In the Optiwave ecosystem, OptiSystem plays a particularly important role here, offering components and examples designed for modelling Quantum Key Distribution (QKD) systems. Lambda Research's tools, above all OSLO and TracePro, can complement such a process on the classical-optics side: designing optical paths, lens systems, coupling, geometric losses and light propagation in real opto-mechanical assemblies. This does not mean, however, that every one of these tools is a full quantum-mechanics simulator — correctly identifying the level of the model is what matters.
What does "simulating a quantum system" actually mean?
In photonics, this term can cover very different tasks: modelling a single-photon source, transmission of weak optical pulses, encoding information in polarization, single-photon detection, analysis of a QKD protocol, propagation through a fiber, transmission through the atmosphere or water, design of the input and receiver optics, and analysis of coupling to a photonic integrated circuit. Not all of these problems require solving the full quantum equation for the entire system.
In engineering practice, a hierarchy of models is very often used: photonic component → optical path → transmission channel → detection → quantum protocol. Each level may require a different tool.
QKD — one of the most practical applications of quantum photonics simulation
One of the areas where system-level simulation already has a concrete application is Quantum Key Distribution. QKD makes it possible to distribute a cryptographic key using the properties of quantum systems. In the classical picture, information is not sent as ordinary power levels interpreted as logical 0s and 1s — an optical state can instead be prepared and measured according to the rules of a specific quantum protocol.
Optiwave currently publishes materials showing the simulation of various QKD algorithms in OptiSystem. Among other things, the vendor lists components such as a Single Photon Pulsed Laser, a Single Photon Detector, a Pulse Polarization Meter and a Polarization Analyzer. This is a significant change compared with older versions of the software — the vendor's archived answers from over a decade ago still mentioned the lack of dedicated QKD components, whereas current versions of OptiSystem have been extended with tools and examples aimed at these applications.
What can a QKD system model look like?
At the system level it can be represented as: data generator → optical source → state preparation → channel → state analysis → detector → key analysis. Depending on the protocol, the individual blocks will look different.
A simulation can be used to study, among other things, the effect of channel losses, the type of transmission medium, the polarization state, source parameters, detector characteristics and transmission errors. What matters most, however, is keeping the correct level of interpretation: a system-level model is primarily meant to study the behaviour of the complete transmission architecture.
A single photon changes how you think about the system
In classical optical telecommunications, we often analyze the signal power, P(t), or the pulse energy. In systems operating at the single-photon level, however, what matters is detection events and their statistics. The energy of a single photon is:
E = hν = hc/λ
where h is Planck's constant, ν is the frequency, c is the speed of light, and λ is the wavelength. For the wavelengths used in telecommunications, the energy of a single photon is very small. That is why even small losses, noise and detector errors can matter far more than in a classical link with a high optical power.
Single-photon detector
The detector is one of the most important elements of a quantum system. An ideal detector would always respond when a photon reaches it, and never when there is no photon. A real device is more complex — depending on the technology, you need to account for detection efficiency, dark counts, response time, the detector's timing limitations, and the probability of a false detection. This is why knowing the power reaching the receiver alone is not enough to correctly assess how a quantum system performs.
Polarization as an information carrier
In many quantum experiments and protocols, the state of the information can be tied to the photon's polarization. In that case, preparing the right state, changes to polarization in the channel, polarization elements, and measuring the state at the receiver all become particularly important. OptiSystem has tools for modelling and analyzing polarization, which makes it possible to build system-level QKD transmission models that use this degree of freedom. The vendor demonstrates this application directly in its training materials on QKD algorithms.
A quantum channel doesn't have to be a fiber
One of the more interesting aspects of the models presented by Optiwave is the ability to study different transmission channels. The vendor lists single-mode fiber, multi-mode fiber, free-space transmission and underwater channels. This means the same QKD problem can be considered for different propagation environments — which matters, because completely different constraints appear in a fiber than in an FSO link.
QKD over fiber
For a fiber channel, one of the main problems is attenuation. If transmission takes place over a long distance, the number of photons reaching the receiver decreases. In classical telecommunications you can try to solve this with an amplifier; in a quantum system the situation is more complicated, because a quantum state cannot simply be copied and amplified without restriction the way a classical signal can. That is why the loss budget becomes one of the key elements of the design. OptiFiber can be used for the earlier analysis of the fiber's own properties, and the behaviour of the complete channel can then be analyzed in OptiSystem.
QKD over free space
The second class of systems is Free-Space Optical channels. Here, besides the transmission of information itself, you need to design the spatial path: source → collimator → transmitter → free space → receiving telescope → detector. Problems such as beam divergence, receiver aperture, axis alignment, geometric losses, atmospheric turbulence, background light and the effect of sunlight all come into play. OptiSystem can be used for system-level analysis of the channel, while at the level of the actual receiver optics, Lambda Research's tools become useful.
OSLO — designing the classical optics for a quantum system
A photonic quantum system still needs classical optical elements. A single-photon detector may need a lens system or fiber coupling, and an FSO transmitter may need a collimator. An experimental system may include lenses, mirrors, beam splitters, apertures, filters and telescopes. Lambda Research's OSLO is used to design and optimize such optical systems — Lambda Research describes OSLO as an environment for professional lens and optical-system design. In practice, OSLO can be used to design a path whose job is to deliver light as efficiently as possible to the next element of the quantum system.
TracePro — where do the photons actually end up?
