CALCULATION METHOD, CALCULATION DEVICE, AND COMPUTER READABLE STORAGE MEDIUM

Abstract

A calculation method reduces a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins. The calculation method includes: adding an auxiliary spin to the Ising model; and setting a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain a new Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.

Description

TECHNICAL FIELD

The present invention relates to a calculation method, a calculation device, and a program.

BACKGROUND ART

A technique has been known that transforms combinatorial optimization problems for searching a best combination from various combinations into an Ising model and solves the problems using annealing machines or Ising machines (See for example, Non Patent Literature 1).

CITATION LIST

Non Patent Literature

• Non Patent Literature 1: K. Takehara, D. Oku, Y. Matsuda, S. Tanaka, and N. Togawa, “A multiple coefficients trial method to solve combinatorial optimization problems for simulated-annealing-based ising machines,” in Proc. IEEE 9th International Conference on Consumer Electronics (ICCE-Berlin). ieeexplore.ieee.org, September 2019, pp. 64-69.

SUMMARY OF INVENTION

Technical Problem

The bit-widths of interaction and external magnetic field coefficients in the Ising model, which are input into annealing machines or Ising machines, are limited in many hardware devices, and thus these hardware devices cannot handle any bit-width value. Using a shift method for truncating the lower bits to reduce the bit-widths leads to a change in a ground state which is the lowest-energy state of the Ising model. This makes it impossible to obtain a desired solution.

The invention has been made in view of the foregoing, and an object of the invention is to provide a calculation method, a calculation device, and a program which reduce a bit-width of an Ising model without changing a ground state.

Solution to Problem

A calculation method according to the invention is a calculation method for reducing a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins. The calculation method includes: adding an auxiliary spin to the Ising model; and setting a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain a new Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.

A calculation device according to the invention is a calculation device configured to reduce a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins. The calculation device includes a processor configured to: add an auxiliary spin to the Ising model; and set a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain anew Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.

A program according to the invention is a program for causing a computer to execute a calculation method which reduces a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins. The program causes the computer to execute: adding an auxiliary spin to the Ising model; and setting a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain a new Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.

Advantageous Effects of Invention

According to the invention, by adding an auxiliary spin to an Ising model and setting a new interaction coefficient between the auxiliary spin and a spin of the Ising model to obtain a new Ising model, it is possible to reduce a bit-width of the Ising model without changing a ground state.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of an example of applying a shift method to an Ising model.

FIG. 2 is a block diagram of a hardware configuration for a calculation device according to embodiments.

FIG. 3 is an illustration diagram of a method for extending an interaction coefficient.

FIG. 4 is an illustration diagram of a method for reducing a bit-width of an interaction coefficient.

FIG. 5 is an illustration of an example of reducing a bit-width of an interaction coefficient.

FIG. 6 is an illustration diagram of a method for reducing a bit-width of an external magnetic field coefficient.

FIG. 7 is an illustration of an example of reducing a bit-width of an external magnetic field coefficient.

FIG. 8 is an illustration of an example of applying a calculation method of the embodiments to an Ising model.

FIG. 9 is an illustration of an example of a number partition problem expressed in terms of an Ising model.

FIG. 10 is an illustration of an example of a vertex cover problem and its optimal solutions.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the invention will be described below with reference to the drawings. In the embodiments, an assumption is made that an integer has a sign bit and hence changing the sign of an integer does not change its bit-width. Therefore, an n-bit integer shows any integer within the range of [−(2ⁿ⁻¹−1), 2ⁿ⁻¹−1].

<Ising Model>

An Ising model is a statistical mechanics model defined on an undirected graph G=(V, E), where V is a set of vertices and E is a set of edges. The Ising model is expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins. A spin σ_i is defined on a vertex i∈V and takes the value of +1 or −1 (up or down). An interaction coefficient J_{i, j} is defined on an edge (i, j)∈E which indicates a weight of connection between σ_i and σ_j. An external magnetic field coefficient h_i is defined on a vertex i∈V which indicates a force on the spin σ_i.

The energy function (or Hamiltonian) H of the Ising model is given by Equation (1).

$\begin{array}{cc}H=-\sum _{\left(i,j\right)\in E}{J}_{i,j}{\sigma }_i{\sigma }_j-\sum _{i\in V}{h}_i{\sigma }_i& \left(1\right)\end{array}$

In Equation (1), a constant value which does not depend on spins is omitted. In the embodiments, J_{i, j} and h_i are assumed to be integers.

An Ising model is stable at the lowest-energy state (ground state). Mapping optimal solutions of combinatorial optimization problems to the ground states of the Ising models may lead to obtaining the optimal solutions.

However, as described above, the bit-widths of the interaction and external magnetic field coefficients of the Ising models, which are input into annealing machines or Ising machines, are limited in many hardware devices. Therefore, when the bit-widths of the interaction and external magnetic field coefficients are larger than the annealing machines or Ising machines can handle, the bit-widths need to be reduced.

<Shift Method>

A shift method is a naive bit-width reduction method for an Ising model. FIG. 1 shows an example of an Ising model 10 defined in terms of four vertices and six edges. In the Ising model 10, J_{1, 2}=3, J_{1, 4}=2, J_{2, 3}=J_{2, 4}=J_{3, 4}=J_{1, 3=1}, h₁=3, h₂=−2, h₃=h₄=0. In FIG. 1, arrows on spins σ₁ and σ₂ represent orientations of the external magnetic fields. Spins of the ground state of the Ising model 10 become (σ₁, σ₂, σ₃, σ₄)=(+1, +1, +1, +1).

Including a sign bit, the bit-width of values “2,” “3,” and “−2” is 3-bits, and the bit-width of a value “1” is 2-bits. The shift method reduces the number of bits by bit shift. By shifting the 3-bit values “2,” “3,” and “−2” of the Ising model 10 by 1-bit (right bit shift), 2-bit values “1,” “1,” and “−1” are obtained, respectively.

