Session Q53: Quantum Networks: Modeling, Components and Demonstrations

Remote Quantum Networking with Single Rare-earth Ions: Bipartite Entanglement and Teleportation

Long range quantum networks will enable secure communication and distributed quantum computing. In this talk we implement a two node network using single ¹⁷¹Yb ions in YVO₄ coupled to nanophotonic cavities. These are excellent optically-addressable qubits with high spin control fidelities, long memory times and stable optical transitions.

We apply a novel entanglement protocol to remote ¹⁷¹Yb ions that demonstrates two key milestones:

1. Optical lifetime-limited entanglement rates and fidelities, even in the presence of spectral diffusion;

2. Entanglement of optically distinguishable spin qubits without optical frequency shifting.

Counteracting both static and dynamic disorder in optical transition frequencies is achieved using the stochastic detection time of a heralding photon to tailor subsequent optical and spin quantum gates in real-time. We also apply a modified version of this protocol to probabilistically teleport quantum states between the two ions.

These results challenge conventional wisdom regarding limitations on entanglement protocols imposed by optical Ramsey coherence times. Furthermore, the narrow optical inhomogeneity of ¹⁷¹Yb:YVO₄ (200 MHz) enables entanglement distribution between any pair of ¹⁷¹Yb qubits with commercially available detector technology. These results showcase single rare-earth ions as a scalable platform for the future quantum internet.   

Remote Quantum Networking with Single Rare-earth Ions: Tripartite Entanglement Heralding

Remote entanglement distribution in quantum networks has applications in connecting quantum computers and secure information transfer. While bipartite entanglement has been shown in multiple systems, tripartite entanglement distribution has only been shown with limited platforms, highlighting the need for more scalable platforms and protocols.

Here, we demonstrate the scalability of a novel entanglement protocol (presented in the previous talk) by generating tripartite entangled states of ¹⁷¹Yb:YVO₄ optically addressed qubits, distributed between two remote nanophotonic cavities.

Specifically, we show that entanglement of three ¹⁷¹Yb ions can be heralded with a single photon protocol. Since the ions possess different optical frequencies, the stochastic photon detection time imprints two random quantum phases, leading to a mixed state. However, with precise measurement of the photon arrival time, the phases can be evaluated and counteracted in real-time via a feedforward phase shift of the qubits. Through this real-time phase compensation, we are able to prepare entangled three-ion W states with a fidelity of 60% and a heralding rate of 3 Hz. This result shows both the scalability of our platform and the robustness of our entanglement protocol.   

Polarization Mode Dispersion Compensation Towards Improved Polarization Based Quantum Networks

Polarization Mode Dispersion (PMD) describes the depolarization effects in single-mode fibers. The compensation of PMD within optical fibers is critical for implementing quantum networks, since it limits the transmission distance and usable spectral bandwidth of polarization-entangled qubits. When PMD is present in optical fiber, it leads to temporal delays depending on the input polarization state. This effect can also be understood as a wavelength-dependent transformation of polarization. We define the principal states of polarization (PSP) for 1st order PMD as an axis on the Poincaré sphere around which wavelength dependent polarization transformations occur. We compensate for PMD using polarization-maintaining fiber aligned with the PSP and characterize the PMD in a 55 km segment of real-world fiber connecting the University of Illinois at Urbana-Champaign to the nearby town of Rantoul. We show significantly different and time varying polarization transformations, even for wavelength differences of 1 nm. We then compensate for the PMD over this bandwidth, corresponding to the bandwidth of entanglement sources we wish to use in quantum networks. We present the comparative results of the polarization transform at wavelengths of 1559.5 nm and 1560.5 nm to confirm the impact of our PMD analysis and compensation system.   

Experimental storage of photonic polarization entanglement in a broadband loop-based quantum memory

We report on an experiment in which one member of a polarization-entangled photon pair is stored in an active "loop and switch" type quantum memory device, while the other propagates through a passive optical delay line. A comparison of Bell's inequality tests performed before and after the storage is used to investigate the ability of the memory to maintain entanglement and demonstrate a rudimentary entanglement distribution protocol. The entangled photons are produced by a conventional Spontaneous Parametric Down Conversion source with center wavelengths at 780 nm and bandwidths of ∼10 THz, while the memory has an even wider operational bandwidth that is enabled by the weakly dispersive nature of the Pockels effect used for polarization-insensitive switching in the loop-based quantum memory platform.   

