Teleportation Systems Toward a Quantum Internet

Quantum teleportation is essential for many quantum information technologies, including long-distance quantum networks. Using fiber-coupled devices, including state-of-the-art low-noise superconducting nanowire single-photon detectors and off-the-shelf optics, we achieve conditional quantum teleportation of time-bin qubits at the telecommunication wavelength of 1536.5 nm. We measure teleportation fidelities of $\ge 90$% that are consistent with an analytical model of our system, which includes realistic imperfections. To demonstrate the compatibility of our setup with deployed quantum networks, we teleport qubits over 22 km of single-mode fiber while transmitting qubits over an additional 22 km of fiber. Our systems, which are compatible with emerging solid-state quantum devices, provide a realistic foundation for a high-fidelity quantum Internet with practical devices.

Popular Summary

A functional quantum Internet, a network in which information stored in qubits is shared over long distances through entanglement, would change the fields of secure communication, data storage, precision sensing, and computing. High-fidelity quantum teleportation is essential for secure long-distance communications and a practical quantum Internet. This work presents—for the first time—sustained, long-distance (44 km of fiber) teleportation of time-bin qubits featuring state-of-the-art fidelity ($>90\mathrm{\%}$) and narrow-band photons with narrow-band entangled photon pairs. The experimental results are supported by an analytical model that accurately accounts for experimental imperfections.

The measurements are performed on the Caltech and Fermilab Quantum Network test beds (CQNET, FQNET), two teleportation systems that have been designed, built, commissioned, and deployed by Caltech’s multidisciplinary multi-institutional public-private research program on Intelligent Quantum Networks and Technologies (IN-Q-NET). IN-Q-NET was jointly founded in 2017 by Caltech, AT&T, Fermi National Accelerator Laboratory, and the Jet Propulsion Laboratory.

These unique quantum network test beds use state-of-the-art solid-state light detectors in a compact fiber-based setup and feature near-autonomous data acquisition, control, monitoring, synchronization, and analysis. The teleportation systems, which are compatible both with existing telecommunication infrastructure and with emerging quantum processing and storage devices, represent a significant milestone towards a practical quantum Internet. These networks are currently being used to improve the fidelity and rate of entanglement distribution, with an emphasis on complex quantum communication protocols and fundamental science. The networks are accessible to multidisciplinary researchers for research and development purposes and will serve both fundamental quantum information science and the development of advanced quantum technologies.

Figure 1

A schematic diagram of the quantum teleportation system consisting of Alice, Bob, Charlie, and the data acquisition (DAQ) subsystems. See the main text for descriptions of each subsystem. One cryostat is used to house all SNSPDs: it is drawn as two for ease of explanation. The detection signals generated by each of the SNSPDs are labelled 1–4 and collected at the TDC, with 3 and 4 being time multiplexed. All individual components are labeled in the legend, with single-mode optical fibers (electronic cables) in gray (green), and with uni- and bichromatic (i.e., unfiltered) optical pulses indicated.

Figure 2

Entanglement visibility. The temperature of the interferometer is varied to reveal the expected sinusoidal variations in the rate of coincidence events. A fit reveals the entanglement visibility $V_{\mathrm{ent}}=96.4\pm 0.3\mathrm{\%}$ (see the main text for details). The uncertainties here and in all measurements are calculated assuming Poisson statistics.

Figure 3

Hong-Ou-Mandel (HOM) interference. A relative difference in arrival time is introduced between photons from Alice and Bob at Charlie’s BS. HOM interference produces a reduction of the threefold coincidence detection rate of photons as measured with SNSPDs after Charlie’s BS and at Bob. A fit reveals (a) $V_{\mathrm{HOM}}=70.9\pm 1.9\mathrm{\%}$ and (b) $V_{\mathrm{HOM}}=63.4\pm 5.9\mathrm{\%}$ when lengths of fiber are added (see the main text for details).

Figure 4

The quantum teleportation of $|+⟩$. Teleportation is performed (b) with and (a) without an additional 44 km of single-mode fiber inserted into the system. The temperature of the inteferometer is varied to yield a sinusoidal variation of the threefold coincidence rate at each output of the MZI (blue and red points). A fit of the visibilities (see Sec. 3a) measured at each output ($V_{+,1}$, $V_{+,2})$ of the MZI gives an average visibility $V_{+}=(V_{+,1}+V_{+,2})/2$ of (a) $69.7\pm 0.91\mathrm{\%}$ without the additional fiber and (b) $58.6\pm 5.7\mathrm{\%}$ with the additional fiber.

Figure 5

Quantum teleportation fidelities for ${|e⟩}_A$, ${|l⟩}_A$, and ${|+⟩}_A$, including the average fidelity. The dashed line represents the classical bound. Fidelities using QST are shown using blue bars, while the minimum fidelities for qubits prepared using $|n=1⟩$, $F_e^d$, $F_l^d$, and $F_{+}^d$, including the associated average fidelity $F_{\mathrm{avg}}^d$, respectively, using a DSM are shown in gray. (a),(b) The results without and with additional fiber, respectively. The uncertainties are calculated using Monte Carlo simulations with Poissonian statistics.

Figure 6

A schematic depiction of the distingushability between Alice and Bob’s photons at Charlie’s BS. The distinguishability is modeled by means of a virtual BS with a transmittance $\zeta$. Indistinguishable photons contribute to interference at the Charlie’s BS while distinguishable photons are mixed with vacuum, leading to a reduction of HOM visibility and teleportation fidelity (see the main text for further details).

Figure 7

The evaluation of the photon indistinguishability using an analytical model. (a) The quantum teleportation fidelity of $|+⟩$. (b) The HOM interference visibility, each with varied mean photon number ${\mu }_A$ of Alice’s qubits. Fits of analytical models to the data reveal $\zeta =90\mathrm{\%}$ indistinguishability between Alice and Bob’s photons at Charlie’s BS. Bob produces ${\mu }_B$ photon pairs on average, while ${\eta }_i$ and ${\eta }_s$ are the probabilities for an individual idler (signal) photon to arrive at Charlie’s BS and be detected at Bob’s detector, respectively.

Figure 8

The elements of the density matrices of teleported $|e⟩$, $|ł⟩$, and $|+⟩$ states with the additional 44 km of fiber in the system.The black points are generated by our teleportation system and the blue bars with red dashed lines are the values assuming ideal teleportation.

Figure 9

The elements of the density matrices of teleported $|e⟩$, $|ł⟩$, and $|+⟩$ states. The black points are generated by our teleportation system and the blue bars with red dashed lines are the values assuming ideal teleportation.