By applying the shift method to the Ising model 10 expressed in a bit-width of 3-bits [−3, +3] as described above, an Ising model 12 (right of FIG. 1) expressed in a bit-width of 2-bits [−1, +1] is obtained. However, as shown in FIG. 1, the Ising model 12 has four different ground states, one of which matches the ground state of the Ising model 10. Unfortunately, the ground states of both Ising models do not match exactly.

EMBODIMENTS

In the following embodiments, a technique for reducing a bit-width without changing a ground state of an Ising model is proposed.

First, referring to FIG. 2, a hardware configuration of a calculation device 100 for executing a calculation method of the embodiments will be described.

As shown in FIG. 2, the calculation device 100 includes a processor 102, a memory 104, and a storage device 106, an input unit 108, a display 110, and a network interface 112, and these devices are connected via a bus.

The processor 102 includes a central processing unit (CPU) or other such device, and executes various processes in accordance with programs stored on the memory 104 and the storage device 106. The calculation method to be executed by the processor 102 will be detailed below.

As the processor 102, instead of employing a general-purpose computer such as CPU, a dedicated computer such as application specific integrated circuits (ASIC) or field programmable gate array (FPGA) for executing the calculation method of the embodiments may be employed.

The memory 104 includes a read only memory (ROM) and a random access memory (RAM). ROM stores a boot program such as BIOS. When the processor 102 reads out the program stored on ROM or the program stored on the storage device 106, these programs are loaded into RAM.

The storage device 106 includes a computer-readable storage medium, and stores a program for executing the calculation method of the embodiments, data required for the program execution, and other such data. Examples of the computer-readable storage medium include a hard disk drive (HDD), a solid state drive (SSD), and an optical disc.

Note that the programs to be executed by the processor 102 may be stored on an external computer connected via a network so that the processor 102 can read out the programs from the external computer via the network interface 112.

The input unit 108 includes an input device such as a mouse and a keyboard. The display 110 includes a liquid crystal display (LCD) or other such device, and displays results of the process executed by the processor 102.

The network interface 112 is an interface for connecting the calculation device 100 to a network such as a local area network (LAN), a wide area network (WAN), and/or the Internet.

Referring now to FIGS. 3 to 8, the calculation method that is executed by the processor 102 will be described.

Before explaining a method for reducing a bit-width of an interaction coefficient of an Ising model, a method which extends an interaction coefficient by adding an auxiliary spin will be described with reference to FIG. 3.

As shown in FIG. 3, two spins σ_i and σ_j are assumed to be connected by an interaction coefficient J_{i, j} in an Ising model (Model 3-1). First, an auxiliary spin σ_x is added to Model 3-1. Next, an edge (i, x) between σ_i and σ_x and an edge (j, x) between σ_j and σ_x are added. The interaction coefficient J_{i, j} and its absolute value |J_{i, j}| are newly set on the added edges (i, x) and (j, x), respectively. By adding σ_x to Model 3-1 to extend J_{i, j} between σ_i and σ_j as described above, an extended Ising model (Model 3-2) is obtained.

Let an energy of the Ising model (Model 3-1) shown in FIG. 3 be denoted by H₁. The energy H₁ is represented by Equation (2). Terms which are independent of the interaction coefficient J_{i, j} are omitted below.

H ₁ =−J _i,jσ_iσ_j  (2)

Let an energy of the extended Ising model (Model 3-2) shown in FIG. 3 be denoted by H′₁. When J_{i, j}>>0, H′₁ is represented by Equation (3).

$\begin{array}{cc}\begin{array}{c}{H}_1^{\prime }=-J_{i,j}{\sigma }_i{\sigma }_x-J_{i,j}{\sigma }_x{\sigma }_j\\ =-J_{i,j}{\sigma }_x\left({\sigma }_i+{\sigma }_j\right)\end{array}& \left(3\right)\end{array}$

The auxiliary spin σ_x takes the value of +1 or −1. Assuming that σ_x takes the value which minimizes H′₁, Equation (3) is represented by Equation (4).

$\begin{array}{cc}\begin{array}{c}{H}_1^{\prime }=-J_{i,j}❘{\sigma }_i+{\sigma }_j❘\\ =-J_{i,j}\left({\sigma }_i{\sigma }_j+1\right)\\ =-J_{i,j}{\sigma }_i{\sigma }_j-J_{i,j}\end{array}& \left(4\right)\end{array}$

The energy difference between Model 3-1 and Model 3-2 (i.e., the difference between Equation (2) and Equation (4)) becomes |J_{i, j}|.

When J_{i, j}<0, H′₁ is represented by Equation (5).

$\begin{array}{cc}\begin{array}{c}{H}_1^{\prime }=-J_{i,j}{\sigma }_i{\sigma }_x+J_{i,j}{\sigma }_x{\sigma }_j\\ =-J_{i,j}{\sigma }_x\left({\sigma }_i-{\sigma }_j\right)\end{array}& \left(5\right)\end{array}$

Assuming that σ_x takes the value which minimizes H′₁, Equation (5) is represented by Equation (6).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_1=J_{i,j}❘{\sigma }_i-{\sigma }_j❘\\ =J_{i,j}\left(-{\sigma }_i{\sigma }_j+1\right)\\ =-J_{i,j}{\sigma }_i{\sigma }_j+J_{i,j}\end{array}& \left(6\right)\end{array}$

The energy difference between Model 3-1 and Model 3-2 (i.e., the difference between Equation (2) and Equation (6)) becomes |J_{i, j}|.

As described above, assuming that ax takes the value which minimizes H′₁, the energy difference between the original Ising model (Model 3-1) and the extended Ising model (Model 3-2) is always |J_{i, j}|. Hence, the energy difference between the ground state of Model 3-1 and the ground state of Model 3-2 has to be |J_{i, j}. The value of |J_{i, j}| is independent of spin states and hence the spins σ_i and σ_j giving the ground state in Model 3-1 give also the ground state in Model 3-2. In other words, by adding the auxiliary spin and the edges to the original Ising model as shown in FIG. 3, the same ground state can be obtained when the focus is put on the original spins.

