Quantum circuit, quantum computer, and method of manufacturing quantum circuit

1 . A quantum circuit comprising:

a plurality of first optical waveguides formed on a substrate and each of which includes a single-photon source;

a plurality of second optical waveguides formed on a substrate and each of which includes a single-photon source;

a first multiplexer formed on the substrate and configured to condense first photons propagated through the plurality of first optical waveguides;

a second multiplexer formed on the substrate and configured to condense second photons propagated through the plurality of second optical waveguides;

a branching element configured to introduce the first photons condensed by the first multiplexer and the second photons condensed by the second multiplexer and branch the first photons and the second photons in a first direction and a second direction;

a first detector configured to detect the first photons and the second photons branched in the first direction; and

a second detector configured to detect the first photons and the second photons branched in the second direction.

2 . The quantum circuit according to claim 1 , wherein the single-photon source includes a complex defect formed by an impurity atom in a diamond single crystal and a vacancy adjacent to the impurity atom.

3 . The quantum circuit according to claim 2 , wherein the impurity atom is at least one selected from nitrogen (N), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb).

4 . The quantum circuit according to claim 2 , wherein

the complex defect functions as one qubit,

the complex defect has an electronic spin state and a nuclear spin state that are coupled to each other, and

the complex defect includes, among a plurality of quantum levels in the electronic spin state and the nuclear spin state, two levels of two series in which an energy difference between the levels is smaller than an energy line width of a laser to be emitted to the single-photon source, and the levels are close to an extent that the levels are able to be resonantly excited by the laser.

5 . The quantum circuit according to claim 1 , further comprising:

a microwave pulse signal generator configured to individually apply a microwave pulse signal to each of the single-photon sources included in the plurality of first optical waveguides and the plurality of second optical waveguides; and

a magnetic field generator configured to apply a static magnetic field to the single-photon sources included in the plurality of first optical waveguides and the plurality of second optical waveguides.

6 . The quantum circuit according to claim 1 , wherein

the first multiplexer includes:

a plurality of third optical waveguides coupled to the plurality of first optical waveguides; and

a first planar waveguide that guides the first photons and the second photons propagated through the plurality of third optical waveguides to an output terminal of the first multiplexer, and

the second multiplexer includes:

a plurality of fourth optical waveguides coupled to the plurality of second optical waveguides; and

a second planar waveguide that guides the first photons and the second photons propagated through the plurality of fourth optical waveguides to an output terminal of the second multiplexer.

7 . The quantum circuit according to claim 6 , wherein

the plurality of third optical waveguides has a lower refractive index than the plurality of first optical waveguides,

the plurality of fourth optical waveguides has a lower refractive index than the plurality of second optical waveguides,

an optical waveguide arranged closer to a center side has a higher refractive index among the plurality of third optical waveguides, and

an optical waveguide arranged closer to a center side has a higher refractive index among the plurality of fourth optical waveguides.

8 . The quantum circuit according to claim 7 , wherein

the plurality of first optical waveguides and the plurality of second optical waveguides are formed with aluminum nitride,

the plurality of third optical waveguides, the plurality of fourth optical waveguides, the first planar waveguide, and the second planar waveguide are formed with silicon oxynitride,

the optical waveguide arranged closer to the center side has a higher concentration of nitrogen and the optical waveguide arranged closer to an outer side has a higher concentration of oxygen among the plurality of third optical waveguides, and

the optical waveguide arranged closer to the center side has a higher concentration of nitrogen and the optical waveguide arranged closer to an outer side has a higher concentration of oxygen among the plurality of fourth optical waveguides.

9 . The quantum circuit according to claim 1 , wherein

each of the plurality of first optical waveguides shares the single-photon source with one of the plurality of second optical waveguides, and

each of the plurality of first optical waveguides and the plurality of second optical waveguides has an optical switch that switches whether to propagate the first photons and the second photons to the first multiplexer or the second multiplexer.

10 . The quantum circuit according to claim 1 , wherein the first detector and the second detector are single-photon detectors.

11 . The quantum circuit according to claim 1 , further comprising:

an input unit configured to:

select one of the single-photon sources respectively included in the plurality of first optical waveguides,

select one of the single-photon sources respectively included in the plurality of second optical waveguides, and

emit an optical pulse to the selected single-photon sources.

12 . The quantum circuit according to claim 1 , wherein, in the branching element,

the first direction and the second direction are orthogonal to each other, and

a probability of branching in the first direction and the second direction is 50%.

13 . A quantum computer comprising:

a quantum circuit that includes:

a plurality of first optical waveguides formed on a substrate and each of which includes a single-photon source;

a plurality of second optical waveguides formed on a substrate and each of which includes a single-photon source;

a first multiplexer formed on the substrate and configured to condense first photons propagated through the plurality of first optical waveguides;

a second multiplexer formed on the substrate and configured to condense second photons propagated through the plurality of second optical waveguides;

a branching element configured to introduce the first photons condensed by the first multiplexer and the second photons condensed by the second multiplexer and branch the first photons and the second photons in a first direction and a second direction;

a first detector configured to detect the first photons and the second photons branched in the first direction; and

a second detector configured to detect the first photons and the second photons branched in the second direction; and

an input unit configured to select one of the single-photon sources respectively included in the plurality of first optical waveguides and one of the single-photon sources respectively included in the plurality of second optical waveguides, and emit an optical pulse to the selected single-photon sources; and

a control device configured to control emission of the optical pulse.