A single-photon source based on cavity quantum electrodynamics considering the re-excitation problem

Single photons play a crucial role in quantum information processing and have a multitude of applications, from quantum computation to quantum communication. While there are several physical implementations of single-photon sources, the cavity QED system is a physical system that can generate photons on demand. In single-photon generation by this system, the use of a Λ-type three-level atom allows control of the photon waveform. In this method, there is a process that inhibits ideal photon generation due to the relaxation of excited atoms to the initial state, which degrades the performance of quantum information processing utilizing photons. To address this issue, we propose a novel photon generation scheme using a four-level atom.

We demonstrate that the relaxation process described above can be made arbitrarily small by manipulating experimentally tunable parameters.   

A scalable multi-partite high-dimensional quantum network for high-rate entanglement distribution and secure communications

Using wavelength and time multiplexing for quantum key distribution networks to serve multiple users over a single link received tremendous interest since it enables using a single link to communicate with multiple parties with minimal equipment. However, most of the recent implementations are suffering from rate limitations due to the usage of qubits carrying binary information, susceptibility against error, and noise with low source and detector efficiencies. Here, we have demonstrated a four-user network that uses high-dimensional arrival time bin encoding that can exceed state-of-the-art implementations in performance and work under high background noise. To overcome these limitations to achieve scalable quantum key distribution networks, we employed high-dimensional entanglement on an energy-time basis to encode multiple bits of information per entangled photon pair shared between two users. Furthermore, we employ nonlocal dispersion compensation modules to reduce the timing errors acquired during propagation over long distances and to monitor channel security using time and energy correlations.

Compared to similar works using wavelength and time, we obtained a gain in secure key rates over an order of magnitude to two orders of magnitude, up to 16 kbits/s at the source and 3.2 kbits/s at 21 km distance. We achieved a non-binary photon information efficiency of up to 2.137 bits/pair while withstanding a quantum bit error rate of 30%, where the binary protocols are susceptible to bit error rates above 10%. Using the dispersive basis for security measurements, we obtained a Holevo leakage only up to 0.3507 bits/pair. The protocol is robust against noisy channels, while it offers full and simultaneous connectivity among the users using a central entanglement provider. The network can be implemented with commercial telecommunications equipment where each user only needs a single channel and detector to be connected, reducing the cost further. Furthermore, the QKD implementation is also a figure of merit for other quantum communication tasks and can be used for high-fidelity entanglement distribution, superdense coding, teleportation, or distributed quantum computing.   

Advancements in Superconducting Quantum Networks for Scalability Beyond Single Cryostat

Superconducting quantum circuits have made remarkable progress in scalable quantum computation. However, the pursuit of scaling these circuits encounters challenges such as chip size limitations, cooling capacity constraints, and increasing wiring complexity. To address these issues, a distributed architecture emerges as a promising solution, enabling scalability beyond a single chip or cryostat, while emphasizing low-loss, long-distance connections. Recent achievements have extended the record to 30 meters [1].

In this talk, we will present our experimental progress in developing a superconducting network featuring a 64-meter-long channel [2]. Leveraging several key technical breakthroughs [3], we have achieved deterministic high-fidelity entangled pair generation and executed quantum teleportation across the network. Our findings not only demonstrate the feasibility of implementing long-distance quantum communication in superconducting architectures but also underscore the potential for realizing practical, large-scale quantum computation systems.

[1] S. Storz et al., Nature 617, 265 (2023).

[2] J. Qiu et al., arXiv:2302.08756.

[3] J. Niu et al., Nat. Electron. 6, 235 (2023).   

Fast Optical Switching for Quantum Information

Fast, low-loss optical switches are a key technology in facilitating fundamental applications such as high efficiency single-photon generation and quantum routing. Additionally, fast switches form an integral component in protocols such as time-bin encoding and long-distance entanglement swapping for quantum communication, and also enable the efficient creation of exotic photonic states for quantum metrology purposes. Electro-optic (EO) switching via the Pockels effect in bulk crystals is low loss but slow (up to ~10 MHz) due to kilovolt-level voltage demands. On-chip EO modulators operate in the few-volt regime (enabling GHz switching rates) but suffer from high insertion losses. An elegant alternative that can attain both low loss and up to THz switching rates is an all-optical approach that utilizes cross-phase modulation of a signal field by a pump field in an optical fiber. The effectiveness of this technique relies on the polarization of the pump being maintained during the pump-signal interaction, which can be implemented by using two equal-length segments of orthogonally-oriented polarization-maintaining fiber. Here, we present progress towards realizing such a device for telecom-wavelength single photons.   

