The coexistence of classical and quantum signals over the same optical fiber is critical for quantum networks operating within the existing communications infrastructure. Here, we characterize the quantum channel that results from distributing approximate single-photon polarization-encoded qubits simultaneously with classical light of varying intensities through a 25 km fiber-optic channel. We use spectrally resolved quantum process tomography with a newly developed Bayesian reconstruction method to estimate the quantum channel from experimental data, both with and without classical noise. Furthermore, we show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel if the noise is dominated by Raman scattering. These results aid future development of quantum repeater designs and quantum error-correcting codes which benefit from realistic channel error models.
Quantum communication networks based on fiber optics are restricted in length since efficient quantum repeaters are not yet available. A free-space channel between a satellite in orbit and Earth can circumvent this problem. We have constructed a system to demonstrate the feasibility of quantum communication between space and earth using photons hyperentangled in their polarization and time-bin degrees of freedom. With this system, we have implemented superdense teleportation (SDT) with a fidelity of 0.94±0.02. To increase the efficiency of SDT, we have developed an active, polarization-independent switch compatible with SDT. We characterized the performance of its switching efficiency. Finally, we have constructed a novel two-level interferometer for time-bin qubit creation and analysis in orbit, and bounded its stability.
We can envision an eventual global multi-node quantum network, with hubs located around the planet. This, however, is still a far reach from current state of the art. Here we discuss some of our approaches to bridge the gap. Specifically, we are pursuing airborne and satellite-based free-space quantum communication. Free-space platforms naturally lend themselves to reconfiguration - likely required by a future quantum-secure network -- as nodes may be easily moved/reoriented to target new nodes. We are implementing a multi-copter drone-based quantum cryptography link, including fast, high-resolution optical stabilization; compact, independent sources; and lightweight single-photon detection. Having access to an agile, reconfigurable QKD networking system will enable quantum cryptography to reach applications prohibited by current approaches, such as temporary networks in seaborne, urban, or even battlefield situations. By using transmitters and receivers at higher altitudes, deleterious effects weather events like fog and turbulence can be mitigated. At longer scale, we are pursuing a quantum link from the International Space Station to earth, which will use hyperentanglement to enable a variety of advanced quantum communication protocols, including multi-bit-per-photon key distribution and "superdense" teleportation. With our table-top experiment we have investigated the effects of loss and turbulence, and demonstrated a system to compensate for the otherwise devastating effect of the Doppler effect from the rapidly moving ISS platform.
Superdense Teleportation (SDT) is a suitable protocol to choose for an advanced demonstration of quantum communication in space. We have taken further steps towards the realization of SDT in such an endeavor. Our system uses polarization and time-bin hyperentanglement via non-degenerate spontaneous parametric downconversion to implement SDT of 4-dimensional equimodular states. Previously, we have shown high fidelity (>90%) SDT implementation and the feasibility to perform SDT on an orbiting platform by correcting the Doppler shift. Here we discuss new analysis of the received state reconstruction performance in the presence of high channel loss and multiple pair events. Additionally, initial characterization of a waveguide-based entanglement source intended for space will be presented.
Establishing a quantum communication network would provide advantages in areas such as security and information processing. Such a network would require the implementation of quantum teleportation between remote parties. However, for photonic "qudits" of dimension greater than two, this teleportation always fails due to the inability to carry out the required quantum Bell-state measurement. A quantum communication protocol called Superdense Teleportation (SDT) can allow the reconstruction of a state without the usual 2-photon Bell-state measurements, enabling the protocol to succeed deterministically even for high dimensional qudits. This technique restricts the class of states transferred to equimodular states, a type of superposition state where each term can differ from the others in phase but not in amplitude; this restricted space of transmitted states allows the transfer to occur deterministically. We report on our implementation of SDT using photon pairs that are entangled in both polarization and temporal mode. After encoding the phases of the desired equimodular state on the signal photon, we perform a complete tomography on the idler photon to verify that we properly prepared the chosen state. Beyond our tabletop demonstration, we are working towards an implementation between a space platform in low earth orbit and a ground telescope, to demonstrate the feasibility of space-based quantum communication. We will discuss the various challenges presented by moving the experiment out of the laboratory, and our proposed solutions to make Superdense Teleportation realizable in the space setting.
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