Quantum Optics Educational Kits

Quantum Optics Educational Kits

• Designed for Educational, Demonstration, and Classroom Use
• Complete Photonics Kit Includes Hardware, Tools, and Software (Computer Not Included)
• Extensive Manual and Teaching Materials Provided
• Easy to Assemble and Use
• Choose from Educational Kits Containing Imperial or Metric Components

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The kit contains several methods and parts that ensure a student-friendly alignment process, including an axicon. With the alignment laser, the axicon generates a light cone with the same opening angle as the photon-pairs, thus helping greatly in alignment. This image shows how the aligned detectors stand in the laser light cone.

Thorlabs Educational Products

Thorlabs' line of educational products aims to promote physics, optics, and photonics by covering many classic experiments, as well as emerging fields of research. Each educational kit includes a manual that contains both detailed setup instructions and extensive teaching materials. These lab kits are being offered at the price of the included components, with the educational materials offered for free. Technical support from our educational team is available both before and after purchase.

Purchasing Note: Both English and German language manuals are available for our educational kits. The imperial educational kit contains the English manual and US-style power cords. The appropriate manual and power cords will be included in the metric kit based on your shipping location. The power supplies and other electronic devices in both the metric and the imperial kit accept voltages of 230 VAC and 120 VAC. Please contact Tech Support if you would like the German manual included with the imperial version of the kit. As with all products on our website, taxes are not included in the price shown below.

Laser Safety

Thorlabs' EDU-QOP1(/M) and EDU-QOPA1(/M) kits use the L405P20 laser diode, which requires that all users be trained and follow all necessary safety protocols, including wearing the LG3 laser safety glasses shipped with the kit. A detailed calculation of the minimum laser safety requirements is included in the manual. More information about the laser classification system and Thorlabs' laser safety products can be found on the Laser Safety tab.

Quantum Optics Kit and Polarization-Entanglement Extension Kit Basics

• Quantum Optics Educational Kit Basics
• Polarization-Entanglement Extension Kit Basics

Quantum Optics Educational Kit Basics

The EDU-QOP1(/M) Quantum Optics Educational Kit allows students to investigage the quantum properties of light, including the generation of heralded single photons, the techniques used to detect them, and several experiments that demonstrate fundamental quantum mechanical concepts, such as the superposition of states. Below is a brief overview of this kit; please see the Experiments tab for a detailed list of the included experiments and the EDU-QOP1(/M) manual for more details.

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Figure 1: An Overview of the Kit, Set Up for the Single-Photon Quantum-Eraser Experiment

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Figure 2: Photon Number Probabilities for a Fock State (Left), a Coherent State (Center), and a Thermal State (Right), all with the Same Mean Photon Number <n> = 5.

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Figure 3: An idler and a signal beam are generated from a pump beam in a birefringent crystal using type-I SPDC

Quantum Description of Light

Quantum Mechanics describes light as a quantized excitation of the electromagnetic field, where the smallest possible excitation is a single photon (with quantum number n = 1). Fock states are non-classical light states with a well-defined photon number (see the left graph of Figure 2). The term “single photons” refers to Fock states with photon number n = 1.

Classical light, on the other hand, does not exhibit such a well-defined photon number. For example, light generated by a laser is in a coherent state. For a given mean photon number, the actual photon number follows a Poisson distribution (see the middle graph of Figure 2). The attenuation of a laser can only reduce the mean photon number, while the underlying photon statistics remain Poissonian. Hence, an attenuated laser is not a single-photon source for quantum optics experiments.

Other classical light sources, such as LEDs or blackbody radiation, are described by thermal states, which are mixed quantum states, and they exhibit an even larger variance of the photon number compared to laser light (see the right graph of Figure 2).

Generation of Single Photons

A range of different sources can be used to generate single photons (Fock States with n = 1). Nowadays, most experiments use a process called Spontaneous Parametric Down-Conversion (SPDC) to generate photon pairs. The EDU-QOP1(/M) kit uses a type-I SPDC single-photon source, as it is rather simple to set up and offers stable operation, making it ideal for lab courses.

