubit define a predetermined coupling strength and wherein the predetermined coupling strength between each said first respective superconducting charge qubit and each said second respective superconducting charge qubit in said plurality of superconducting charge qubits collectively define a computational problem to be solved, and
(ii) each superconducting charge qubit in said plurality of superconducting charge qubits is capable of being in one of at least two configurations including:
a first configuration in accordance with an initialization Hamiltonian H 0 ; and
a second configuration in accordance with a problem Hamiltonian H P ; and
(B) an electrometer coupled to a superconducting charge qubit in said plurality of superconducting charge qubits.
3. The apparatus of claim 2 wherein a superconducting charge qubit in said plurality of superconducting charge qubits comprises:
a mesoscopic island made of superconducting material;
a superconducting reservoir; and
a Josephson junction connecting said mesoscopic island to said superconducting reservoir.
4. The apparatus of claim 3 wherein said Josephson junction is a split Josephson junction.
5. The apparatus of claim 4 wherein said superconducting charge qubit further comprises a flux source configured to apply flux to the split Josephson junction.
6. The apparatus of claim 2 further comprising a generator capacitively coupled to a superconducting charge qubit in said plurality of superconducting charge qubits by a capacitor.
7. The apparatus of claim 6 wherein said generator is configured to apply a plurality of electrostatic pulses to said superconducting charge qubit and wherein the plurality of electrostatic pulses additionally defines the computational problem.
8. The apparatus of claim 2 further comprising a variable electrostatic transformer disposed between a first superconducting charge qubit and a second superconducting charge qubit in the plurality of superconducting charge qubits such that the predetermined coupling strength between the first superconducting charge qubit and the second superconducting charge qubit is tunable.
9. The apparatus of claim 2 wherein each respective first superconducting charge qubit in said plurality of superconducting charge qubits is arranged with respect to a respective second superconducting charge qubit in the plurality of superconducting charge qubits such that said plurality of superconducting charge qubits collectively form a non-planar graph.
10. A method for computing using a quantum system comprising a plurality of superconducting charge qubits, wherein the quantum system is coupled to an electrometer and the quantum system is capable of being in any one of at least two configurations, the at least two configurations including:
a first configuration characterized by an initialization Hamiltonian H 0 , and
a second configuration characterized by a problem Hamiltonian H P having a ground state, and wherein the plurality of superconducting charge qubits are arranged with respect to one another, with a predetermined number of couplings between respective pairs of superconducting charge qubits in the plurality of charge qubits, such that the plurality of superconducting charge qubits, coupled by the predetermined number of couplings, collectively define a computational problem to be solved; the method comprising:
initializing the quantum system to the initialization Hamiltonian H O ;
adiabatically changing the quantum system until it is described by the ground state of the problem Hamiltonian H P ; and
reading out the quantum state of each superconducting charge qubit in the quantum system through the electrometer, thereby solving the computational problem to be solved.
11. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits is coupled to a second superconducting charge qubit in the plurality of superconducting charge qubits by a capacitor such that the predetermined coupling strength between the first superconducting charge qubit and the second superconducting charge qubit is predetermined and is a function of the physical properties of the capacitor.
12. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits is coupled to a generator by a device configured to provide a tunable effective charging energy, wherein said device comprises a capacitor, and wherein the method further comprises:
tuning the value of the effective charging energy of the first superconducting charge qubit by varying the gate voltage on the capacitor of said device.
13. The method of claim 10 , wherein a superconducting charge qubit in said plurality of superconducting charge qubits comprises a split Josephson junction having a variable effective Josephson energy, the method further comprising tuning the value of the effective Josephson energy of the superconducting charge qubit by varying a flux applied to the split Josephson junction.
14. The method of claim 13 , wherein said first configuration is reached by setting the effective Josephson energy of said superconducting charge qubit to a maximum value.
15. The method of claim 14 , wherein said adiabatically changing step comprises changing the configuration of the system from the first configuration characterized by said initialization Hamiltonian H 0 , to the second Hamiltonian characterized by a problem Hamiltonian H P in the presence of tunneling on a superconducting charge qubit in said plurality of superconducting charge qubits.
16. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits is characterized by (i) an effective Josephson energy that is tunable and (ii) an effective charging energy that is tunable, wherein:
a minimum value of the effective Josephson energy is less than the effective charging energy of the first superconducting charge qubit;
a minimum value of the effective Josephson energy is less than a strength of a coupling between the first superconducting charge qubit and a second superconducting charge qubit in the plurality of superconducting charge qubits;
the effective charging energy is, at most, equal to a maximum value of the effective Josephson energy of the first superconducting charge qubit; and
a strength of a coupling between the first superconducting charge qubit and a second superconducting charge qubit in the plurality of superconducting charge qubits is, at most, equal to a maximum value of the effective Josephson energy of the first superconducting charge qubit.
17. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits is characterized by (i) an effective Josephson energy that is tunable and (ii) an effective charging energy that is tunable, and wherein:
said adiabatically changing step comprises adiabatically tuning the effective Josephson energy of the first superconducting charge qubit such that the effective Josephson energy of the first superconducting charge qubit reaches a minimum value when the quantum system is described by the ground state of the problem Hamiltonian H P .
18. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits has a first basis state and a second basis state and wherein, when the quantum system is described by the ground state of the problem Hamiltonian H P , tunneling between the first basis state and the second basis state of said first superconducting charge qubit does not occur.
19. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits has a first basis state and a second basis state and wherein, when the quantum system is described by the ground state of the problem Hamiltonian H P , the tunneling between the first basis state and the second basis state of said first superconducting charge qubit does occur, and wherein said reading out step comprises probing an observable of the sigma-x Pauli matrix σ X .
20. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits is characterized by (i) an effective Josephson energy that is tunable and (ii) an effective charging energy that is tunable, wherein:
a minimum value of the effective Josephson energy is less than the effective charging energy of the first superconducting charge qubit;
a minimum value of effective Josephson energy is less than a strength of a coupling between the first superconducting charge qubit and a second superconducting charge qubit in the plurality of superconducting charge qubits;
the effective charging energy is greater than a maximum value of the effective Josephson energy of the first superconducting charge qubit; and
a strength of a coupling between the first superconducting charge qubit and a second superconducting charge qubit in the plurality of superconducting charge qubits is, at most, equal to the maximum effective Josephson energy of the first superconducting charge qubit.
21. The method of claim 20 , wherein said initializing step comprises setting the effective charging energy of the first superconducting charge qubit to a minimum value.
22. The method of claim 21 , wherein said adiabatically changing step comprises: adiabatically tuning the effective Josephson energy of the first superconducting charge qubit such that the effective Josephson energy is at a minimum value when the quantum system is described by the ground state of the problem Hamiltonian H P , and
adiabatically increasing the effective charging energy of the first superconducting charge qubit.
23. The method of claim 10 , wherein a first superconducting charge qubit in said plurality of superconducting charge qubits is characterized by an effective Josephson energy that is tunable and wherein:
said initializing step comprises setting the effective Josephson energy of the first superconducting charge qubit to a minimum value, and
said adiabatically changing step comprises:
adiabatically tuning the effective Josephson energy of the first superconducting charge qubit such that the effective Josephson energy is greater than a minimum value for a period of time before the quantum system is described by the ground state of the problem Hamiltonian H P , and
adiabatically tuning the effective Josephson energy of the first superconducting charge qubit such that the effective Josephson energy is at a minimum value when the quantum system is described by the ground state of the problem Hamiltonian H P .