Raster multiplexing in photonic circuits

1. A circuit comprising:

a number (N) of input paths, wherein N is greater than 1;

a plurality of output paths including a raster group of output paths, wherein the raster group of output paths has a number (R) of output paths, wherein R is at least 2;

an optical switching network coupled between the input paths and the output paths, the optical switching network comprising a plurality of active optical switches arranged to selectably couple a photon from any one of the input paths to any one of the output paths; and

control logic coupled to the optical switching network and configured to:

receive, for a time bin of a plurality of time bins, an input signal indicative of whether a photon is present on each input path;

select, based on the input signal, one of the input paths as an active input path for the time bin;

select one of the output paths as an active output path for the time bin, wherein output paths in the raster group are selected according to a fixed order such that each output path in the raster group of output paths is selected as the active output path once during a raster period consisting of R consecutive time bins; and

generate control signals to set a state of the active optical switches for the time bin such that a photon from the active input path is coupled to the active output path.

2. The circuit of claim 1 further comprising:

a plurality of delay lines, each delay line introducing a different amount of delay,

wherein each delay line is coupled to a different one of the R output paths in the raster group of output paths.

3. The circuit of claim 2 wherein the control logic is configured to select the output paths in an order such that photons entering the optical switching network during a set of R consecutive time bins arrive at respective outputs of the delay lines in the same time bin.

4. The circuit of claim 1 wherein the optical switching network is a generalized Mach-Zehnder interferometer (GMZI) and the active optical switches include active phase shifters.

5. The circuit of claim 1 wherein each input path and each output path comprises a waveguide.

6. The circuit of claim 1 wherein each input path and each output path comprises a pair of waveguides.

7. The circuit of claim 1 wherein each input path is coupled to an output of a different one of a set of N heralded single photon sources and wherein the input signal includes heralding signals from the heralded single photon sources.

8. The circuit of claim 1 wherein the plurality of output paths further includes at least one additional output path separate from the raster group of output paths.

9. A circuit comprising:

a number (N) of source circuits, each source circuit having an output path to propagate a photon, wherein N is greater than 1;

a downstream circuit having a number (R) of input paths to receive photons, wherein R is at least 2; and

a raster multiplexer circuit having:

a number N of multiplexer input paths, each multiplexer input path coupled to an output path of one of the source circuits;

a plurality of multiplexer output paths including a raster group of multiplexer output paths, wherein the raster group of multiplexer output paths includes R multiplexer output paths, each multiplexer output path in the raster group of multiplexer output paths being coupled to one of the input paths of the downstream circuit;

an optical switching network coupled between the multiplexer input paths and the multiplexer output paths, the optical switching network comprising a plurality of active optical switches arranged to selectably couple a photon from any one of the multiplexer input paths to any one of the multiplexer output paths; and

control logic coupled to the optical switching network and configured to:

receive, for each of a plurality of time bins, an input signal indicative of whether the output path of each source circuit is propagating a photon;

select, based on the input signal, one of the multiplexer input paths as an active multiplexer input path for the time bin;

select one of the multiplexer output paths as an active multiplexer output path for the time bin, wherein multiplexer output paths in the raster group are selected according to a fixed order such that each multiplexer output path in the raster group of multiplexer output paths is selected as the active multiplexer output path once during a raster period consisting of R consecutive time bins; and

generate control signals to set a state of the optical active switches such that a photon from the active multiplexer input path is coupled to the active multiplexer output path.

10. The circuit of claim 9 wherein the downstream circuit is a Bell state generator.

11. The circuit of claim 9 wherein the source circuits are heralded single photon source circuits.

12. The circuit of claim 9 wherein the source circuits are entanglement circuits that generate entangled systems of photons that encode qubits.

13. The circuit of claim 12 wherein the qubits are encoded using a dual-rail encoding and wherein each multiplexer input path and each multiplexer output path comprises a pair of waveguides.

14. The circuit of claim 12 wherein the downstream circuit includes a second optical switching network coupled to a plurality of fusion circuits.

15. A circuit comprising:

a number (N) of source circuits, each source circuit having an output path to propagate a photon, wherein N is at least 2;

a number (R) of downstream circuits, each downstream circuit having a number (M) of input paths to receive photons, wherein R is at least 2 and M is at least 2; and

a set of M raster multiplexer circuits, each raster multiplexer circuit having:

a set of N multiplexer input paths, each multiplexer input path coupled to an output path of one of the N source circuits;

a plurality of multiplexer output paths including a raster group of multiplexer output paths, wherein the raster group of multiplexer output paths includes R multiplexer output paths, each multiplexer output path in the raster group of multiplexer output paths being coupled to one of the input paths of a different one of the R downstream circuits; and

an optical switching network coupled between the multiplexer input paths and the multiplexer output paths, the optical switching network comprising a plurality of active optical switches arranged to selectably couple a photon from any one of the multiplexer input paths to any one of the multiplexer output paths; and

control logic coupled to the raster multiplexer circuits and configured to:

receive, for each of a plurality of time bins, an input signal indicative of whether the output path of each source circuit is propagating a photon;

select, for each of the raster multiplexer circuits, one of the multiplexer input paths as an active multiplexer input path for the time bin, the selection being based at least in part on the input signal;

select, for each of the raster multiplexer circuits, one of the multiplexer output paths as an active multiplexer output path for the time bin, wherein, for each raster multiplexer circuit, multiplexer output paths in the raster group are selected according to a fixed order such that each multiplexer output path in the raster group of multiplexer output paths is selected as the active multiplexer output path once during a raster period consisting of R consecutive time bins and wherein all M of the multiplexer output paths that couple to a same one of the R downstream circuits are selected as the active multiplexer output paths for a same time bin; and

generate control signals to set a state of the active switches in the optical switching network of each of the R raster multiplexer circuits such that, in each of the R raster multiplexer circuits, a photon from the active multiplexer input path is coupled to the active multiplexer output path.

16. The circuit of claim 15 further comprising:

a plurality of delay lines, each delay line introducing a different amount of delay,

wherein each delay line is coupled to a different one of the multiplexer output paths in the raster group of multiplexer output paths.

17. The circuit of claim 16 wherein the control logic is further configured to select the output paths in an order such that photons entering the optical switching network during a set of R consecutive time bins arrive at respective outputs of the delay lines in the same time bin.

18. The circuit of claim 17 wherein the source circuits are heralded single photon source circuits.

19. The circuit of claim 18 wherein each downstream circuit is a Bell state generator.

20. A method comprising:

receiving, for each time bin in a series of time bins, a plurality of input signals indicating whether photons are present on each of a plurality of input paths of an optical circuit;

selecting, for each time bin in the series of time bins, an active input path for the optical circuit based at least in part on the input signals;

selecting, for each time bin in the series of time bins, an active output path for the optical circuit from a plurality of output paths that includes a raster group of a number (R) of output paths, wherein R is at least 2, wherein output paths in the raster group are selected according to a fixed order such that each output path in the raster group of output paths is selected as the active output path once during a raster period consisting of R consecutive time bins; and

controlling, for each time bin in the series of time bins, a plurality of active switches in the optical circuit to couple a photon from the active input path to the active output path.

21. The method of claim 20 wherein each output path in the raster group of output paths is coupled to a delay circuit that introduces a different number of clock cycles of delay and wherein the output paths are selected in an order such that photons entering the optical circuit during a set of R consecutive cycles arrive at respective outputs of the delay lines in a same clock cycle.