Linear optics quantum information (LOQI) is a branch of quantum information science that leverages photons—the fundamental particles of light—as carriers of quantum information by encoding qubits in their quantum states. By manipulating these photons with linear optical components such as beamsplitters and wave plates, researchers can perform quantum operations crucial to quantum computing, secure communication, and enhanced metrology. Photons are particularly well-suited to quantum communication due to their low decoherence over distances, setting LOQI apart from other quantum technologies that rely on atoms or superconducting circuits. This quality makes LOQI a central component in building scalable and practical quantum networks and information systems.

In addressing the need for secure and efficient information transmission, LOQI offers a distinct advantage over classical systems, which are susceptible to eavesdropping. Quantum communication protocols, like quantum key distribution (QKD), leverage LOQI’s properties to ensure secure data transmission based on the principles of quantum mechanics. Additionally, LOQI provides a photonic-based platform for quantum computing, enabling scalable and integrative solutions suitable for larger quantum networks and enhancing the potential of photonic quantum processing.

LOQI experiments typically involve creating photons and directing them through optical setups to manipulate their quantum states. Essential to these setups are single-photon detectors, such as single-photon avalanche photodiodes (SPADs) and superconducting nanowire single-photon detectors (SNSPDs), and precise timing electronics, such as Swabian Instruments’ Time Tagger, to track photon arrival times and measure correlations. Timing accuracy at picosecond or sub-nanosecond levels is critical in LOQI for examining quantum interference and entanglement characteristics. An additional critical capability is photon number resolution (PNR), which plays a pivotal role in these experiments. PNR, achieved through timing-based discrimination or the intrinsic multi-photon sensitivity of SNSPDs, allows researchers to detect and characterize multiphoton events. This capability enhances the fidelity of quantum measurements and is essential for investigating quantum interference, entanglement, and state reconstruction.

Figure 1 depicts a typical setup for linear optics quantum information experiments involving several key components and processes, each designed to generate, manipulate, and detect photons in a way that preserves and measures their quantum states. The setup includes a photon source, typically a laser at low power producing single photons or a nonlinear crysta generating entangled photon pairs through processes such as spontaneous parametric down-conversion (SPDC) or four-wave mixing (FWM). The photons are then directed through various linear optical elements, such as beamsplitters, polarizers, wave plates, and phase shifters, which enable precise manipulation of the photons’ polarization, phase, or path-based quantum states. After this, an interferometer, such as a Mach-Zehnder or a Hong-Ou-Mandel (HOM) interferometer, is often incorporated into the setup to observe quantum interference effects by splitting and recombining photon paths, leading to interference patterns that reveal information about the quantum state. At the end of the optical path, single-photon detectors, typically single-photon avalanche photodiodes (SPADs) and superconducting nanowire single-photon detectors (SNSPDs), are used to detect the arrival of single photons with very high quantum efficiency, low dark counts, and high timing resolution.

The photons’ arrival time has to be recorded with picosecond-level precision. This timing data is essential for analyzing time-correlated events, such as entangled photon pairs or photon coincidences, that verify quantum properties. Advanced time-to-digital converters (TDCs), like Swabian Instruments’ Time Tagger, allow for precise and simultaneous measurement of the photons’ arrival time from multiple detectors.

The critical hardware characteristics of linear optics quantum information experiments:

• Timing accuracy and precision. The lower the timing jitter of the timing electronics is, the better the resolution and precision of the measurement will be. Sub-nanosecond or even picosecond timing resolution is often required to distinguish closely spaced photon events.
• Photon Number Resolution (PNR). Experiments in LOQI increasingly rely on (pseudo-)PNR to distinguish between single-photon and multiphoton states. Picosecond timing resolution on multiple independent channels is required to leverage the intrinsic PNR capability of SNSPDs and/or to implement pseudo-resolution techniques based on precise timing data.
• High photon count rates. HMinimizing dead time is crucial to avoid data loss, particularly when working with high photon count rates and very low dead time detectors, like SNSPDs, which can have a dead time as low as 10 ns.
• Flexible setup with many-channels measurements. Linear optics quantum information experiments incorporate numerous detectors to identify multi-coincidence events. The ability to easily scale the setup by adding more detectors is a key feature, allowing for greater complexity in potential LOQI experiments.