Next, the method for reducing a bit-width of an interaction coefficient will be described with reference to FIG. 4. As shown in FIG. 4, two spins σ_i and σ_j are assumed to be connected by an interaction coefficient J_{i, j} in an Ising model (Model 4-1).

First, J_{i, j} is partitioned into a first interaction coefficient J′_{i, j} and a second interaction coefficient J″_{i, j} such that J_{i, j}=J′_{i, j}+J″_{i, j}. As shown in FIG. 4, both J′_{i, j} and J″_{i, j} are provided to connect between σ_i and σ_j. When the method which extends an interaction coefficient shown in FIG. 3 is applied to one of J′_{i, j} and J″_{i, j} (J′_{i, j} in FIG. 4), an extended Ising model (Model 4-2) is obtained.

As described above, if an auxiliary spin which minimizes the energy of the extended Ising model is added to the original Ising model to extend the interaction coefficient, the extended Ising model shares the same ground state with the original Ising model. By repeating the process shown in FIG. 4 until the bit-width decreases to the point where the bit-width can be handled by the processor 102, it is possible to reduce the bit-width of the interaction coefficient of the original Ising model without changing the ground state.

For example, if J_{i, j}=2J′_{i, j}, an energy H₁₀ of Model 4-1 is represented by Equation (7). Terms which are independent of the interaction coefficient J_{i, j} are omitted below.

H ₁₀=−2J′_i,jσ_iσ_j  (7)

J_{i, j}=2J′_{i, j} is partitioned into the same two interaction coefficients J′_{i, j} (J′_{i, j}=J″_{i, j}). Let an energy of Model 4-2 be denoted by H′₁₀. When J′_{i, j}>0, H′₁₀ is represented by Equation (8).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_{10}=-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_x-{J^{\prime }}_{i,j}{\sigma }_x{\sigma }_j\\ =-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}{\sigma }_x\left({\sigma }_i+{\sigma }_j\right)\end{array}& \left(8\right)\end{array}$

Since σ_x takes the value which minimizes H′₁₀, Equation (8) is represented by Equation (9).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_{10}=-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}❘{\sigma }_i+{\sigma }_j❘\\ =-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}\left({\sigma }_i{\sigma }_j+1\right)\\ =-2{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}\end{array}& \left(9\right)\end{array}$

From Equation (7) and Equation (9), H₁₀=H′₁₀ is satisfied except for a constant value |J′_{i, j}| which does not depend on spins. That is, it is possible to reduce the bit-width of the interaction coefficient J_{i, j} (=2J′_{i, j}) by 1-bit without changing the ground state.

When J′_{i, j}<0, the energy H′₁₀ of Model 4-2 is represented by Equation (10).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_{10}=-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_x+{J^{\prime }}_{i,j}{\sigma }_x{\sigma }_j\\ =-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j-{J^{\prime }}_{i,j}{\sigma }_x\left({\sigma }_i-{\sigma }_j\right)\end{array}& \left(10\right)\end{array}$

Since σ_x takes the value which minimizes H′₁₀, Equation (10) is represented by Equation (11).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_{10}=-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j+{J^{\prime }}_{i,j}❘{\sigma }_i-{\sigma }_j❘\\ =-{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j+{J^{\prime }}_{i,j}\left(-{\sigma }_i{\sigma }_j+1\right)\\ =-2{J^{\prime }}_{i,j}{\sigma }_i{\sigma }_j+{J^{\prime }}_{i,j}\end{array}& \left(11\right)\end{array}$

From Equation (7) and Equation (11), H₁₀=H′₁₀ is satisfied except for a constant value |J′_{i, j}| which does not depend on spins. That is, it is possible to reduce the bit-width of the interaction coefficient J_{i, j} (=2J′_{i, j}) by 1-bit without changing the ground state.

Next, referring to FIG. 5, an example of reducing a bit-width of an interaction coefficient will be described. FIG. 5 shows the example of reducing a bit-width of an interaction coefficient J_{i, j} between spins σ_i and σ_j from 5-bits (including a sign bit) to 3-bits.

Let a value of J_{i, j} be “14” (5-bits including the sign bit) in an original Ising model (Model 5-1) (Step 50). First, “14” in Model 5-1 is partitioned into “3” (3-bits) and “11” (5-bits) (Step 52). Then, an auxiliary spin a₁ is added for “3” connecting σ_i and σ_j, an interaction coefficient between a₁ and σ_j is set to be “3,” and an interaction coefficient between a₁ and σ_j is also set to be “3,” thereby obtaining a new Ising model (Model 5-2) (Step 54).

Next, the value of J_{i, j} “11” in Model 5-2 is partitioned into “3” (3-bits) and “8” (5-bits) (Step 56). Then, an auxiliary spin a₂ is added for “3” connecting σ_i and σ_j, an interaction coefficient between σ_i and a₂ is set to be “3,” and an interaction coefficient between a₂ and σ_j is also set to be “3,” thereby obtaining a new Ising model (Model 5-3) (Step 58).

Next, the value of J_{i, j} “8” in Model 5-3 is partitioned into “3” (3-bits) and “5” (4-bits) (Step 60). Then, an auxiliary spin a₃ is added for “3” connecting σ_i and σ_j, an interaction coefficient between σ_i and a₃ is set to be “3,” and an interaction coefficient between a₃ and σ_j is also set to be “3,” thereby obtaining a new Ising model (Model 5-4) (Step 62).

