Superdense Coding over Optical Fiber Links with Complete Bell-State Measurements

Adopting quantum communication to modern networking requires transmitting quantum information through a fiber-based infrastructure. We report the first demonstration of superdense coding over optical fiber links, taking advantage of a complete Bell-state measurement enabled by time-polarization hyperentanglement, linear optics, and common single-photon detectors. We demonstrate the highest single-qubit channel capacity to date utilizing linear optics, $1.665\pm 0.018$, and we provide a full experimental implementation of a hybrid, quantum-classical communication protocol for image transfer.

Figure 1

Our complete Bell-state measurement is achieved using linear optics and time-polarization hyperentanglement. Two-photon interference is orchestrated to ensure Bell-state detection signatures do not involve two photons coincident on a single detector. $\mathrm{bs}\equiv \text{beam}$ splitter, and $\mathrm{pbs}\equiv \text{polarizing}$ beam splitter.

Figure 2

Hong-Ou-Mandel interference at a beam splitter leads to unique outputs for the four Bell states. This leads to partial Bell-state discrimination, perfectly identifying ${\mathrm{\Psi }}^{+}$ and ${\mathrm{\Psi }}^{-}$ from $\mathrm{\Phi }$ states. The ${\mathrm{\Phi }}^{+}$ and ${\mathrm{\Phi }}^{-}$ states are left indistinguishable.

Figure 3

To implement superdense coding, Alice and Bob initially each receive one photon from a time-polarization hyperentangled photon pair. Alice performs one of four operations on her photon, which encodes two bits on the nonlocal two-photon Bell state. Alice transmits her photon to Bob, who performs a Bell-state measurement, i.e., decodes two bits. $\mathrm{bs}\equiv \text{beam}$ splitter, $\mathrm{pbs}\equiv \text{polarizing}$ beam splitter, and $\mathrm{PPKTP}\equiv \text{potassium}$ titanyl phosphate.

Figure 4

The conditional probabilities $p(y|x)$ of receiving state $y$ given the sent state $x$ using our apparatus give the highest channel capacity to date, $1.665\pm 0.018$, by encoding on a single qubit and decoding using linear optics.

Figure 5

(Left) The original four-color $100\times 136\text{pixel}$ 3.4 kB image. (Right) The image received using superdense coding. The calculated fidelity was 87%.