Time-bin qubit converter

1. A system comprising:

a first photonic integrated circuit comprising:

a qubit encoder configured to receive a spatial-mode qubit and convert the spatial-mode qubit to a temporal-mode qubit, the qubit encoder including an optical switch controlled by a first clock signal having a frequency that is at least twice of a clock frequency of a qubit source for generating the spatial-mode qubit;

an optical interconnect configured to receive and transmit the temporal-mode qubit; and

a second photonic integrated circuit comprising:

a qubit decoder configured to receive the temporal-mode qubit and convert the temporal-mode qubit back into the spatial-mode qubit.

2. The system of claim 1 , further comprising:

a first control circuit configured to control the qubit encoder; and

a second control circuit configured to control the qubit decoder,

wherein the second control circuit is synchronized with the first control circuit.

3. The system of claim 1 , wherein the qubit encoder includes:

a dual-rail qubit bus optically coupled to the qubit source, the dual-rail qubit bus including two waveguides, wherein a first waveguide of the dual-rail qubit bus includes a delay line and the optical switch is optically connected to the two waveguides of the dual-rail qubit bus.

4. The system of claim 1 , wherein the qubit source comprises a clock generator configured to generate a second clock signal using pump laser pulses.

5. The system of claim 4 , further comprising an electrical interconnect channel or an optical interconnect channel configured to send the second clock signal to the second photonic integrated circuit.

6. The system of claim 5 , wherein the qubit encoder comprises a first dual-rail qubit bus that comprises a first set of waveguides, one of the first set of waveguides comprising a first delay, and wherein the qubit decoder comprises a second dual-rail qubit bus that comprises a second set of waveguides, one of the second set of waveguides comprising a second delay.

7. The system of claim 6 , further comprising a phase shifter to compensate for phase differences between the first delay and the second delay.

8. The system of claim 7 , wherein the phase shifter is in the optical switch of the qubit encoder in the first photonic integrated circuit.

9. The system of claim 1 , wherein the spatial-mode qubit corresponds to a dual-rail encoded photonic qubit.

10. The system of claim 1 , wherein the optical interconnect comprises optical fiber.

11. The system of claim 1 , wherein the optical interconnect is a waveguide of an optical interposer that is coupled to the first photonic integrated circuit and the second photonic integrated circuit.

12. A photonic integrated circuit device comprising:

a dual-rail qubit bus optically coupled to a photonic qubit source, the dual-rail qubit bus including two waveguides, wherein a first waveguide of the dual-rail qubit bus includes a waveguide delay line that is disposed in a first layer of the photonic integrated circuit device;

an optical switch, disposed in a second layer of the photonic integrated circuit device and optically connected to the two waveguides of the dual-rail qubit bus; and

a control circuit configured to switch the optical switch to optically connect output portions of the two waveguides to a single output waveguide of the photonic integrated circuit device at different times, wherein the control circuit is synchronized with the photonic qubit source,

wherein the optical switch is controlled by a clock signal characterized by a frequency that is at least twice of a clock frequency of the photonic qubit source.

13. The photonic integrated circuit device of claim 12 , wherein the photonic qubit source includes a clock generator configured to generate a clock signal using pump laser pulses.

14. The photonic integrated circuit device of claim 13 , wherein the waveguide delay line is characterized by a delay that is equal to or less than a half of a clock period of the clock signal.

15. The photonic integrated circuit device of claim 12 , wherein the waveguide delay line is characterized by a delay that is between 25 ps and 500 ps.

16. The photonic integrated circuit device of claim 12 , wherein the control circuit is controlled by a feedforward control signal generated by the photonic qubit source when photon pairs are generated.

17. The photonic integrated circuit device of claim 12 , wherein the optical switch includes a Mach-Zehnder interferometer.

18. A method comprising:

converting a spatial-mode qubit to a temporal-mode qubit using an optical delay and a switch of a first photonic integrated circuit, the switch controlled by a clock signal characterized by a frequency that is at least twice of a clock frequency of a qubit source for generating the spatial-mode qubit;

transmitting, on an optical interconnect, the temporal-mode qubit from the first photonic integrated circuit to a second photonic integrated circuit, the optical interconnect being coupled to the first photonic integrated circuit and the second photonic integrated circuit; and

converting, on the second photonic integrated circuit, the temporal-mode qubit back into the spatial-mode qubit.

19. The method of claim 18 , wherein the optical interconnect comprises optical fiber.