Quantum secret sharing is the procedure of securely distributing information between multiple parties by exploiting the features of quantum mechanics. Many variants exist, but in this work, we report a high-dimensional realization of a single-photon secret sharing scheme for distributing classical keys amongst many nodes. The implementation, which makes use of twisted light, is realized for as high as 11 dimensions and for as many as 10 participants: the highest reported to date and which is easily extendable to even higher dimensions and many participants. Such a result is an important first step towards a future quantum network.
Quantum secret sharing (QSS) is a cryptographic multiparty communication technique in which a secret is divided and shared among N parties and then securely reconstructed by (N-1) cooperating parties, making it perfect for storing and sharing highly sensitive data. Challenges in high dimensional state preparation, transformation and detection, the key steps of any QSS protocol, have so far hindered experimental realisation. Here, by taking advantage of the high-dimensional encoding space accessible by a photon's orbital angular momentum, we present a toolbox for realising practical high-dimensional single photon QSS schemes that are easily scalable in both dimension and number of participants. Our implementations realised a new record in both the number of participants (N=10) and the dimensionality (d=11), with the latter facilitating the transfer of 2.89 bits of information per photon. This work is an important step towards securely distributing information across a network of nodes.
In this work, Stokes polarimetery is used to extract the polarization structure of optical fields from only four measurements as opposed to the usual six measurements. Here, instead of using static polarization optics, we develop an all-digital technique by implementing a Polarization Grating (PG) which projects a mode into left- and right-circular states which are subsequently directed to a Digital Micromirror Device (DMD) which imparts a phase retardance for full polarization acquisition. We apply our approach in real-time to reconstruct the State of Polarization (SoP) and intra-modal phase of optical modes.
The global quantum network requires the distribution of entangled states over long distances, with significant advances already demonstrated using polarization, reaching approximately 1200 km in free space and 100 km in optical fiber. While Hilbert spaces with higher dimensionality, e.g., spatial modes of light, allows higher information capacity per photon, such spatial mode entanglement transport requires custom multimode fiber and is limited by decoherence induced mode coupling. Here we circumvent this by transporting multi-dimensional spatial entangled states down conventional single-mode fiber (SMF). We achieve this by entangling the spin-orbit degrees of freedom of a bi-photon pair, passing the polarization (spin) photon down the SMF while accessing multiple orbital angular momentum (orbital) sub-spaces with the other, thereby realizing multi-dimensional spatial entanglement transport. We show high fidelity hybrid entanglement preservation down 250 m of SMF across multiple 2 x 2 dimensions, which we confirm by quantum state tomography and Bell violation measures. This work offers an alternative approach to spatial mode entanglement transport that facilitates deployment in legacy networks across conventional fiber optic links.
Youngs double slit experiment is one of the most celebrated achievements in quantum and classical optics; it provides experimental proof of the wave-particle duality of light. When the paths of the double slit are marked with orthogonal polarizations, the path information is revealed and no interference pattern is observed. However, the path information can be erased with a complimentary analysis of the polarization. Here we use hybrid entanglement between photons carrying orbital angular momentum and polarization to show that, just as in Young's experiment, the paths (OAM) marked with polarization do not lead to interference. However, when introducing the eraser (polarizer) which projects the polarization of one of the entangled photons onto a complementary polarization basis, the OAM (paths) are allowed to interfere, leading to the formation of azimuthal fringes whose frequency is proportional to the OAM content carried by the photon.
Combining the multiple degrees of freedom of photons has become topical in quantum communication and information
processes. This provides advantages such as increasing the amount of information that is be packed into
a photon or probing the wave-particle nature of light through path-polarisation entanglement. Here we present
two experiments that show the advantages of using hybrid entanglement between orbital angular moment (OAM)
and polarisation. Firstly, we present results where high dimensional quantum key distribution is demonstrated
with spatial modes that have non-separable polarisation-OAM DOF called vector modes. Secondly, we show
that through OAM-polarisation entanglement, the traditional which-way experiment can be performed without
using the traditional physical path interference approach.
High-dimensional encoding using higher degrees of freedom has become topical in quantum communication protocols. When taking advantage of entanglement correlations, the state space can be made even larger. Here, we exploit the entanglement between two dimensional space and polarization qubits, to realize a four-dimensional quantum key distribution protocol. This is achieved by using entangled states as a basis, analogous to the Bell basis, rather than typically encoding information on individual qubits. The encoding and decoding in the required complementary bases is achieved by manipulating the Pancharatnam-Berry phase with a single optical element: a q-plate. Our scheme shows a transmission fidelity of 0.98 and secret key rate of 0.9 bits per photon. While the use of only static elements is preferable, we show that the low secret key rate is a consequence of the filter based detection of the modes, rather than our choice of encoding modes.