Quantum Communication

Figure 1. Schematic example of a qantum 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.

What is quantum key distribution (QKD)?

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 (LOQI) 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.

Timing electronics required for quantum communication

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:

Timing Jitter. Lower electronic timing jitter directly improves measurement resolution and precision. To maintain optimal performance, it is essential to consider the jitter from SNSPDs, typically around a few ps (RMS), and ensure that the timing electronics have jitter levels below this threshold.

Dead Time. Minimizing dead time is critical to avoid data loss, particularly in high photon count rate experiments. With SNSPDs currently achieving a minimum dead time of around 10 ns, Swabian Instruments Time Taggers offer significantly shorter dead times, ensuring seamless data acquisition even in advanced experimental setups.

Data Transfer Rate. Higher data transfer rates enable the collection of more photon events within shorter timeframes, boosting the statistical accuracy of measurements. If data transfer rates are insufficient, overflow and data loss can occur. A robust data filtering system, such as the one offered by Swabian Instruments, prevents data loss and ensures efficient data acquisition.

Additional advantages of high-performance timing electronics:

Remote Synchronization.The ability to find coincidences between the events measured by Alice and Bob is often critical to extracting meaningful data. It requires that their measurements share a precisely aligned time base, even if they are separated by many kilometers. Tools like GPS disciplined oscillators (GPSDOs) and the White Rabbit (WR) protocol enable nanosecond-level synchronization over metropolitan distances.

Time-Bin and Phase-Based QKD. Accurate timing ensures precise quantum state measurement, enhancing both data rates and signal fidelity.

Photon Number Resolution. High timing resolution allows for advanced analysis, such as resolving photon numbers based on SNSPD output pulse rising edge shapes which depends on the number of detected photons.

The Time Tagger series also includes software with direct access to time tag streams via USB 3.0, delivering up to 90 Mtags/s bandwidth. Powerful analysis tools, including coincidence detection, correlation measurements, and conditional filtering, help users optimize their systems and achieve the highest possible performance.

Swabian Instruments’ Time Taggers - an advanced solution for QKD

Versatile & Intuitive Software Engine

Time Tagger software allows for direct access to the time tag stream, 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.

Remote Synchronization

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

Clock synchronization is often essential when comparing remote measurements. The Time Tagger series offers multiple synchronization tools, such as SoftwareClock and PulsePerSecondMonitor. These, together with TimeTaggerNetwork and White Rabbit nodes, can be used to synchronize measurements with subnanosecond 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.

High-Performing & Scalable Hardware

Swabian Instruments Time Tagger Series offer state-of-the-art time-to-digital converters optimized for single photon counting. It offers:

Low timing jitter. The Time Tagger Ultra (down to 3 ps) and the Time Tagger X (1.5 ps) offer superb performance, which combines well with even the world’s best commercially available SNSPDs.
Exceptional Channel Capacity Supports up to 20 channels (Time Tagger Ultra and Time Tagger X), enabling complex and scalable experimental setups.
Seamless Expansion Effortlessly combine multiple Time Taggers using our Synchronizer to expand input capacity without compromising performance. Leverage the benefits of the rack mount kit with two synchronized Time Tagger Ultra.
Superior Compatibility Optimized for high-end single photon detection systems, ensuring maximum precision and reliability.

Relevant news

The First Photonic Quantum Computer in Germany - Swabian Instruments' Role in this Breakthrough

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!

Remote Synchronization of Time Taggers

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.