Superconducting quantum circuit

1 . A superconducting quantum circuit comprising:

first to fourth qubits; and

a coupler that couples the first to fourth qubits by a four-body interaction,

wherein the coupler includes:

a loop circuit connected between one end and an other end of the coupler; and

a capacitor connected in parallel to the loop circuit,

wherein the loop circuit includes:

n (n is a positive integer greater than or equal to 2) first Josephson junctions spaced apart from each other and arranged in series; and

a second Josephson junction arranged in parallel with the n first Josephson junctions, the second Josephson junction having a junction size smaller than that of the first Josephson junction,

wherein the first and second qubits are capacitively coupled to the one end of the coupler, respectively, and

the third and fourth qubits are capacitively coupled to the other end of the coupler, respectively, and

wherein a magnitude of a coupling coefficient of the four-body interaction by the coupler is made configurable based on circuit parameters including at least the n and α (0<α<1) that is a ratio of a Josephson energy of the second Josephson junction and that of the first Josephson junction.

2 . The superconducting quantum circuit according to claim 1 , wherein in the coupler, the α is set to a predetermined value close to 1/n within a range smaller than 1/n an inverse of the n.

3 . The superconducting quantum circuit according to claim 1 , comprising

a magnetic field generation part,

wherein the magnetic field generation part, when a DC current from a current control part is passed therethrough, generates a magnetic flux Φ 0 /2 or −Φ 0 /2 (where Φ 0 (=h/(2e)) is a magnetic flux quantum: h is a Planck's constant and e is an elementary charge), the magnetic flux penetrating the loop circuit of the coupler.

4 . The superconducting quantum circuit according to claim 1 , wherein, out of the n first Josephson junctions and the second Josephson junction of the loop circuit,

odd number (2k+1, where k is a predetermined non-negative integer) junctions are set to π-junctions and rest are set to zero-junctions, and

a phase difference when going around the loop circuit is ±π×(2k+1),

the loop circuit being in a state biased with a magnetic flux ±Φ 0 /2×(2k+1) (where Φ 0 (=h/(2e)) is a magnetic flux quantum: h is a Planck's constant and e is an elementary charge) without applying any magnetic flux that penetrates the loop circuit.

5 . The superconducting quantum circuit according to claim 1 , wherein the loop circuit of the coupler includes

a first electrode and a second electrode arranged opposite to each other, and spaced apart from a ground pattern in a region surrounded by the ground pattern in a wiring layer on a substrate,

wherein the n (n being two or more positive integers) first Josephson junctions and the second Josephson junction are arranged in parallel between the first electrode and the second electrode, respectively,

wherein the first electrode includes

first and second opposing portions extended towards the first and second qubits from locations of the first electrode different from a side where the first electrode is opposed to the second electrode, respectively, the first and second opposing portions having ends capacitively coupled to ends of coupler connection portions of the first and second qubits, respectively, and

wherein the second electrode includes

third and fourth opposing portions extended towards the third and fourth qubits from locations of the second electrode different from a side where the second electrode is opposed to the first electrode, respectively, the third and fourth opposing portions having ends capacitively coupled to ends of coupler connection portions of the third and fourth qubits, respectively.

6 . The superconducting quantum circuit according to claim 5 , where each of the first to fourth qubits includes

a Josephson parametric oscillator,

wherein the Josephson parametric oscillator includes:

a superconducting quantum interference device (SQUID) including

two Josephson junctions at both ends where a first superconducting line and a second superconducting line that constitute a loop intersect;

a capacitor connected in parallel to the SQUID; and

a line configured to inductively couple to the SQUID to generate a magnetic flux that penetrates the loop of the SQUID,

wherein one of the first and second superconducting lines of the SQUID is set to a ground potential, and

an other of the first and second superconducting lines of the SQUID is connected to the coupler connection portion, and

wherein the Josephson parametric oscillator is configured to perform parametric oscillation according to a microwave current supplied to the line.

7 . The superconducting quantum circuit according to claim 6 , wherein circuit parameter defining the coupling coefficient of the four-body interaction includes, in addition to the α and the n,

C which is a value of capacitance of a capacitive coupling between the coupler connection portion of each of the first to fourth qubits and each of the first to fourth opposing portions of the coupler;

C J which is a value of capacitance of the capacitor connected in parallel to the SQUID of each of the first and fourth qubits; and,

C g which is a value of capacitance between the first and second electrodes of the coupler, with a magnitude relationship thereamong set to C J >C g >C.

8 . The superconducting quantum circuit according to claim 6 , wherein with respect to circuit parameters including:

the α and the n;

C which is a value of the capacitance of the capacitive coupling between the coupler connection portion of each of the first and fourth qubits and each of the first and fourth opposing portions of the coupler;

C J which is a value of the capacitance of the capacitors connected in parallel to the SQUID of each of the first and fourth qubits;

C g which is a value of the capacitance between the first and second electrodes of the coupler; and

a difference between a resonant angular frequency ω− of the coupler and a resonant angular frequency ω of each of the first to fourth qubits,

the coupling coefficient of the four-body interaction is given by

(

ω

⁢

ω

-

)

2

(

ω

-

ω

-

)

4

.

C

4

C

J

2

(

C

g

+

C

)

3

·

{

1

-

n

3

⁢

α

n

2

(

1

-

n

⁢

α

)

}

.

9 . The superconducting quantum circuit according to claim 6 , wherein a capacitance C of a coupling capacitor between each of the first to fourth qubits and the coupler is set smaller as compared with a capacitance C J of each of the capacitors of the first to fourth qubit to weaken a two-body interaction in which two of the first to fourth qubits interact with each other.

10 . The superconducting quantum circuit according to claim 1 , wherein the α (0<α<1) is a ratio of the junction size of the second Josephson junction to that of the first Josephson junction.

11 . A superconducting quantum circuit apparatus including

a superconducting quantum circuit,

wherein the superconducting quantum circuit comprises:

first to fourth qubits; and

a coupler that couples the first to fourth qubits by a four-body interaction,

wherein the coupler includes:

a loop circuit connected between one end and an other end of the coupler; and

a capacitor connected in parallel to the loop circuit,

wherein the loop circuit includes:

n (n is a positive integer greater than or equal to 2) first Josephson junctions spaced apart from each other and arranged in series; and

a second Josephson junction arranged in parallel with the n first Josephson junctions, the second Josephson junction having a junction size smaller than that of the first Josephson junction,

wherein the first and second qubits are capacitively coupled to the one end of the coupler, respectively, and

the third and fourth qubits are capacitively coupled to the other end of the coupler, respectively, and

wherein a magnitude of a coupling coefficient of the four-body interaction by the coupler is made configurable based on circuit parameters including at least the n and α (0<α<1) that is a ratio of the Josephson energy of the second Josephson junction and that of the first Josephson junction,

wherein the superconducting quantum circuit constitutes a quantum computer including, as a unit structure, the first to fourth qubits that are each configured to perform Josephson parametric oscillation and the coupler.

12 . The superconducting quantum circuit apparatus as according to claim 11 , including a plurality of the unit structures, wherein the unit structure constitutes a quantum computer in which at least one of the first through fourth qubits are shared by one or more other the unit structures.