A different problem appears once a real assembly has been built. An idealized model might assume: source → lens → detector. In reality, light can reflect off the housing, windows, mounts, edges, filters and mechanical surfaces. For a high-power system, a small parasitic signal may be negligible; in a single-photon system, the situation can be completely different. If the receiver is meant to respond to a very small number of photons, even very weak unwanted light can affect the measurement. TracePro uses ray tracing to analyze real optical systems and lets you account for absorption, reflection, transmission and scattering of light, which is why it can be useful when designing the opto-mechanical part of quantum apparatus.
Stray light in quantum systems
In a classical design, stray light often affects contrast, SNR or image quality. In systems detecting very weak signals, it can become an even more serious problem. Light coming from a pump laser, optical electronics, external illumination, or reflections off the enclosure walls can reach the detector via an unwanted path. TracePro lets you analyze such paths and identify the surfaces responsible for an unwanted optical flux — the software has tools for analyzing stray light, scattering, polarization and energy distributions in a complete geometry. This is not a simulation of a quantum state, but it is a very important part of designing a real quantum-optical experiment.
Coupling light
Another critical location is the free-space↔fiber or free-space↔photonic-chip interface. Coupling losses reduce the number of photons available further along the experiment. That is why quantum designs may require especially careful optimization of beam diameter, aperture, focal length, focus position, fiber position and alignment tolerances. OSLO and TracePro can complement Optiwave's tools here, which handle propagation and system-level analysis.
Photonic integrated circuits for quantum technologies
Another direction of development is integrating quantum functions onto a chip. A photonic integrated circuit can include waveguides, splitters, couplers, interferometers, resonators, modulators and coupling structures. At the level of these components' geometry, electromagnetic solvers are needed. In the Optiwave ecosystem, this role is filled by tools such as OptiOmega and other solutions for modelling photonic structures — OptiOmega is currently being developed as an environment that uses, among other methods, FDTD, FEFD and mode analysis to design photonic integrated circuits.
It is worth restating the distinction, however: an electromagnetic solver determines the classical electromagnetic fields and characteristics of a component. This does not automatically mean a full simulation of a many-body quantum state.
Why this distinction matters
In marketing materials, terms like "quantum photonics" and "quantum simulation" can easily be used too broadly. In reality, there are three quite different levels. The first is the classical physics of a component — for example computing a waveguide mode or a coupler's transmission. The second is a photonic system that includes models of quantum elements — for example QKD with a single-photon source and detector. The third is a full simulation of quantum-state dynamics — for example the evolution of a multi-photon entangled state in Hilbert space.
OptiSystem is particularly relevant at the second level. OSLO and TracePro work mainly at the level of the classical optics of the whole path, while Optiwave's electromagnetic tools let you design the physical components. OSLO or TracePro should therefore not be presented as quantum-mechanics solvers, nor should every model built in OptiSystem be assumed to be equivalent to a full quantum simulation.
From component to QKD system
A practical design process can be built hierarchically:
- Photonic component — designing a waveguide, coupler, resonator or other element.
- Classical optics — designing lenses, collimators, coupling and apertures in OSLO.
- Full opto-mechanical geometry — analyzing losses, unwanted paths and stray light in TracePro.
- Quantum transmission system — sources, channels, polarization and detectors in OptiSystem.
- Experiment — comparing the simulation with a measurement of the real system.
This approach lets you separate problems that are not worth solving with a single, enormous model.
Simulating the fiber, or the whole protocol?
If the question is "how does the propagating mode change?", you need a component-level model. If the question is "how much signal gets through the lens system?", you need an optical analysis. If the question is "will an unwanted reflection reach the detector?", you need ray tracing. But if the question is "how do the channel's parameters affect the QKD system's performance?", then OptiSystem is the right level. Choosing the tool should always follow from the design question, not simply from the fact that a device is labelled "quantum".
Simulation before the experiment
Experiments using single photons can require costly apparatus and precise alignment. Simulation lets you check in advance the loss budget, the range of parameters, the effect of the channel, sensitivity to polarization, coupling efficiency, possible stray-light paths and the detector's requirements. It does not eliminate the experiment, but it lets you define its conditions more precisely. This is exactly where design tools deliver the most value — they reduce the number of iterations that have to be done on the physical bench.
New horizons in quantum photonics
Optiwave is clearly developing OptiSystem toward QKD applications. In the OptiSystem 21.1 release, the vendor listed QKD as one of the application areas for new examples and features, alongside FSO, LiDAR and sensing systems. Developing such models matters, because quantum photonics is gradually moving from individual experiments to increasingly complex engineering systems. And with that come the same problems we already know from classical photonics: losses, tolerances, coupling, polarization, opto-mechanical design, detector characterization, validation and the integration of many components. The difference is that the power margin is often much smaller, and how the signal is interpreted can follow directly from the laws of quantum physics.
Summary
Simulating quantum systems does not mean a single type of calculation. A QKD project, or any other photonic quantum system, can require, all at once: a component-level solver → optical design → ray-tracing analysis → system-level simulation → experiment. In Optiwave's solutions, the most important tool at the level of QKD systems is OptiSystem, which has dedicated components related to single-photon sources and detection, polarization analysis, and different types of transmission channels. OptiOmega and Optiwave's other component-level tools can be used to design the physical photonic structures that later become part of such a system.
OSLO and TracePro from Lambda Research, in turn, are useful for designing the classical optical part: lenses, collimators, apertures, coupling and complete opto-mechanics. TracePro additionally lets you analyze scattering and stray light, which can be especially important in single-photon systems.
The new horizon, then, is not just "simulating quantum mechanics". It is the ability to design increasingly complete devices, in which quantum physics, classical photonics and real opto-mechanics come together in a single engineering process.