In type-I SPDC, pump light generates two photons inside a nonlinear crystal, as seen in Figure 3. These photons are created simultaneously, so one of the photons can be used to signal the existence of the other, making it possible to perform measurements on single photons. Thus, this type of source is a heralded single-photon source.

The EDU-QOP1 kit uses a β-Barium borate (BBO) birefringent nonlinear crystal designed for a pump wavelength of 405 nm and a degenerate pair wavelength of 810 nm. The crystal is designed for a pair opening angle of 6° to allow spatially separated detection of both photons. For the experiments in this kit, the input pump polarization is horizontal, and the plane containing the BBO crystal's optical axis and the pump's wave vector is also horizontal. Since the polarization of both photons of the pair created by type-I SPDC is always perpendicular to this plane, the photon pair has vertical polarization, as shown in Figure 3.

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Figure 4: The measurement setup to verify operation of a heralded single photon source is shown (Grangier-Roger-Aspect experiment). Coincidences between the three single photon detectors are analyzed.

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Figure 5: Definition of Coincidences

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Figure 7: How the Aligned Detectors Stand in the Laser Light Cone

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Figure 6: Schematic of How Light Passes Through an Axicon

Coincidence Detection

This kit uses three SPDMA single-photon detectors, as seen in the GRA schematic in Figure 4. Similar to a Geiger-Müller counter, an incoming photon creates an electron avalanche in the detector, which is detected and converted into a TTL-level output signal.

The three signal channels are analyzed by an educational-grade time tagger for coincidences between the detected events, as shown in Figure 5: For each event on the Trigger channel T, it checks whether there is another event on channel A or B within a few nanoseconds wide window before or after the triggering event. This period of time is called the coincidence window. If the coincidence condition is met, an event is generated at the coincidence channels T&A or T&B. A triple coincidence occurs if all three detectors register a photon within the coincidence window.

In the Grangier-Roger-Aspect experiment (see the Experiments tab), the triple coincidences are compared to the rate expected for an uncorrelated (classical) light source by using the second order correlation function g⁽²⁾_GRA:

where R_TAB is the triple coincidence rate, R_T is the count rate of the trigger detector T, and R_TA and R_TB are the coincidence rates of detectors T and A and T and B, respectively. As a single photon pair can't trigger events on all three detectors, the triple coincidence rate is close to zero for a heralded single-photon source and thus g⁽²⁾_GRA << 1.

Easy Alignment

The base kit and the extension kit described below have two iris apertures that define a common beam path for the pump and alignment lasers. For each laser, two mirrors are used for beam walking, ensuring fast and repeatable alignment of the system using visible light.

An axicon is placed at the position of the BBO crystal to generate a cone of red light from the alignment laser, which mimics the cone of photon pairs from the BBO crystal. Using this visible indicator, the detectors can be reliably aligned at the correct positions and orientations. The laser light cone can also be used for demonstration purposes.

Lighting Conditions

We recommend using the kit in rooms that can be completely darkened (no windows or shutters) since the best performance is reached in darkness. Sunlight is especially problematic as its spectrum includes the transmission range of the bandpass filter in front of the detectors and thus results in large background count rates. LED lighting with spectra outside of the 810 nm bandpass filters is unproblematic. See the Technical Notes – Environmental Conditions section of the Quantum Optics Kit manual for more information.

Polarization-Entanglement Extension Kit Basics

The EDU-QOPA1(/M) Extension Kit allows the generation of pairs of polarization-entangled photons using a crossed BBO pair crystal and three additional crystals that compensate the spatial and temporal walk-off effects introduced by the BBO pair crystal. The famous Bell test is performed on entangled-photon pairs, proving the non-locality of reality. Below is a brief overview of the extension kit; see the combined EDU-QOP1(/M) and EDU-QOPA1(/M) manual for more details. Note that this extension kit requires the EDU-QOP1(/M) base kit, sold separately below.

