Constructing and programming quantum hardware for quantum annealing processes
Methods, systems, and apparatus, including computer programs encoded on computer storage media, for constructing and programming quantum hardware for quantum annealing processes.
1. A computer implemented method comprising:
determining, by a control system of quantum annealing hardware, values of control parameters for programming the quantum annealing hardware to implement a quantum annealing schedule, wherein the control parameters comprise coupling strengths of programmable inductive couplers between superconducting qubits in a quantum system included in the quantum annealing hardware, wherein the superconducting qubits comprise logical superconducting qubits for use in computation and control superconducting qubits for use in assisting the computation without being computational units, the determining comprising:
obtaining information representing energy states of a total Hamiltonian, wherein the total Hamiltonian characterizes dynamics of the quantum system, the quantum system comprising a quantum annealing subsystem and a quantum governor subsystem that comprises the control superconducting qubits;
calculating an average phonon energy of a bath in which the quantum system is located;
selecting a probability mass function for a ground state fidelity of the quantum annealing subsystem;
determining a quantum-governor distribution of an energy spectrum for the quantum governor subsystem based on i) the information representing the energy states of the total Hamiltonian, ii) the average phonon energy, and iii) the probability mass function; and
selecting the values of the control parameters from the quantum-governor distribution; and
programming, by the control system, the quantum system included in the quantum annealing hardware using the determined values of the control parameters, comprising:
initializing the quantum system according to initialization parameters that encode an optimization problem into an energy spectrum of the total Hamiltonian; and
applying a magnetic field along a z direction to adjust the coupling provided by the inductive couplers between the logical superconducting qubits and the control superconducting qubits included in the quantum annealing hardware according to the determined values of the control parameters;
implementing the quantum annealing schedule to drive the quantum system programmed by the determined values of the control parameters to a ground state of the quantum system; and
measuring an energy of the ground state, wherein the energy encodes a solution to the optimization problem.
2. The method of claim 1 , wherein the information representing the energy states of the total Hamiltonian comprises one or more of energy levels, spacings between energy levels, a distribution of the spacings between energy levels, or an average spacing between energy levels.
3. The method of claim 2 , wherein the average energy level spacing is given by
Δε
(
t
)
_
=
2
∑
i
=
0
N
-
1
❘
"\[LeftBracketingBar]"
ε
i
(
t
)
-
ε
j
(
t
)
❘
"\[RightBracketingBar]"
N
(
N
-
1
)
where ε i (t) represents an energy of an i-th instantaneous eigenstate energy of the total Hamiltonian and N represents a total number of eigenstates of the total Hamiltonian.
4. The method of claim 1 , wherein the information representing the energy states of the total Hamiltonian comprises information representing the energy states of the total Hamiltonian at a time of a quantum phase transition.
5. The method of claim 1 , wherein calculating the average phonon energy of the bath comprises experimentally measuring the average phonon energy of the bath.
6. The method of claim 1 , wherein calculating the average phonon energy of the bath comprises computing kT, where k represents the Boltzmann constant and T represents temperature.
7. The method of claim 1 , wherein the average phonon energy satisfies the following equation:
ω
_
=
∑
0
∞
ω
J
(
ω
)
d
ω
/
(
e
ω
/
kT
-
1
)
∑
0
∞
J
(
ω
)
d
ω
/
(
e
ω
/
kT
-
1
)
,
where J(ω) represents one of an Omhic spectral density:
J
(
ω
)
=
λω
e
-
ω
γ
,
a super-Omhic spectral density:
J
(
ω
)
=
λ
ω
3
e
-
ω
γ
,
a Drude-Lorentz spectral density:
J
(
ω
)
=
2
λ
ω
γ
ω
2
+
γ
2
,
or a flat spectral distribution: J(ω)=1, λ represents a reorganization energy, and γ represents a bath frequency cut-off.
8. The method of claim 1 , wherein calculating the average phonon energy of the bath comprises calculating the average phonon energy using an open quantum system model.