Next, the value of J_{i, j} “5” in Model 5-4 is partitioned into “3” and “2” (both of which are 3-bits) (Step 64). Then, an auxiliary spin a₄ is added for “3” connecting σ_i and σ_j, an interaction coefficient between σ_i and a₄ is set to be “3,” and an interaction coefficient between a₄ and σ_j is also set to be “3.” Consequently, it is possible to obtain an extended Ising model (Model 5-5) in which all the interaction coefficients are expressed by 3-bits or less (Step 66).

Referring now to FIG. 6, a method for reducing a bit-width of an external magnetic field coefficient will be described. As shown in FIG. 6, let an Ising model (Model 6-1) have an external magnetic field coefficient h_i acting on a spin σ_i.

Let h_i be expressed in terms of sum of a first external magnetic field coefficient h′_i and a second external magnetic field coefficient h_x (h_i=h′_i+h_x). First, an auxiliary spin σ_x is added to Model 6-1, h′_i is set as an external magnetic field coefficient on σ_i, and h_x is set as an external magnetic field coefficient on σ_x. Then, an edge (i, x) is added between σ_i and σ_x, and an absolute value of the second external magnetic field coefficient |h_x| is set as an interaction coefficient on the added edge (i, x). Consequently, an extended Ising model (Model 6-2) is obtained.

Next, a condition for matching a ground state of Model 6-2 to a ground state of Model 6-1 will be discussed.

Let an energy of Model 6-1 be denoted by H₂. H₂ is represented by Equation (12). Terms which are independent of the external magnetic field coefficient h_i are omitted below.

H ₂ =−h _iσ_i  (12)

Let an energy of Model 6-2 be denoted by H′₂. When h_x>0 and h_i>h_x, H′₂ is represented by Equation (13).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_2=-h_i^{\prime }{\sigma }_i-h_x{\sigma }_x-h_x{\sigma }_i{\sigma }_x\\ =-h_i^{\prime }{\sigma }_i-h_x{\sigma }_x\left(1+{\sigma }_i\right)\end{array}& \left(13\right)\end{array}$

Since σ_i takes the value of +1 or −1, (1+σ_i) in Equation (13) becomes 2 or 0, i.e., always takes a non-negative value. Hence, H′₂ is minimized when σ_x=+1, and Equation (13) is represented by Equation (14).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_2=-h_i^{\prime }{\sigma }_i-h_x\left(1+{\sigma }_i\right)\\ =-h_i^{\prime }{\sigma }_i-h_x-h_x{\sigma }_i\\ =-\left(h_i^{\prime }+h_x\right){\sigma }_i-h_x\\ =-h_i{\sigma }_i-h_x\end{array}& \left(14\right)\end{array}$

The energy difference between Model 6-1 and Model 6-2 (i.e., the difference between Equation (12) and Equation (14)) becomes |h_x|.

When h_x<0 and h_i<h_x, H′₂ is represented by Equation (15).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_2=-h_i^{\prime }{\sigma }_i-h_x{\sigma }_x+h_x{\sigma }_i{\sigma }_x\\ =-h_i^{\prime }{\sigma }_i-h_x{\sigma }_x\left(1-{\sigma }_i\right)\end{array}& \left(15\right)\end{array}$

Since σ_i takes the value of +1 or −1, (1−σ_i) in Equation (15) becomes 0 or 2, i.e., always takes a non-negative value. Hence, H′₂ is minimized when σ_x=−1, and Equation (15) is represented by Equation (16).

$\begin{array}{cc}\begin{array}{c}{H^{\prime }}_2=-h_i^{\prime }{\sigma }_i+h_x\left(1-{\sigma }_i\right)\\ =-h_i^{\prime }{\sigma }_i+h_x-h_x{\sigma }_i\\ =-\left(h_i^{\prime }+h_x\right){\sigma }_i+h_x\\ =-h_i{\sigma }_i+h_x\end{array}& \left(16\right)\end{array}$

The energy difference between Model 6-1 and Model 6-2 (i.e., the difference between Equation (12) and Equation (16)) becomes |h_x|.

As described above, assuming that ax takes the value which minimizes H′₂, the energy difference between the original Ising model (Model 6-1) and the extended Ising model (Model 6-2) is always |h_x|. Hence, the energy difference between the ground state of Model 6-1 and the ground state of Model 6-2 has to be |h_x|. The value of |h_x| is independent of spin states and hence the spin σ_i giving the ground state in Model 6-1 gives also the ground state in Model 6-2. In other words, by adding the auxiliary spin and the edge to the original Ising model as shown in FIG. 6, the same ground state can be obtained when the focus is put on the original spin.

By repeating the process shown in FIG. 6 until the bit-width decreases to the point where the bit-width can be handled by the processor 102, it is possible to reduce the bit-width of the external magnetic field coefficient of the original Ising model without changing the ground state.

Next, referring to FIG. 7, an example of reducing a bit-width of an external magnetic field coefficient will be described. FIG. 7 shows the example of reducing a bit-width of an external magnetic field coefficient h_i acting on a spin σ_i from 5-bits (including a sign bit) to 3-bits.

Let a value of h_i be “14” (5-bits including the sign bit) in an original Ising model (Model 7-1) (Step 70). Since the value of h_i “14” in Model 7-1 is expressed in terms of sum of “3” (3-bits) and “11” (5-bits), an auxiliary spin a₁ is added first to Model 7-1, an external magnetic field coefficient on σ_i is set to be “11,” and an external magnetic field coefficient on a₁ is set to be “3.” Then, an edge is added between σ_i and a₁, and a new interaction coefficient on the added edge is set to be “3,” thereby obtaining a new Ising model (Model 7-2) (Step 72).

Since the value of h_i “11” in Model 7-2 is expressed in terms of sum of “3” (3-bits) and “8” (5-bits), an auxiliary spin a₂ is then added to Model 7-2, an external magnetic field coefficient on σ_i is set to be “8,” and an external magnetic field coefficient on a₂ is set to be “3.” Then, an edge is added between σ_i and a₂, and a new interaction coefficient on the added edge is set to be “3,” thereby obtaining a new Ising model (Model 7-3) (Step 74).

