One-Way Quantum Repeater Based on Near-Deterministic Photon-Emitter Interfaces
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-06-30 |
| Journal | Physical Review X |
| Authors | Johannes Borregaard, Hannes Pichler, Tim Schröder, Mikhail D. Lukin, Peter Lodahl |
| Institutions | Harvard University, University of Copenhagen |
| Citations | 141 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Novel Architecture: Proposes a one-way quantum repeater utilizing photonic tree-cluster states, designed to overcome the rate limitations imposed by two-way communication time in conventional repeaters.
- Minimal Resource Overhead: The architecture requires only two stationary memory spin qubits and one quantum emitter per repeater station, a reduction of two orders of magnitude compared to previous matter-based one-way protocols.
- High Communication Rate: Achieves projected secret bit rates up to ~70 kHz over 1000 km, provided fast spin-spin Controlled-Phase (CZ) gates (10 ns) and high detection efficiency (95%) are met.
- Near-Deterministic Interface: Protocol relies on solid-state quantum emitters (e.g., quantum dots or SiV/NV centers) strongly coupled to nanophotonic cavities, requiring a high cooperativity factor (C = 100) for low operational errors (Δr < 0.1%).
- High-Speed Photonics Required: Implementation necessitates integrated electro-optic photonic circuits capable of fast switching rates (> 2 GHz) for time-bin qubit measurement and routing.
- Loss Tolerance: The tree-cluster encoding provides robustness against transmission loss, significantly increasing the effective transmission probability compared to single-photon transmission.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Target Distance (L) | 1000 | km | Maximum simulated communication distance. |
| Secret Bit Rate (Fast Gate) | ~70 | kHz | Achieved over 1000 km using 10 ns CZ gates. |
| Secret Bit Rate (Modest Gate) | ~13 | kHz | Achieved over 1000 km using 100 ns CZ gates. |
| Repeater Station Spacing (Lo) | 2.6 | km | Optimized spacing for 10 ns TCZ, 1000 km total distance. |
| Required Detection Efficiency (ηd) | 95 | % | Includes coupling and frequency conversion efficiency. |
| Tolerable Re-encoding Error (Δr) | 0.1 | % | Maximum operational error rate for optimal performance. |
| Spin-Spin CZ Gate Time (TCZ) | 10 to 100 | ns | Range of feasible gate times for solid-state systems. |
| Photon Emission Time (Tph) | 1 | ns | Target time based on Purcell enhancement (100 ps lifetime). |
| Required Cooperativity (C) | 100 | - | Needed for spin-photon CZ gate error less than 10-4. |
| Required Optical Switching Rate | > 2 | GHz | Necessary for fast time-bin qubit routing and measurement. |
| Maximum Fiber Delay Length (ldel) | ~374 | m | Required for detection order sequencing (100 ns TCZ case). |
| Attenuation Length (Latt) | 20 | km | Standard telecom optical fiber loss assumption. |
| Memory Qubits per Station | 2 | - | Stationary spin systems (S1, S2). |
| Quantum Emitters per Station | 1 | - | Cavity-coupled emitter (E). |
Key Methodologies
Section titled âKey Methodologiesâ- Deterministic Tree-Cluster Generation: Photonic tree-cluster states are generated sequentially using repeated excitation of a single quantum emitter (E) coupled to a cavity, mediated by Controlled-Phase (CZ) gates involving two stationary memory spin qubits (S1, S2).
- Spin-Photon CZ Gate: The core re-encoding operation uses a cavity-mediated spin-photon CZ gate. The photonic time-bin qubit state (0 or 1) is reflected off the cavity, acquiring a Ï-phase shift conditional on the state of an auxiliary spin qubit, requiring C = 100.
- Heralded Loss-Tolerant Re-encoding: The message qubit is transferred from the incoming photon to an auxiliary spin qubit. This transfer is heralded by detecting the photon in the x-basis. If the photon is lost, the operation aborts without perturbing the root qubit of the new tree, allowing immediate re-attempt with the next incoming photon.
- High-Speed Photonic Circuitry: Measurements of photonic qubits in the x-basis require integrated Mach-Zehnder Interferometers (MZIs) utilizing electro-optic (EO) modulation to achieve switching rates exceeding 2 GHz.
- Time-Bin Encoding: Qubits are encoded in the presence of a single photon in one of two non-overlapping spatiotemporal modes (time bins). This encoding is preferred over polarization encoding for robust transmission through optical fibers.
- Modular Hybrid Integration: The proposed repeater station architecture involves a hybrid photonic platform consisting of multiple chips: one hosting stationary qubits, one for electro-optic routing and frequency conversion (to telecom C-band), and separate chips for 1st-level and 2nd/3rd-level photon detection.
Commercial Applications
Section titled âCommercial Applicationsâ- Long-Distance Quantum Key Distribution (QKD): Provides a high-rate, scalable solution for unconditionally secure communication over existing telecom fiber infrastructure, overcoming the distance and rate limitations of current QKD systems.
- Quantum Network Backbone: Serves as a critical component for building continental-scale quantum networks, enabling the linkage of distributed quantum processors and sensors.
- Solid-State Quantum Emitter Development: Drives the engineering requirements for high-performance solid-state systems:
- Quantum dots or SiV/NV centers with high Purcell enhancement (Tph ~ 1 ns).
- High-efficiency chip-to-fiber coupling (> 95%).
- Long-coherence spin memories (nuclear spins) operating at low temperatures.
- Integrated Photonics and EO Modulation: Requires and advances the technology for fast, integrated electro-optic switches and MZIs operating in the GHz regime, relevant for high-speed classical and quantum optical processing.
- Frequency Conversion: Necessitates efficient frequency conversion modules to transduce the emitted photons (often visible/near-IR) to the low-loss telecom C-band (1550 nm).
View Original Abstract
We propose a novel one-way quantum repeater architecture based on photonic\ntree-cluster states. Encoding a qubit in a photonic tree-cluster protects the\ninformation from transmission loss and enables long-range quantum communication\nthrough a chain of repeater stations. As opposed to conventional approaches\nthat are limited by the two-way communication time, the overall transmission\nrate of the current quantum repeater protocol is determined by the local\nprocessing time enabling very high communication rates. We further show that\nsuch a repeater can be constructed with as little as two stationary qubits and\none quantum emitter per repeater station, which significantly increases the\nexperimental feasibility. We discuss potential implementations with diamond\ndefect centers and semiconductor quantum dots efficiently coupled to photonic\nnanostructures and outline how such systems may be integrated into repeater\nstations.\n