Session P08: Quantum networks

High Purity Single Photons Entangled with an Atomic Qubit

Trapped ion quantum networks feature identical stationary qubits that can interact locally via phonons within ion-trap modules and remotely via photonic flying qubits that connect separate modules. The single-photon purity, and ion-photon entanglement fidelity are of crucial importance for such quantum information networks, but there is often a tradeoff between these attributes and the rate of heralded entanglement. We demonstrate a single-photon source based on single trapped $^{138}$Ba$^+$ ion subject to pulsed excitation, and we report a second-order coherence of $g^{(2)}(0) = (8.1\pm2.3)\times10^{-5}$ without any background subtraction, and present results on the effect of integration time. Next, we show entanglement between the stationary ion spin qubit and flying photon polarization qubit and explore the errors due to polarization mixing at large collection solid angle in a free space system. We introduce spatial filtering of the emitted light to optimize the tradeoff between the photon generation rate and the entanglement fidelity. This technique improves the ion-photon entanglement fidelity from $F=0.884(4)$ to $F=0.930(4)$.   

Optimized non-Gaussian communication strategies for a phase diffusion channel

Noise in realistic communication channels limits the amount of information that we can communicate. Many studies on how to maximize information transfer over noisy channels have focused on Gaussian noise. However, there are realistic situations in which communication takes place over a channel where the noise is not Gaussian. One such non-Gaussian channel is the phase diffusion channel, where optimal encoding and decoding methods and information rates are not known. However, optimized non-Gaussian communication strategies can potentially enhance information transfer beyond what is possible with conventional methods in this noisy channel. We demonstrate an optimized strategy for a phase diffusion channel which consists of a binary coherent state alphabet and a non-Gaussian measurement based on a coherent displacement of the input state followed by photon number resolving detection. In this optimized strategy, both the input state alphabet and measurement are optimized together to provide robustness and enhanced sensitivity for communications over noisy channel. We find that this optimized strategy achieves information transfer rates which surpass what is possible with conventional encoding and measurement methods for communications with binary coherent states.   

Drone-based Quantum Key Distribution

Quantum communication channels offer the ability to transmit provably secure messages, link together distant quantum computers, and support the deployment of advanced networked quantum sensors. Although many applications of quantum communication channels can tolerate wired connections, e.g., fiber-optic cables, other potential applications benefit from wireless quantum communication links. Small mobile platforms, such as multi-rotor drones, offer the ability for rapid reconfiguration and facilitate operation in both urban and rural environments. Here, we discuss our progress towards developing a quantum key distribution channel over a free-space link between two multi-rotor drones in flight. Establishing such a quantum communication channel requires an advanced control system to perform optical alignment in-flight, as well as developing and packaging a suitable quantum light source, optics, single-photon detectors, time-tagger, and synchronization electronics all into a small Size, Weight, and Power (SWaP) footprint for use on a drone payload.   

Hyperentangled Time-bin and Polarization Quantum Key Distribution

Fiber-based quantum key distribution (QKD) networks are currently limited to metropolitan distances without quantum repeaters. To reach longer distances, satellite-based QKD links have been proposed to extend the network domain. We have developed a quantum communication system suitable for a satellite-to-ground link. With this system, using polarization entangled photons, we have executed the QKD protocol developed by Bennett, Brassard, and Mermin in 1992 (BBM92), achieving quantum bit error rates (QBER) below 2$\%$. More importantly, we demonstrate low QBER execution of a higher dimensional hyperentanglement-based QKD protocol (HEQKD) using photons simultaneously entangled in polarization and time-bin. We verify the security of our protocol using a rigorous, modern finite-key analysis, and show it is suitable for a space-to-ground link, after incorporating a Doppler shift compensation scheme. Finally, we demonstrate a distinct advantage to using HEQKD over BBM92.   

A Free-Space Quantum Optical Link for Daytime Operation using Atomic Line Filters

Quantum key distribution (QKD) can provide secure optical data links using the established BB84 protocol, though solar background limits the performance through free space. Despite time-gating the photon signal, limiting the field of view through geometrical design of the optical system, and spectral rejection using interference filters, the solar background continues to dominate under daytime atmospheric conditions. We overcome this limitation by introducing an atomic line filter (ALF) based on a warm rubidium vapor combined with an attenuated laser source tuned to the passband of the filter. By adjusting the optical rotation through the vapor, the ALF transmits a narrow spectral region ($\Delta\nu\sim1$ GHz) between crossed polarizers that improves upon the $\Delta\nu\sim$50 GHz bandpass of an interference filter. We generate 1 ns pulses at 10 MHz along four polarization channels attenuated to a photon occupancy of $\mu$=0.5 per pulse. We observe quantum bit error rates (QBERs) less than the functional threshold of $\sim11\%$ under daytime conditions simulated in a laboratory with link losses up to 28 dB. We project that with spatial filtering from a telescope system ($<$100 $\mu$rad), this link could function with losses up to 35 dB under the brightest daytime scenarios.   

