Figure 1. Schematic example of a Quantum Key Distribution setup between two parties (Alice and Bob) using Swabian Instruments Time Taggers. Each party has the ability to detect single photons transmitted through the quantum channel (red) using superconducting nanowire single photon detectors (SNSPDs). The SNSPD output clicks are digitized at high rates and very low jitter using a Time Tagger X.
Quantum communication describes cutting-edge techniques to distribute information using the quantum properties of single photons. One of the most important applications is providing secure channels of communication using a technique known as quantum key distribution (QKD). Unlike classical public-key cryptography, which is susceptible to decryption by future quantum computing-based decryption algorithms, QKD offers unconditional security. Thanks to the quantum properties of photons, such as polarization and phase, the communicating parties can be alerted when any eavesdropper disturbs the photon's state.
Many quantum communication protocols have been developed, but in the most generalized form, they involve at least two parties (Alice and Bob) who want to exchange information and who can establish both quantum and classical communication channels. Protocols explicitly take into account that an eavesdropper (Eve) may have full access to both channels but is nevertheless unable to extract the secret key that Alice and Bob are exchanging. The quantum channel typically requires a single photon source at either party or an entangled photon source to which both parties have access. Photons can either be transferred by optical fiber, through free space communication systems, or via space using QKD satellites.
The performance of the quantum channel is often quantified in terms of the secret key rate, which is limited by channel losses, the rate at which the source can produce single photons in random states, the efficiency of the single photon detectors, and the implemented protocol. Each of these presents significant challenges for real-world applications, and the high cost of single photon generation and detection. The lack of quantum repeaters for long-distance communication, as well as the lack of protocol standardization prevent the global implementation of the QKD, despite its enormous potential.
Quantum communication is closely related to Linear Optics Quantum Information and has many other applications beyond QKD, for example, in secure voting, networks of quantum sensors, and the exchange of quantum information between quantum computers. By transferring qubit states between geographically separated quantum computers with high fidelity, quantum communication may enable distributed quantum computation in the future.
In practical quantum key distribution (QKD) protocols, detecting high rates of incoming single photons with precision is crucial. This requires advanced single-photon detection and counting capabilities, supported by robust time-tagging hardware.
An example of a modern QKD system is shown in Figure 1. Alice and Bob detect photons with superconducting nanowire single-photon detectors (SNSPDs) or single-photon avalanche diodes (SPADs), both of which have recently seen significant improvements in performance. To match these advancements, time-tagging hardware has evolved in parallel with the characteristics of these single-photon detectors. Swabian Instruments' Time Tagger offers low noise, high data rates, and precision to fully leverage detector capabilities and maximize the secret key rate.
The critical hardware characteristics of a time-to-digital converter required for QKD experiments:
Additional advantages of high-performance timing electronics:
Time Tagger software allows for direct access to the time tag stream, which is only limited by the bandwidth of USB 3.0 (~90 Mtags/s), which connects the Time Tagger hardware to the measurement PC. Our flexible and highly performant API offers many powerful tools to help you analyze your data, such as coincidence, conditional filters, dead times, and delays. These help to avoid dark counts, detect afterpulsing, and extract the maximum amount of useful signal from your data.
Quantum communication frequently involves performing measurements at remotely separate locations. The Time Tagger series offers a powerful tool: TimeTaggerNetwork. You can flexibly control servers from a central location and stream data over the internet. Data taken at multiple locations can be merged, compared with local measurements, and analyzed using our powerful API.
Clock synchronization is often essential when comparing remote measurements. The Time Tagger series offers multiple synchronization tools, such as `ReferenceClock` and PulsePerSecondMonitor. These, together with TimeTaggerNetwork and White Rabbit nodes, can be used to synchronize measurements with nanosecond accuracy and picosecond jitter levels. As our API is not tied to any particular synchronization hardware, GPSDO may alternatively be used to synchronize remote time taggers remotely.
Swabian Instruments Time Tagger series offers state-of-the-art time-to-digital converters optimized for single photon counting. It offers:
The first photonic quantum computer built in Paderborn, Germany! At Swabian Instruments, we’re proud to have enabled this breakthrough with our software-based Photon Number Resolution (PNR) on single SNSPDs!
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Key breakthroughs in quantum networking, science, telecommunications, finance, and computing rely on precise synchronization of time measurements across multiple locations. Swabian Instruments has demonstrated remote synchronization of Time Taggers leveraging Safran White Rabbit network nodes to achieve measurements with high timing resolution and picosecond precision.
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