Quantum multiplexing with the orbital angular momentum of light

The orbital angular momentum (OAM) of photons offers a suitable support to carry the quantum data of multiple users. We present two optical setups that send the information of $n$ quantum communication parties through the same free-space optical link. Those qubits can be sent simultaneously and share path, wavelength, and polarization without interference, increasing the communication capacity of the system. The first solution, a qubit combiner, merges $n$ channels into the same link, which transmits $n$ independent photons. The second solution, the OAM multiplexer, uses controlled-not ($\mathrm{CNOT}$) gates to transfer the information of $n$ optical channels to a single photon. Additional applications of the multiplexer circuits, such as quantum arithmetic, as well as connections to OAM sorting are discussed.

Figure 1

Hologram representation. The exact value of $\Delta \ell$ is included below the symbol.

Figure 2

Basic optical building blocks. Left: 50:50 beam splitter. Right: Dove prisms.

Figure 3

W sorting interferometer.

Figure 4

Interferometer with two Dove prisms with a relative angle of $\frac{\alpha }{2}$. The representation will always include the angle $\alpha$ of the relative rotation of the lower arm beam with respect to the upper arm beam.

Figure 5

Path dual rail to OAM qubit conversion.

Figure 6

OAM qubit merger.

Figure 7

OAM multiplexing block for the $i\text{th}$ qubit.

Figure 8

OAM demultiplexing block for the $i\text{th}$ qubit.

Figure 9

Nested increasing level (OAM number) MUX and DEMUX operations.

Figure 10

Adder block.