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Figure 8: An overview of the parts included with the polarization-entanglement extension kit. Red labels indicate items that are included with the EDU-QOPA1(/M) kit, and black labels indicate items from the EDU-QOP1(/M) kit.

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Figure 9: A Crossed BBO Pair Crystal Generates Polarization-Entangled Photon Pairs

Entangled States

For systems composed of more than one subsystem, quantum mechanics allows for pure system states that cannot be expressed as the product of pure states of the subsystems: These system states are called entangled states. Entangled-state subsystems exhibit strong correlations that cannot be explained with classical physics.

An example entangled state is a pair of photons generated by Spontaneous Parametric Down-Conversion (SPDC) in a crossed pair of type-I BBO crystals, as shown in Figure 9. While photons generated in the first crystal of the pair would both have vertical (V V) polarization and photons generated in the second crystal would both have horizontal (H H) polarization, as shown, the pair crystal makes those pairs indistinguishable, resulting in a polarization-entangled state of the generated photon pair.

In this entangled state, the polarization of the photons is not only unknown but unknowable. However, once the polarization of one of the photons is measured, the second photon instantly changes its polarization state to the measurement result of the first, even if they are far apart in space. This makes quantum mechanics inherently non-local.

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Figure 10: "Spooky Action at a Distance" in the Collapse of an Entangled State

Hidden Variable Theories and the Bell Test

This “spooky action at a distance” (Einstein), in which the measurement of one particle of an entangled pair changes the state of the other particle instantly, even if they are far apart in space, is shown in Figure 10. It sparked a vigorous discussion in the physics community in the first half of the 20th century that continues to this day. Hidden-variable theories were believed to have the potential to explain these predictions from quantum mechanics without resorting to "spooky action at a distance".

In a milestone publication, John Bell proposed an experimental measurement and an inequality based on its results, called the Bell test. Local hidden-variable theories predict that the inequality will be fulfilled, but quantum physics predicts it will be violated. The Bell test can be performed with the EDU-QOPA1(/M) extension kit.

Bell Test Setup

The basic setup for the Bell test in this kit is a remarkably simple addition to the base kit. All that is required is a source of polarization-entangled photons and a pair of linear polarizers, one in front of each of the two detectors in the path of the photons that make up the pair; please see Figure 11 (the SCCs and the TCC will be discussed below).

The Bell test consists of 16 measurements of the coincidence rate between the two detectors with a different pair of polarizer settings for each measurement. The recorded coincidence rates are combined into a single quantity, S. The CHSH inequality, a form of the Bell inequality introduced by Clauser, Horne, Shimony, and Holt, states that S ≤ 2 for all local hidden variable theories. However, the quantum mechanical prediction for the Bell test is S = 2 * 2^1/2, violating the CHSH inequality and thus ruling out local hidden variable theories as an explanation of reality.

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Figure 11: Bell Test Schematic. TCC = Temporal Compensation Crystal, SCC = Spatial Compensation Crystal.

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Figure 12: Entangled-State Phase Probability Distributions

Walk-Off Effects and Compensation

The values of S above are calculated for an idealized setup. In a real-life setup, several effects lower the perceivable entanglement; the most important among them for this kit are temporal and spatial walk-off.

The entangled state generated by SPDC is not perfect. Each pair of photons is in a different entangled state depending on the wavelength of the generating pump photon (temporal walk-off) and the emission angle (spatial walk-off) because the phase Φ in the general entangled state:

depends on these parameters. The pump laser's bandwidth and the adjustment of the detector irises result in a range of entangled states with different phases that get integrated during the measurement of S. As the range of possible entangled-state phases, Φ, approaches 2π, the observability of entanglement is lost; see Figure 12 for details.

