Quantum network devices face a set of requirements defined by laws of quantum physics. These requirements are unique to quantum communications and are technologically challenging. To ensure a sustainable path forward, major technological goals need to be identified not only for each component type but for the network as a whole. This work requires a holistic understanding of how components fit into quantum networks.
At this stage, when quantum networks are limited to simple configurations with highly focused experimental goals, such an understanding requires analysis of case studies of the quantum network applications under different assumptions. In the future, much larger and technically diverse quantum networks will be developed, and far more advanced testbed-based metrology will be required. The metrological effort spans multiple NIST operating units (OUs). Quantum network metrology will greatly accelerate the development of compatible heterogenous quantum network systems and ultimately the realization of a globally optimized quantum internet.
To aid component development, the metrology community develops a characterization framework. The characterization of some components predates the quantum network efforts, while some other quantum network-specific components are new. For instance, a range of quantum sources of light and faint-light detectors have been available prior to quantum networking. NIST has world-class experience in characterization of quantum states of light, including entanglement characterization.
NIST also calibrates photon-counting detectors. These practical devices are complex and cannot be fully characterized with just one parameter. The specific advantages and disadvantages of these devices are application-specific. Therefore, NIST is currently identifying and specifying key performance metrics that would make these devices most useful as quantum network components. Concurrently, best measurement practices are being researched. We envision dissemination of these practices and deployable metrology-quality reference standards for use by other institutions and industry.
The development of other network components, such as transducers and quantum memories, is in its infancy. Proper metrology efforts aid development and will ensure successful integration of new technologies with existing quantum networking hardware. We use component prototypes and network simulation to develop characterization techniques.
Sources of single photons based on different technologies are developed and characterized at NIST. Here we focus on characterization of parameters that are common to most sources, although capabilities exist to also characterize parameters specific to a particular underlying technology.
Single-photon emission probability: The single photon emission probability characterizes the probability of exactly one photon in a well-defined mode at a given time. It is measured using our well-characterized, high-efficiency single-photon detectors.
Spectral/Spatial mode structure: In addition to well identified parameters, such as wavelength, bandwidth, temporal, and spatial characteristics, the mode structure of the single photon output determines the possible use of the source. In the context of networking, a single spatial mode (i.e. confinement to a single mode fiber) is required. In addition, photons in a single temporal mode may be preferred for protocols requiring indistinguishable photons. We measure mode structure with different techniques including dispersion-based single-photon spectrometers, compressive imaging, photon-number resolved counting, and others.
Photon-number statistics: It is important for many applications that the output of a single photon source emits no two-photon or multiphoton states, or as few as possible. Sources are characterized using their photon statistics and purity of photon number using transition edge sensors and multiplexed click detectors.
Indistinguishability: Some network protocols require single photon interference (a.k.a. Hong-Ou-Mandel interference). The degree of such interference determines how indistinguishable the states are. When two photon states are fully indistinguishable, the interference leads to a full coalescence of single photon states to a state of two photons, whereas partially indistinguishable states do not fully coalesce.
Detectors also are based on different underlying technologies. Below we list device-independent parameters that are most important for quantum networks. NIST scientists established characterization methods for those parameters. Note that in some special cases, device-specific characterization may be more appropriate. Some device-specific characterization methods were developed at NIST.
First-order effects (detection efficiency, dark counts): Detection efficiency is measured using detector substitution and correlated (Klychko) method. Dark counts are measured in the absence of background radiation, including blackbody radiation. Both detection efficiency and dark counts are defined for a detector that is fully armed; therefore the history of prior detections is irrelevant.
Second-order effects (afterpulsing/recovery characterization): The transient process that occurs after a photon detection includes a recovery (dead) time and afterpulsing; these phenomena depend on the prior detection history of the detector. We measure these effects using the second-order assumption (i.e., that these effects can be fully described by the time since the last detection). We verify this assumption by measuring higher-order detection history events.
Latency and timing jitter: A measurable classical signal appears with a delay with respect to the absorption of a photon at the detector. We characterize the latency and timing jitter of single-photon detectors with picosecond accuracy.
Detector tomography: We perform photon-number resolving detector tomography to estimate each detector's positive-operator valued measure (POVM). With an accurate estimate of a detector's POVM and sufficient data, detectors with significant imperfections can be used to accurately characterize quantum states of light.
Future development and growth of heterogenous multi-mode quantum networks will present unique metrological challenges. For instance, because the complexity of quantum state tomography scales exponentially with the number of nodes in a system, advanced nonclassical metrology of the network as a complete system may be required. Other issues, such as synchronization of multiple remote nodes and co-existence of classical and quantum states in the network, are the examples of classical but technologically hard problems.
In anticipation of these needs, we are exploring the new holistic characterization methods and the required underlying technology through theoretical research, simulation, and emulation. We expect that this research will produce guidance on the required quality and quantity of network probing components. We will also determine which network metrology and monitoring tasks require classical measurements and when quantum methods should be used. The understanding of the practical network conditions and limitations is important for future network development and growth. For instance, such understanding will help in developing entanglement distillation schemes to overcome loss, incorporating hybrid continuous/discrete variable approaches to enable non-Gaussian detection schemes, and establishing a workable path for multipartite entanglement distribution.
At the same time, an advanced quantum measurement node is envisioned. When a network is equipped with such a node, real-time, in-situ characterization of network conditions can be enabled. In addition to remote characterization of other nodes, such an advanced measurement tool could evaluate how the network performs at hard tasks, such as multipartite entanglement distribution, multipartite distributed quantum measurement, and distributed quantum computation.
PI contacts: Thomas Gerrits, Sae Woo Nam, Varun Verma, Adriana Lita, John Lehman, Michelle Stephens, Marty Stevens, Rich Mirin, Kevin Silverman, Alan Migdall, Sergey Polyakov, Josh Bienfang, Zachary Levine, Oliver Slattery, Lijun Ma, Paulina Kuo, Scott Glancy, Manny Knill, Jolene Splett, Kevin Coakley