Polarization-Independent Photon Storage System with Variable Time Delay

Quantum optical memories are a key component for a variety of quantum information applications, from extending quantum communication channels to building high-efficiency single-photon sources. However, current broad bandwidth photon storage systems we've seen operate at somewhat low efficiency and short storage times (on the order of 10 ns). Here we develop a system with multiplexed free-space storage cavities, able to store single photons with high efficiency over variable delays [N x 12.5 ns, 1 $\le $ N $\le $ 999], and over several nanometers bandwidth. The system can store multiple photons simultaneously and can potentially store qubits encoded in various degree of freedoms (e.g., polarization, timing, and spatial modes). We have demonstrated a process fidelity \textgreater 98{\%} for storage times up to 125 ns and \textgreater 90{\%} for up to 500 ns. A future goal for this experiment is to achieve storage of hyperentanglement. While previous hyperentangled photon storage systems only achieved 5{\%} efficiency, we have currently demonstrated fiber-coupled transmission above 65{\%} for delay times up to 125 ns, and free-space transmission above 50{\%} for delay times up to 5 $\mu $s.   

Quantum repeaters with optimized memory storage time

We propose an optimized access time protocol (OAP) and quantum repeater architecture that maximizes the entanglement generation rate. Access time is defined as the total time the quantum memories store the quantum state before they are either used for entanglement length doubling or discarded due to their poor entanglement quality. The OAP uses hierarchically optimized access times for all nesting levels. The optimal access time depends on the parameters of quantum memory quality, the entanglement generation probability, and the swapping success probability. By restricting the access time, the OAP provides an average remote entangled state with a high measure of entanglement since the time for memory decoherence is limited.   

Entanglement State Transfer Between the Random Access Quantum Memories

The quantum network requires the high capability of interfaces and channels. The random access quantum memory (RAQM) is one promising candidate, of which the individual micro memory cells can be entangled and store the quantum state under programmable control. Here, we demonstrate a protocol to transfer the entanglement state between the two types of random access quantum memory, which are based on the DLCZ protocol and EIT effect individually with the help of acousto-optic deflectors (AODs). The measured state fidelity indicates the entanglement is preserved during the process. The experiment results confirm that the quantum links between RAQMs is intrinsically high dimensional quantum channel, which makes a significant step towards quantum information process and quantum network.   

Tests of nonlocality with hyperentangled photons

Entanglement represents a valuable resource for enhanced communication. It is often necessary to both verify and quantify the entanglement shared through a quantum channel. Tests of nonlocality, such as Bell tests and steering, can be used to certify shared entanglement. For pairs of entangled qubits, the standard CHSH Bell inequality is a sufficient entanglement certification. However, the CHSH inequality is unable to measure increasing degrees of entanglement, as exists in hyperentangled photons. We use a bipartite Bell test with four settings to demonstrate that a measured 4-dimensional state is genuinely hyperentangled, in polarization and time-bin, by showing that its Bell parameter exceeds the maximum value possible with a pair of entangled qubits. In addition, we present a scheme for using quantum steering to verify that a state contains multipartite entanglement.   

Pulse-Enhanced Two-Photon Interference with Solid State Quantum Emitters

The ability to generate distributed entanglement across distant quantum nodes is essential for the construction of scalable quantum networks and for various quantum information processing operations such as quantum teleportation and Bell inequality tests[1, 2]. For solid state spin qubits, entangling two qubits can be achieved through photon interference on a beam splitter. This operation operation can have its efficiency drastically reduced by fluctuations in the uncorrelated environments of the respective qubits. We simulate the two-photon interference operation in a Hong-Ou-Mandel-type experiment for two distant solid state quantum emitters that are driven by suitable pulse sequences. We find that besides their emission/absorption spectrum having little dependence on their environments[3, 4], photon indistinguishability can be restored to optimal values allowing for highly improved efficiency of photon-mediated QIP operations. [1]P. Humphreys et al, Nature \textbf{558}, 268 (2018); [2] D. D. Awschalom, Nat. Photonics \textbf{12}, 516 (2018); [3] H. F. Fotso et al, Phys. Rev. Lett. \textbf{116}, 033603 (2016); [4] H. F. Fotso, J. Phys, B \textbf{52}, 2 (2018).