Compensation crystals counteract these effects, with one temporal compensation crystal (SCC) between the pump and the crossed BBO pair crystal and one spatial compensation crystal (SCC) after the pair crystal in the path of each of the photons. The compensation crystals largely reverse the walk-off effects, securing a 10x narrower phase range for the photon pairs reaching the detector, allowing the measurement of S-values on the order of S = 2.6 to be achieved, comfortably violating the CHSH inequality.

Figures 13 and 14 show theoretical predictions of the entangled-state phase maps at the detector without and with spatial compensation, respectively. Note that the range of possible phases is on the order of 2π without spatial walk-off compensation. Without the compensation crystals, the Bell test would not reveal a violation of the CHSH inequality without shutting the detector irises to limit the detector's effective area, significantly decreasing the signal-to-noise ratio. 

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Figure 14: Entangled-State Phase Map at the Detector With SCCs

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Figure 13: Entangled-State Phase Map at the Detector Without SCCs

Quantum Optics Kit and Polarization-Entanglement Extension Kit Experiments

• Quantum Optics Educational Kit Experiments
• Polarization-Entanglement Extension Kit Experiments

Quantum Optics Educational Kit Experiments

Polarization-Entanglement Extension Kit Experiments

The "Additional Experiments" section of the manual for the EDU-QOP1(/M) Quantum Optics Educational Kit includes a detailed description of how to extend the kit to a two-qubit quantum computer that runs the Deutsch algorithm. The Deutsch algorithm is a quantum computing algorithm designed to determine whether an unknown input function, operating on one bit, is either constant or balanced. While a classical computer would need to calculate both function values f(0) and f(1) and do a comparison, the Deutsch algorithm can theoretically solve this problem in a single run.

The following additional components (not included) are required to build the algorithm based on the EDU-QOP1(/M) kit:

• Two LCC1511-B Half-Wave LC Retarders
• Two KLC101 K-Cube^® Controllers
• One TPS002 Power Supply
• Two RS1.5P8E (RS1.5P4M) Ø1" (Ø25.0 mm) Pedestal Pillar Posts
• Two RS4M Ø25.0 mm Post Spacers
• Two 8-32 (M4) Setscrews, 3/4" (20 mm) Long (Packs of 50 are Available by Ordering Item # SS8S075 (SS4MS20))
• Two RSP1D(/M) Ø1" (Ø25.4 mm) Rotation Mounts

The Deutsch algorithm can be set up with half-wave plates (HWPs) and two beamsplitters (BSs) in a Mach-Zehnder interferometer configuration with a one-to-one correspondence between logic gates in the quantum circuit and the optical elements. The colored logic gates in Figure 1 correspond to the color-coded elements in the optical setup in Figure 2 below. Figure 1 shows a quantum circuit that solves the Deutsch problem with two qubits, |x> and |y>, as input. Figure 2 shows a Mach-Zehnder interferometer configuration, where |H> and |V> are polarization states representing the |y> qubit, and |A> and |B> are path states representing the |x> qubit.

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Figure 1: Quantum circuit to solve the Deutsch problem, based on two qubits labeled x and y. It uses three Hadamard gates (H) and a two-qubit gate (U_f), as described in the manual. The result is obtained from measuring the state of qubit x (M).

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Figure 2: Scheme for an optical implementation of the Deutsch algorithm in a Mach-Zehnder interferometer configuration. The color-highlighted elements correspond to the colored elements in Figure 1, with the result being measured at the avalanche photodiode (APD) detector.

Our optical implementation of the Deutsch algorithm using the EDU-QOP1(/M) kit is configured as a Michelson interferometer, which simplifies the alignment and phase control of the interferometer. Instead of the two beamsplitters in Figure 2, only one beamsplitter is required. As the optical elements in both interferometer arms are passed twice in the Michelson configuration, the half-wave plates in the interferometer are replaced with quarter-wave plates. Apart from that, the setup and interpretation for the Deutsch algorithm remain identical.