Since the value of h_i “8” in Model 7-3 is expressed in terms of sum of “3” (3-bits) and “5” (4-bits), an auxiliary spin a₃ is then added to Model 7-3, an external magnetic field coefficient on σ_i is set to be “5,” and an external magnetic field coefficient on a₃ is set to be “3.” Then, an edge is added between σ_i and a₃, and a new interaction coefficient on the added edge is set to be “3,” thereby obtaining a new Ising model (Model 7-4) (Step 76).

Since the value of h_i “5” in Model 7-4 is expressed in terms of sum of “3” and “2” (both of which are 3-bits), an auxiliary spin a₄ is then added to Model 7-4, an external magnetic field coefficient on σ_i is set to be “2,” and an external magnetic field coefficient on a₄ is set to be “3.” Then, an edge is added between σ_i and a₄, and a new interaction coefficient on the added edge is set to be “3.” Consequently, it is possible to obtain an extended Ising model (Model 7-5) in which all the external magnetic field coefficients are expressed by 3-bits or less (Step 78).

By the calculation method of the embodiments, it is possible to formulate the number of auxiliary spins to be added when bit-widths of an original Ising model are reduced by 1-bit.

Let J_{i, j} and h_i of the original Ising model be represented by n-bits including a sign bit and be reduced by 1-bit to become any (n−1)-bit integer (n−1≥2) within the range of [−(2ⁿ⁻²−1), 2ⁿ⁻²−1].

Let the number of auxiliary spins to be added when J_{i, j} is reduced by 1-bit be denoted by s_{i, j}. s_{i, j} is represented by Equation (17).

$\begin{array}{cc}{s}_{i,j}=\lceil \frac{❘J_{i,j}❘}{2^{n-2}-1}\rceil -1& \left(17\right)\end{array}$

Let the number of auxiliary spins to be added when h_i is reduced by 1-bit be denoted by s_i. s_i is represented by Equation (18)

$\begin{array}{cc}{s}_i=\lceil \frac{❘h_i❘}{2^{n-2}-1}\rceil -1& \left(18\right)\end{array}$

From Equation (17) and Equation (18), when the Ising model whose bit-width is represented by n-bits is reduced by 1-bit using the calculation method of the embodiments, the total number s of auxiliary spins to be added is represented by Equation (19).

$\begin{array}{cc}s=\sum _{\left(i,j\right)\in E}{s}_{i,j}+\sum _{i\in V}{s}_i& \left(19\right)\end{array}$

As shown in FIG. 8, by applying the calculation method of the embodiments to the Ising model 10 expressed in a bit-width of 3-bits ([−3, +3]) shown in FIG. 1 to reduce the bit-width by 1-bit, an extended Ising model 80 (right of FIG. 8) is obtained which is expressed in a bit-width of 2-bits [−1, +1].

In the extended Ising model 80 shown in FIG. 8, solid edges mean that interaction coefficients are +1, solid arrows mean that external magnetic field coefficients are +1, and dotted arrows mean that external magnetic field coefficients are −1.

From Equation (17) to Equation (19), the total number s of the auxiliary spins added to the Ising model 10 is 6. Specifically, auxiliary spins a₁ and a₂ are added to reduce h₁ (=3) by 1-bit, auxiliary spins a₃ and a₄ are added to reduce J_{1, 2} (=3) by 1-bit, an auxiliary spin a₅ is added to reduce h₂ (=−2) by 1-bit, and an auxiliary spin a₆ is added to reduce J_{1, 4} (=2) by 1-bit, thereby obtaining the extended Ising model 80.

In the extended Ising model 80, J_{1, 2}=J_{1, 3}=J_{1, 4}=J_{2, 3}=J_{2, 4}=J_{3, 4}=+1, h₁=+1, h₂=−1, and h₃=h₄=0. Consequently, the spins of the ground state of the extended Ising model 80 become (σ₁, σ₂, σ₃, σ₄)=(+1, +1, +1, +1) except for the auxiliary spins, and thus match the spins of the ground state of the original Ising model 10.

Examples of applying the calculation method of the embodiments to random Ising models and two combinatorial optimization problems (number partition problem and vertex cover problem) will now be described below.

<Random Ising Model>

An assumption is made that the following random Ising model is a connected graph obtained by randomly generating vertices and edges, the number of vertices ranges from 5 to 20, and the edge density is set to be around 1.0. The interaction coefficients and the external magnetic field coefficients are assumed to be randomly generated such that their values take a uniform distribution of a given bit-width. The bit-width is assumed to range from 5-bits to 11-bits.

Table 1 shows examples of applying the calculation method of the embodiments to Ising models which are randomly generated as described above.

	TABLE 1
		#Spins
	Bit	#Reduction bits
Name	width	0	1	2	3	4	5	6	7	8	9
#1 (|V| = 5, E| = 10)	5	5	15	34	109
#2 (|V| = 5, E| = 10)	6	5	16	37	88	271
#3 (|V| = 7, E| = 21)	6	7	19	48	118	363
#4 (|V| = 10, |E| = 45)	6	10	43	118	291	909
#5 (|V| = 10, |E| = 44)	7	10	33	87	203	495	1527
#6 (|V| = 13, |E| = 78)	7	13	52	152	359	882	2703
#7 (|V| = 13, |E| = 78)	8	13	52	121	280	638	1523	4630
#8 (|V| = 13, |E| = 77)	9	13	55	145	330	708	1554	3660	11050
#9 (|V| = 15, |E| = 103)	9	15	76	204	455	998	2181	5154	15544
#10 (|V = 15, E| = 105)	10	15	68	189	412	880	1868	4054	9526	28675
#11 (|V = 20, E| = 190)	11	20	117	327	754	1602	3331	6982	15043	35218	105825

In Table 1, #1 to #11 in “Name” column indicate graphs of different Ising models. In “Name” column, |V| indicates the number of vertices, and |E| indicates the number of edges. “Bit-Width” column indicates the bit-widths of the original Ising models. “#Reduction bits” column indicates the number of bits reduced by applying the calculation method of the embodiments to the original Ising models. “#Spins” column indicates the total number of spins of the extended Ising models after reducing the bit-widths of the original Ising models. When the number of the reduction bits is “0,” the total number of spins is identical to that of the original Ising models. A blank in “#Spins” column indicates that the bit-widths cannot be reduced anymore. Tables described below share the same structure with Table 1.