Multimode Character of Quantum States Released from a Superconducting Cavity

Quantum state transfer by propagating wave packets of electromagnetic radiation requires tunable couplings between the sending and receiving quantum systems and the propagation channel or waveguide. The highest fidelity of state transfer in experimental demonstrations so far has been in superconducting circuits. Here, the tunability always comes together with nonlinear interactions, arising from the same Josephson junctions that enable the tunability. The resulting non-linear dynamics correlates the photon number and spatio-temporal degrees of freedom and leads to a multi-mode output state, for any multi-photon state. In this work, we study as a generic example the release of complex quantum states from a superconducting resonator, employing a flux tunable coupler to engineer and control the release process.

We quantify the multi-mode character of the output state and discuss how to optimize the fidelity of a quantum state transfer process with this in mind.   

Quantum Key Distribution Testbed For Acceleration of Secure Quantum Communication

Current developments in quantum computer technology and platforms are driving the need for secure networking environments that take advantage of the existing networking infrastructure that is in place today. As a result, we see a growing need to experiment and implement Quantum Key Distribution (QKD) methodologies utilizing the existing fiber networking systems. We present a novel test methodology that utilizes existing photonics hardware, coupled with single-photon detectors and quantum control software to enable designers to rapidly prototype their QKD designs. The presentation will focus on how the test bed can be used to evaluate quantum correction schemes as well as evaluate their quantum-based components for use in a classical based networks.   

Implementation of the RRDPS QKD protocol using time-domain Fourier transform via space-time duality

Quantum Key Distribution (QKD) is a secure communication method that provides protection against adversaries with unlimited power. The Round Robin Differential Phase Shift (RRDPS) QKD protocol has distinctive features that eliminate the need for signal disturbance monitoring, unlike other commonly used QKD protocols. Some experiments have shown that the RRDPS protocol is superior to the decoy-state BB84 protocol, particularly under high error rates.

In the RRDPS protocol, the pulse train containing phase information is divided into two parts. One part is randomly delayed to interfere with the other, and the interference signal is used to generate the key. A conventional method utilizes optical switches to achieve variable-delay interferometry. However, optical switches have a typical response time of microseconds, which restricts the key rate.

To address this issue, we have developed a new and simple optical circuit that uses Fourier transform and phase modulation to generate a variable delay to the pulse. This method eliminates the slow optical switches and boosts the transmission rate of the RRDPS protocol.

Furthermore, we use the space-time duality technique to implement the Fourier transform in the time domain, rather than the space domain. This technique offers a simple and cost-effective solution for electrical control of pulse delay, paving the way for practical applications.   

Hardware modelling in space-based quantum networks

A global quantum internet involves connecting quantum nodes separated by thousands of kilometers. A satellite-based quantum repeater network may offer significant advantages over a ground station-based network given that free-space photon propagation leads to only a polynomially decreasing transmission with distance compared to an exponential decrease in optical fibers [1,2].

To date, most works have considered hardware agnostic models of satellite-based quantum repeaters which has confirmed the promise of the approach but important questions regarding realistic hardware performance in space remain unanswered. Here, I will present a semi-analytical model of a satellite-based quantum network that takes into account hardware losses, both classical (link budget) and quantum (quantum memory time, imperfect qubit operations, etc.). The semi-analytical flavor of the model allows for easy adaptation to different situations and optimization of various elements such as satellite positions. Furthermore, I will present results on achievable quantum communication rates focusing on quantum hardware based on individually trapped neutral atoms which are particularly promising for satellite quantum payloads since they allow for laser cooling and have long coherence times.

[1] C. Liorni et al., “Quantum repeaters in space”, New Journal of Physics, vol.23, 2021

[2] J. Wallnoffer et al., “Simulating quantum repeater strategies for multiple satellites”, Communications Physics, vol. 5, 2022."   

A quantum switch for itinerant microwave single photons with superconducting quantum circuits

A classical switch determines which of several inputs to the device passes through the output [1]. In analogy, a quantum switch that coherently links the input to a superposition of several outputs plays an important role in QIP, especially when operating at the fundamental limit where a single photon can be controlled [2]. In this work, we realize such a quantum switch in the microwave regime with superconducting quantum circuits. A microwave cavity, dispersively coupled with a transmon qubit, is used to coherently switch the input single microwave photons emitted from another superconducting circuit. We show that the propagation path of single photons can be coherently manipulated by the qubit and confirm the existence of entanglement between the qubit and the photons’ propagation path. We further demonstrate that the quantum switch can effectively generate heralded entanglement between the qubit and the propagating photonic qubit, thus providing a versatile approach to generate reconfigurable multi-node entanglement in one step. As such, our work provides a prime building block for microwave quantum networks and modular superconducting quantum computing.

[1] M. Pechal, et. al., Physical Review Applied 6, 024009 (2016).

[2] H. J. Kimble, Nature 453, 1023 (2018).