9. The method of claim 1 , wherein the total Hamiltonian is given by
H
t
o
t
=
I
(
t
)
∑
i
N
σ
i
x
+
G
(
t
)
(
∑
i
=
1
N
G
h
i
G
σ
i
z
+
∑
i
,
j
=
1
N
G
J
ij
G
σ
i
z
σ
j
z
+
∑
i
=
1
N
G
∑
j
=
1
N
A
J
ij
G
A
σ
i
z
σ
j
z
)
+
P
(
t
)
(
-
∑
i
N
h
i
σ
i
z
+
∑
i
j
N
J
ij
σ
i
z
σ
j
z
)
wherein i represents an i th superconducting unit, h i and J ij represent control parameters included in a problem Hamiltonian and take real values that are associated with a coupling strength between superconducting units, h i G , J ij G and J ij GA represent control parameters included in a quantum governor Hamiltonian and take real values that are associated with a coupling strength between superconducting units, I(t), G(t), and P(t) represent time-dependent control parameters, and σ i x , σ i z represent Pauli-x and Pauli-z operators for superconducting unit i.
10. The method of claim 1 , wherein the quantum-governor distribution is represented by an exponential family of random matrixes selected using a random matrix theory model.
11. The method of claim 10 , wherein the exponential family is parameterized with respect to controllable quantum annealing hardware parameters, the parameters comprising coupling between qubits.
12. The method of claim 1 , wherein in a second phase of the quantum annealing schedule, an average energy level spacing of the quantum governor subsystem satisfies » ω where represents a modified average energy level of the quantum annealing subsystem and quantum governor subsystem and ω represents the average phonon energy.
13. The method of claim 1 , wherein in a third phase of the quantum annealing schedule, an average energy level spacing of the quantum governor subsystem satisfies ≈ ω where represents a modified average energy level of the quantum annealing subsystem and quantum governor subsystem and ω represents the average phonon energy.
14. A control system for programming quantum annealing hardware, the control system comprising:
one or more computers; and
one or more computer-readable media coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising:
determining, by a control system of quantum annealing hardware, values of control parameters for programming the quantum annealing hardware to implement a quantum annealing schedule, wherein the control parameters comprise coupling strengths of programmable inductive couplers between superconducting qubits in a quantum system included in the quantum annealing hardware, wherein the superconducting qubits comprise logical superconducting qubits for use in computation and control superconducting qubits for use in assisting the computation without being computational units, the determining comprising:
obtaining information representing energy states of a total Hamiltonian, wherein the total Hamiltonian characterizes dynamics of the quantum system, the quantum system comprising a quantum annealing subsystem and a quantum governor subsystem that comprises the control superconducting qubits;
calculating an average phonon energy of a bath in which the quantum system is located;
selecting a probability mass function for a ground state fidelity of the quantum annealing subsystem;
determining a quantum-governor distribution of an energy spectrum for the quantum governor subsystem based on i) the information representing the energy states of the total Hamiltonian, ii) the average phonon energy, and iii) the probability mass function; and
selecting the values of the control parameters from the quantum-governor distribution; and
programming, by the control system, the quantum system included in the quantum annealing hardware using the determined values of the control parameters, comprising:
initializing the quantum system according to initialization parameters that encode an optimization problem into an energy spectrum of the total Hamiltonian; and
applying a magnetic field along a z direction to adjust the coupling provided by the inductive couplers between the logical superconducting qubits and the control superconducting qubits included in the quantum annealing hardware according to the determined values of the control parameters;
implementing the quantum annealing schedule to drive the quantum system programmed by the determined values of the control parameters to a ground state of the quantum system; and
measuring an energy of the ground state, wherein the energy encodes a solution to the optimization problem.
15. The control system of claim 14 , wherein in a second phase of the quantum annealing schedule, an average energy level spacing of the quantum governor subsystem satisfies » ω where represents a modified average energy level of the quantum annealing subsystem and quantum governor subsystem and ω represents the average phonon energy.
16. The control system of claim 14 , wherein in a third phase of the quantum annealing schedule, an average energy level spacing of the quantum governor subsystem satisfies ≈ ω where represents a modified average energy level of the quantum annealing subsystem and quantum governor subsystem and ω represents the average phonon energy.