For example, in Table 1, the Ising model of #1 has five vertices, ten edges, and a bit-width of 5-bits. By applying the calculation method of the embodiments to the Ising model of #1, the total number of spins becomes 15 when the bit-width is reduced by 1-bit, the total number of spins becomes 34 when the bit-width is reduced by 2-bits, and the total number of spins becomes 109 when the bit-width is reduced by 3-bits. Table 1 indicates that the more reduced the bit-widths of the Ising models of #1 to #11, the larger the total number of spins.

<Number Partition Problem>

The number partition problem (hereinafter referred to as NPP) is defined as follows: Given M={m₁, m₂, . . . , m_N} as a set of N natural numbers, partition M into two subsets M_i and M₂, and judge whether the sum of natural numbers belonging to M₁ and the sum of natural numbers belonging to M₂ are the same or not. NPP is known to be NP-complete.

For example, given a set M={1, 2, 4, 7}, M is partitioned into subsets M₁={1, 2, 4} and M₂={7}. The sum of natural numbers in M₁ is equal to that in M₂.

NPP can be expressed in terms of Ising models. N spins σ_i (N=|M|, 1≤i≤N) are prepared, and the energy function (or Hamiltonian) is denoted by H_NPP. NPP can be formulated into an Ising model as given by Equation (20) and Equation (21).

$\begin{array}{cc}{H}_{\mathrm{NPP}}={\left(\sum _{i=1}^N{m}_i{\sigma }_i\right)}^2& \left(20\right)\end{array}$ $\begin{array}{cc}{\sigma }_i=\{\begin{array}{cc}1& m_i\mathrm{is}\mathrm{assigned}\mathrm{to}a\mathrm{subset}{M}_1.\\ -1& m_i\mathrm{is}\mathrm{assigned}\mathrm{to}a\mathrm{subset}{M}_2.\end{array}& \left(21\right)\end{array}$

As shown in Equation (21), σ_i (1≤i≤N) becomes +1 when m_i is assigned to the subset M₁, and becomes −1 when m_i is assigned to the subset M₂. The minimum value of H_NPP is zero in Equation (20). When H_NPP is zero, the sum of natural numbers belonging to M₁ and the sum of natural numbers belonging to M₂ are the same.

FIG. 9 shows an example of NPP expressed in terms of the Ising model when the set M={1, 2, 4, 7} is given as described above. From Equation (20), an interaction coefficient J_{i, j} of the Ising model 90 shown in FIG. 9 is 2m_im_j. A spin σ_i (1≤i≤N) corresponds to m_i (1≤i≤N). As described above, the set M can be partitioned into M₁={1, 2, 4} and M₂={7}, and H_NPP takes the minimum value when (σ₁, σ₂, σ₃, σ₄)=(+1, +1, +1, −1) or (σ₁, σ₂, σ₃, σ₄)=(−1, −1, −1, +1) as shown in FIG. 9.

Table 2 shows examples of applying the calculation method of the embodiments to Ising models for NPPs.

	TABLE 2
		#Spins
	Bit	#Reduction bits
Name	width	0	1	2	3	4	5	6	7	8	9	10	11	12
#12 ({i | i ∈ [1, 3]})	5	3	4	8	22
#13 ({f(1, j) | j ∈ [0, 4]})	6	5	7	14	33	99
#14 ({1, 2, 4, 7})	7	4	5	9	18	43	124
#15 ({f(2, j) | j ∈ [0, 4]})	8	5	7	13	28	60	137	411
#16 ({i | i ∈ [1, 8]})	8	8	14	30	66	146	352	1072
#17 ({i | i ∈ [1, 12]})	10	12	13	28	68	154	337	749	1772	5380
#18 ({f(3, j) | j ∈ [0, 5]})	11	6	10	11	21	40	83	174	378	879	2655
#19 ({i | i ∈ [1, 20]})	11	20	36	102	257	583	1256	2656	5795	13620	41060
#20 ({f(1, j) | j ∈ [0, 10]})	15	11	12	14	21	38	76	154	313	639	1307	2716	5836	13644
#21 ({f(10, j) | j ∈ [0, 10]})	21	11	13	18	31	59	120	243	488	990	1988	3991	8008	16063
#22 ({f(1, j) | j ∈ [0, 20]})	29	21	24	31	47	77	151	291	587	1180	2371	4754	9541	16063
#23 ({f(1, j) | j ∈ [0, 23]})	33	24	25	27	35	52	90	171	335	667	1323	2668	5335	10699

In Table 2, #12 to #23 in “Name” column indicate different sets M. A set M of N natural numbers (N mod 3=0 or 2) is created based on f (i, j) defined by Equation (22) which is the Fibonacci sequence. When sets M are created based on Equation (22), NPPs always have solutions.

$\begin{array}{cc}f\left(i,j\right)=\{\begin{array}{cc}f\left(i,j-2\right)+f\left(i,j-1\right)& 2\le j\\ i& j=0\mathrm{or}1\\ 0& \mathrm{others}\end{array},& \left(22\right)\end{array}$

#13, #15, #18, and #20 to #23 in Table 2 indicate the sets created based on f(i, j) of Equation (22). The set of #14 corresponds to the Ising model 90 with the bit-width of 7-bits shown in FIG. 9. For example, by applying the calculation method of the embodiments to the Ising model 90, the total number of spins becomes 5 when the bit-width is reduced by 1-bit, and the total number of spins becomes 124 when the bit-width is reduced by 5-bits. Thus, the bit-width reduction for NPPs by the calculation method of the embodiments is also found to increase in the total number of spins.

