Short answer: Photons can carry orbital angular momentum (OAM), which gives access to many more than two states per photon. Three experiments use OAM for communication tasks: a 2019 transmission of four OAM states through a 1.2 km air-core fibre, a 2020 secret-sharing protocol with up to ten participants, and a 2023 remote transport of high-dimensional OAM states with fidelities of about 0.88 and 0.80. All are laboratory experiments.
Why use more than two states per photon?
The 2019 Physical Review Applied abstract says quantum networks are the ultimate target in quantum communication, with many connected users sharing information carried by quantum systems. Qubits are the usual information units, but the authors say high-dimensional states, qudits, are a richer resource that goes beyond the limitations of qubits. Generation and manipulation of such states have improved over ten years, but reliable transmission between remote locations remains the main challenge.
What did the fibre experiment show?
The authors show that a recent air-core fibre supporting OAM modes can be used to transmit D-dimensional states faithfully. Four OAM quantum states and their superpositions are created, propagated in a 1.2 km fibre and detected with high fidelities. They also implement three quantum key distribution protocols as concrete applications, to show practical use. The abstract text we read breaks off here, and no numerical fidelity is quoted in this post.
What did the secret-sharing experiment show?
The 2020 Physical Review A abstract describes secret sharing as allowing three or more parties to share secret information that can be decrypted only through collaboration, complementing quantum key distribution. The authors use hybrid spin and orbital angular momentum states to access a high-dimensional encoding space, and demonstrate a protocol they describe as easily scalable in dimension and participants. They first demonstrate it in two dimensions with up to ten participants, then in three dimensions with three participants. They reconstruct secrets depicted as images with a fidelity of up to 0.979. They also say their scheme uses conventional linear optics to emulate the quantum gates needed for transitions between basis modes, allowing them to exceed the 1 bit per photon limit of two-dimensional protocols. Security analysis is not covered in the abstract.
What did the 2023 transport experiment show?
The 2023 Nature Communications paper uses spatial-mode-engineered frequency conversion to realise the remote transport of high-dimensional OAM states between two distant parties without direct transmission of the information carriers. The authors use perfect vortices to prepare high-dimensional maximal OAM entanglement, and use nonlinear sum-frequency generation with a strong coherent wave packet and a single photon for Bell-like state measurements. They achieve average transport fidelities of 0.879 plus or minus 0.048 for 3-dimensional and 0.796 plus or minus 0.066 for 5-dimensional OAM mutually unbiased bases. They also construct a quantum imaging strategy with interaction-free light, and say future advances in nonlinear frequency conversion are expected to help, which is their outlook.
How to read this
The three experiments address transmission, secret sharing and state transport, with different fidelities that measure different things, so they cannot be ranked. Fidelity here is the authors' measure of how close the received state is to the intended one. This post is based on abstracts, and full texts were not checked. It makes no claim about deployed networks or products.
FAQ
What is OAM?
A property of light with helical phase structure that can take many values, per the 2023 and 2012 abstracts.
How long was the fibre?
1.2 km, in the 2019 air-core fibre experiment.
How many parties shared a secret?
Up to ten, in two dimensions.
What fidelity was reported for transport in 2023?
0.879 for 3-dimensional and 0.796 for 5-dimensional OAM bases.
All field notes