Raster multiplexing in photonic circuits
Circuits and methods that implement multiplexing for photons propagating in waveguides are disclosed, in which an input photon received on a selected one of a set of input waveguides can be selectably routed to one of a set of output waveguides. The output waveguide can be selected on a rotating or cyclic basis, in a fixed order, and the input waveguide can be selected based at least in part on which one(s) of a set of input waveguides is (are) currently propagating a photon.
1 . A system comprising:
a multiplexer circuit having:
a plurality of input paths to receive photonic qubits;
a plurality of output paths including a first subset of output paths having a number (R) of output paths and a second subset of output paths having the number R of output paths, wherein R is at least 2; and
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 photonic qubit from any one of the input paths to any one of the output paths;
a plurality of downstream circuits, each downstream circuit having a first input port and a second input port;
a first optical switching circuit having a set of R input paths coupled to the first subset of the output paths of the multiplexer circuit and a plurality of output paths coupled to the first input ports of the downstream circuits via delay circuits that introduce different amounts of delay;
a second optical switching circuit having a set of R input paths coupled to a second subset of the output paths of the multiplexer circuit and a plurality of output paths coupled to the second input ports of the downstream circuits; and
control logic coupled to the multiplexer circuit, the first optical switching circuit, and the second optical switching circuit and configured to control the multiplexer circuit, the first optical switching circuit, and the second optical switching circuit such that a first photonic qubit output on one of the first subset of output paths of the multiplexer circuit and a second photonic qubit output on one of the second subset of output paths of the multiplexer circuit arrive in temporal alignment at the first and second input ports of one of the downstream circuits, the control logic being further configured to:
receive, for a time bin of a plurality of time bins, an input signal indicative of whether a photonic state encoding a qubit is present on each input path of the multiplexer circuit;
select, based on the input signal, one of the input paths of the multiplexer circuit as an active input path for the time bin;
select one of the output paths of the multiplexer circuit as an active output path for the time bin, wherein output paths in the first and second subsets of output paths are selected according to a fixed order such that each output path in the first subset of output paths and each output path in the second subset of output paths is selected as the active output path once during a raster period consisting of 2R consecutive time bins; and
generate control signals to set a state of the optical switching network of the multiplexer circuit for the time bin such that a photon from the active input path is coupled to the active output path.
2 . The system of claim 1 wherein the control logic is further configured to:
select, for each time bin, an active output path for each of the first optical switching circuit and the second optical switching circuit such that a qubit output on the active output path of the first optical switching circuit and a qubit output on the active output path of the second optical switching circuit arrive in temporal alignment at one of the downstream circuits.
3 . The system of claim 1 wherein the plurality of output paths of the first optical switching circuit and the plurality of output paths of the second optical switching circuit each include 2R output paths.
4 . The system of claim 3 wherein the delay circuits introduce different delays in a range from 0 to 2(R−1) time bins.
5 . The system of claim 1 wherein the plurality of downstream circuits includes a plurality of fusion circuits configured to perform a joint measurement operation that consumes a pair of qubits received at the first and second input ports and produces measurement data.
6 . The system of claim 5 wherein the joint measurement operation is a Type II fusion operation.
7 . The system of claim 5 wherein the plurality of fusion circuits includes a number of fusion circuits equal to 2R−1.
8 . The system of claim 1 further comprising:
a first single-qubit measurement circuit; and
a second single-qubit measurement circuit,
wherein the first optical switching circuit includes an output path coupled to an input path of the first single-qubit measurement circuit and the second optical switching circuit includes an output path coupled to an input path of the second single-qubit measurement circuit.
9 . The system of claim 1 wherein each input path and each output path of the multiplexer circuit and the first and second optical switching circuit comprises a waveguide.
10 . The system of claim 1 wherein each input path and each output path of the multiplexer circuit and the first and second optical switching circuit comprises a pair of waveguides.
11 . The system of claim 1 wherein the optical switching network of the multiplexer circuit is a generalized Mach-Zehnder interferometer (GMZI) and the active optical switches include active phase shifters.
12 . The system of claim 2 wherein each of the first and second optical switching circuits includes a generalized Mach-Zehnder interferometer (GMZI).
13 . A method comprising:
receiving, at a first optical switching circuit, a first group of qubits via a first plurality of optical paths, wherein during each of a plurality of successive time bins in a first set of time bins, each optical path in the first plurality of optical paths receives one qubit of the first group of qubits;
receiving, at a second optical switching circuit, a second group of qubits via a second plurality of optical paths, wherein during each of a plurality of successive time bins in a second set of time bins subsequent to the first set of time bins, each optical path in the second plurality of optical paths receives one qubit of the second group of qubits;
identifying a target pairing between a first qubit in the first group of qubits and a second qubit in the second group of qubits;
routing, by the first optical switching circuit, the first qubit to a selected one of a plurality of downstream circuits via one of a plurality of output paths of the first optical switching circuit, wherein different ones of the output paths of the first optical switching circuit incorporate different amounts of delay; and
routing, by the second optical switching circuit, the second qubit to the selected one of the plurality of downstream circuits via one of a plurality of output paths of the second optical switching circuit such that the first qubit and the second qubit arrive at the selected one of the plurality of downstream circuits in temporal alignment.
14 . The method of claim 13 further comprising:
providing photonic states encoding qubits from a plurality of source circuits on a plurality of input paths to a multiplexer circuit having a plurality of output paths that includes a first subset of output paths coupled to the first plurality of optical paths and a second subset of output paths coupled to the second plurality of optical paths; and
operating the multiplexer circuit for the first set of time bins and the second set of time bins such that, for each time bin, a qubit from one of the source circuits is delivered to a different one of the plurality of output paths of the multiplexer circuit.
15 . The method of claim 14 wherein the first subset of output paths of the multiplexer circuit and the second subset of output paths of the multiplexer circuit each include a number (R) of output paths and wherein operating the multiplexer circuit includes:
receiving, for each time bin, an input signal indicative of whether a photonic state encoding a qubit is present on each input path of the multiplexer circuit;
selecting, based on the input signal, one of the input paths of the multiplexer circuit as an active input path for the time bin;
selecting one of the output paths of the multiplexer circuit as an active output path for the time bin, wherein output paths in the first and second subsets of output paths are selected according to a fixed order such that each output path in the first subset of output paths and each output path in the second subset of output paths is selected as the active output path once during a raster period consisting of 2R consecutive time bins; and
generating control signals to set a state of an optical switching network of the multiplexer circuit for the time bin such that a photon from the active input path is coupled to the active output path.
16 . The method of claim 15 wherein the different ones of the output paths of the first optical switching circuit incorporate different amounts of delay in a range from 0 to 2(R−1) time bins.
17 . The method of claim 13 further comprising:
operating the downstream circuits to perform joint measurement operations on pairs of qubits received in temporal alignment.
18 . The method of claim 17 wherein the joint measurement operation is a Type II fusion operation.
19 . The method of claim 13 further comprising:
operating one or more of the downstream circuits to perform a single-qubit measurement on a received qubit.