<Vertex Cover Problem>

A vertex cover of an undirected graph G=(V, E) is defined such that at least one of vertices that constitute each edge in E is included in a subset of vertices V⊆V. The vertex cover problem (hereinafter referred to as VCP) is to find the smallest size of V′ and is known to be NP-hard.

FIG. 10 shows an example of VCP. A graph 1000 shown in FIG. 10 has eight vertices (1 to 8) and sixteen edges. The smallest size of V′ of the graph 1000 is a set of five vertices: a subset V′₁ of vertices 1, 3, 4, 6, and 7, and a subset V′₂ of vertices 2, 4, 5, 6, and 8. In other words, at least one of vertices that constitute each edge of the graph 1000 is included in V′₁ or V′₂.

VCP can also be expressed in terms of Ising models. N spins σ_i (N=|V|, 1≤i≤N) are prepared, and the energy function (or Hamiltonian) is denoted by H_VCP. VCP can be formulated into an Ising model as given by Equation (23) and Equation (24).

$\begin{array}{cc}{H}_{\mathrm{VCP}}={\alpha }_{\mathrm{VCP}}\sum _{\left(i,j\right)\in E}\left(1-{\sigma }_i\right)\left(1-{\sigma }_j\right)+{\beta }_{\mathrm{VCP}}\sum _{i\in V}{\sigma }_i& \left(23\right)\end{array}$ $\begin{array}{cc}{\sigma }_i=\{\begin{array}{cc}1& \mathrm{Vertex}i\in V\mathrm{is}\mathrm{in}{V}^{\prime }.\\ -1& \mathrm{Vertex}i\in V\mathrm{is}\mathrm{not}\mathrm{in}{V}^{\prime }.\end{array}& \left(24\right)\end{array}$

In Equation (23), α_VCP and β_VCP are positive weight parameters. As shown in Equation (24), σ_i (1≤i≤N) becomes +1 when a vertex i belongs to V′, and becomes −1 when a vertex i does not belong to V′.

The first and second terms in Equation (23) indicate a penalty function and an objective function, respectively. When at least one of vertices that constitute each edge in E belongs to V′, the first term in Equation (23) becomes zero. If the size of V′ is the smallest, H_VCP is minimized when the second term of Equation (23) is minimized, and thus optimal solutions can be obtained. Note that β_VCP≤α_VCP is set to obtain solutions satisfying the constraints called feasible solutions.

Table 3 shows examples of applying the calculation method of the embodiments to Ising models for VCPs.

	TABLE 3
		#Spins
	Bit	#Reduction bits
	Name	width	0	1
	se 1 (V| = 8, |E = 20)	3	8	14
	cage 1 (|V| = 10, |E = 30)	3	10	20
	cage 2 (|V| = 14, |E = 42)	3	14	28
	se 2 (V| = 16, |E| = 42)	3	16	28
	cage 3 (|V| = 24, |E = 72)	3	24	48
	ccc 1 (|V| = 24, |E = 72)	3	24	48
	bfly 1 (V| = 24, E| = 96)	3	24	72

“Name” column in Table 3 indicates benchmark graphs which are picked up from AG-Monien Graph Collection on a website below.

“AG-Monien—SuiteSparse matrix collection,”https://sparse.tamu.edu/AG-Monien.Both α_VCP and β_VCP are set to be one in all the graphs in Table 3.

Reduction in the bid-width of each graph shown in Table 3 by 1-bit using the calculation method of the embodiments is also found to increase the total number of spins.

Annealing machines or Ising machines have predetermined number of spins, and can work successfully if the number of spins is not greater than the predetermined number. In fact, as for Table 1 to Table 3, the processor 102 is able to accurately obtain the ground states when the total number of spins does not exceed the predetermined number for the processor 102.

Next, for comparison with the calculation method of the embodiments, Table 4, Table 5, and Table 6 show results of applying the naïve method (shift method) to random Ising models, NPPs, and VCPs, respectively.

	TABLE 4
		#Ground states by naive method/#Original ground states matched
	Bit	#Reduction bits
Name	width	0	1	2	3	4	5	6	7	8	9
#1	5	1/1	2/1	1/0	1/0
#2	6	1/1	1/1	1/1	1/1	1/0
#3	6	1/1	1/1	1/1	2/1	1/0
#4	6	1/1	1/1	1/1	1/1	1/1
#5	7	1/1	1/1	1/1	1/1	1/1	1/0
#6	7	1/1	1/1	1/1	1/1	1/0	2/0
#7	8	1/1	1/1	1/1	1/1	1/1	1 /1	8/0
#8	9	1/1	1/1	1/1	1/1	1/1	1/1	1/0	1/0
#9	9	1/1	1/1	1/1	1/1	1/1	1/1	1/0	1/0
#10	10	1/1	1/1	1/1	1/1	1/1	1/1	1/1	2/1	1/0
#11	11	1/1	1/1	1/1	2/1	1/0	2/1	1/1	1/1	1/1	5/1

TABLE 5

#Ground states by naive method/#Original ground states matched

Bit

#Reduction bits

Name

width

0 and 1

2

3

4

5

6

7

8

9

10

11

12

#12

5

2/2

4/2

6/2

#13

6

4/4

4/4

6/4

20/4

#14

7

2/2

4/2

2/0

4/0

6/0

#15

8

4/4

4/4

4/4

4/4

6/4

20/4

#16

8

14/14

2/2

8/4

8/2

14/4

70/8

#17

10

124/124

4/0

24/8

26/4

2/0

10/0

504/40

924/58

#18

11

4/4

2/2

2/2

2/2

6/2

6/2

2/2

12/2

20/2

#19

11

15272/15272

6/0

6/6

8/0

2/2

4/0

2/0

1584/92

184756/5448

#20

15

16/16

16/16

4/0

20/8

6/0

8/4

4/0

2/0

12/0

42/0

252/12

504/12

#21

21

16/16

16/16

16/16

16/16

8/8

8/8

6/4

14/8

6/4

2/0

2/0

6/0

#22

29

128/128

2/2

6/0

4/2

2/0

14/2

4/0

8/0

2/0

4/0

10/0

2/0

#23

33

256/256

2/2

2/0

2/0

2/0

12/0

2/0

6/0

2/0

6/0

2/0

8/0

	TABLE 6
		#Ground states by naive method/
		#Original ground states matched
	Bit	#Reduction bits
	Name	width	0	1
	se 1	3	2/2	2/2
	cage 1	3	5/5	5/5
	cage 2	3	2/2	2/2
	se 2	3	30/30	16/8
	cage 3	3	8/8	18/8
	ccc 1	3	828/828	11682/828
	bfly 1	3	177/177	6/0

Contents of “Name” column in Table 4 to Table 6 are the same as those in Table 1 to Table 3, respectively. In Table 4 to Table 6, “a”/“b” in “#Ground states by naive method/#Original ground states matched” columns means that “a” is the number of ground states of the Ising model whose bit-width is reduced by the shift method and “b” is the number of the ground states that match the original ground states of the original Ising model, among the ground states obtained after the shift method is applied.

In Table 4 (random Ising model), the ground states of each Ising model whose bit-width is reduced to 3-bits by the shift method are found to be almost identical to the ground state of the original Ising model (see the underlined data in Table 4). On the other hand, the ground states of each Ising model whose bit-width is reduced to 2-bits by the shift method do not match very much the ground state of the original Ising model (see the double-underlined data in Table 4).

In Table 5 (NPP), the number of the ground states of each Ising model whose bit-width is reduced by the shift method is found to be hardly the same as that of the original Ising model. For example, when the bit-width of the Ising model for #14 (Ising model 90 shown in FIG. 9) is reduced by 3-bits or more, the optimal solution of NPP cannot be obtained. In the problems shown in #12 to #19 of Table 5, the number of the ground states of each Ising model whose bit-width is reduced to 2-bits by the shift method is found to be much more than the number of the ground states that match the original ground state of the original Ising model (see the underlined data in Table 5). In this case, the optimal solution of NPP can be hardly obtained.

In Table 6 (VCP), with respect to the graphs with the number of vertices being less than fifteen (se 1, cage 1, cage 2 in Table 6), the ground states of the Ising model whose bit-width is reduced by 1-bit by the shift method are equal to those of the original Ising model. However, when the size of the graph increases, the number of the ground states of the Ising model whose bit-width is reduced by 1-bit by the shift method does not match the number of the ground states of the original Ising model. Especially in “bfly 1” (|V|=24, |E|=96) of Table 6, after the bit-width of the original Ising model is reduced only by 1-bit, the optimal solution in VCP cannot be obtained at all.

According to the embodiments, by adding an auxiliary spin to an Ising model and setting a new interaction coefficient between the auxiliary spin and a spin of the Ising model to obtain a new Ising model, the size of coefficients (an interaction coefficient and an external magnetic field coefficient) of the Ising model is dispersed. Accordingly, it is possible to reduce the bit-width of the Ising model without changing the ground state. Thus, in the embodiments, even when the Ising model with the bit-width larger than hardware devices can handle is input, it is possible to accurately obtain the ground state of the Ising model.

The invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention. Other embodiments and variations made by those skilled in the art are intended to be embraced within the scope of the invention.

REFERENCE SIGNS LIST

• • 100: Calculation Device
  • 102: Processor
  • 104: Memory
  • 106: Storage Device
  • 108: Input Unit
  • 110: Display
  • 112: Network Interface

Claims

1. A calculation method for reducing a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins, the calculation method comprising: adding an auxiliary spin to the Ising model; andsetting a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain a new Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.

2. The calculation method according to claim 1, wherein the auxiliary spin takes a value which minimizes an energy of the new Ising model.

3. The calculation method according to claim 1, wherein if an interaction coefficient between two spins of the plurality of spins is expressed in terms of sum of a first interaction coefficient and a second interaction coefficient,reducing the bit-width of the interaction coefficient between the two spins comprises:partitioning the interaction coefficient between the two spins into the first interaction coefficient between the two spins and the second interaction coefficient between the two spins; andsetting the first interaction coefficient between the auxiliary spin and a first spin of the two spins, and setting an absolute value of the first interaction coefficient between the auxiliary spin and a second spin of the two spins, thereby extending the first interaction coefficient between the two spins.

4. The calculation method according to claim 1, wherein if an external magnetic field coefficient acting on one spin of the plurality of spins is expressed in terms of sum of a first external magnetic field coefficient and a second external magnetic field coefficient,reducing the bit-width of the external magnetic field coefficient acting on the one spin comprises:setting the first external magnetic field coefficient on the one spin;setting the second external magnetic field coefficient on the auxiliary spin; andsetting an absolute value of the second external magnetic field coefficient as the new interaction coefficient between the auxiliary spin and the one spin.

5. A calculation device configured to reduce a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins, the calculation device comprising a processor configured to: add an auxiliary spin to the Ising model; andset a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain a new Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.

6. A non-transitory computer-readable storage medium storing a program for executing a calculation method which reduces a bit-width of an Ising model, the Ising model being expressed in terms of a plurality of spins, interaction coefficients between the plurality of spins, and external magnetic field coefficients acting respectively on the plurality of spins, the program causing a computer to execute: adding an auxiliary spin to the Ising model; andsetting a new interaction coefficient between the auxiliary spin and at least one of the plurality of spins to obtain a new Ising model, thereby reducing a bit-width of at least one of the interaction coefficients between the plurality of spins and the external magnetic field coefficients acting respectively on the plurality of spins.