An example measurement is shown in Figure 3. The logic level output of the algorithm is shown, which distinguishes the two classes of functions that the Deutsch algorithm analyzes.

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Figure 3: T&B Coincidence Signal for All Four Functions of the Deutsch Algorithm

For more details, including a more detailed theoretical description, please refer to the “Additional Experiments” chapter of the manual.

Quantum Optics Kit and Polarization-Entanglement Extension Kit Components

• Quantum Optics Educational Kit Components
• Polarization-Entanglement Extension Kit Components

Quantum Optics Educational Kit Components

The EDU-QOP1(/M) Quantum Optics Kit must be mounted on an optical table or breadboard. As these are common in many labs, we have not included a breadboard in this kit. If you need to purchase a breadboard separately, we recommend the B2448FX (B60120AX) optical breadboard with the AV5(/M) damping feet, which are available separately below.

Thorlabs Quantum Optics Kit is available in imperial and metric versions. In cases where the metric and imperial kits contain parts with different item numbers, metric part numbers and measurements are indicated by parentheses unless otherwise noted.

• This kit contains the number of screws indicated in the Qty. column; replacements, which are sold in packages of 50, are available by ordering the Item # listed.
• This kit contains the number of screws indicated in the Qty. column; replacements, which are sold in packages of 25, are available by ordering the Item # listed.
• This kit contains the number of screws indicated in the Qty. column; replacements, which are sold in packages of 100, are available by ordering the Item # listed.
• This kit contains the number of knobs indicated in the Qty. column; replacements, which are sold in packages of 10, are available by ordering the Item # listed.

Polarization-Entanglement Extension Kit Components

The EDU-QOPA1(/M) extension kit requires many of the EDU-QOP1(/M) base kit components, in addition to the components that come with the kit, listed below. The items in the kits must be mounted on an optical table or breadboard. As these are common in many labs, we have not included a breadboard in either kit. If you need to purchase a breadboard separately, we recommend the B2448FX (B60120AX) optical breadboard with the AV5(/M) damping feet, which are available separately below. 

Thorlabs' Polarization-Entanglement Extension Kit is available in imperial and metric versions. In cases where the metric and imperial kits contain parts with different item numbers, metric part numbers and measurements are indicated by parentheses, unless otherwise noted.

• This kit contains the number of screws indicated in the Qty. column; replacements, which are sold in packages of 50, are available by ordering the Item # listed.
• This kit contains the number of screws indicated in the Qty. column; replacements, which are sold in packages of 25, are available by ordering the Item # listed.

Software

Each kit includes a USB stick with the free EDU-QOP1 software package, which is used to set the acquisition parameters and record the measurements. A computer running Windows^® 10 or higher is required, as well as four USB 2.0 ports (or a suitable USB hub) for connecting the coincidence counter and K-Cube^® controllers (included with the kit). Measurements can be saved as .csv files, and measurement parameters are saved as an .xml file.

The software requires an installation of Kinesis^® and Swabian Instruments Time Tagger software packages (all installers are included on the USB stick). The software is written in LabVIEW and built as a Windows-executable file. Please contact Tech Support for a copy of our LabVIEW code if you plan to expand upon our experiments.

Kinesis Software

Version 1.14.52

The Kinesis Software Package includes a GUI for running Thorlabs' Kinesis system controllers.

Also Available:

• Communications Protocol

EDU-QOP1 Software

Version 1.2.0.2

The EDU-QOP1(/M) Software includes a GUI for monitoring detector signals during alignment, setting measurement parameters, and acquiring coincidence counts.

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Figure 1: Alignment Tab of Software

Alignment/Delay Adjust Tab
The Alignment tab, shown in Figure 1, is used to align the setup before taking measurements, while the Delay Adjust tab is used to set the delay of the time tagger’s coincidence counting. The Alignment tab shows count rates of the three detectors T, A, and B in the upper graph, while the lower graph can be selected to show either the coincidence count rates between T&A and T&B or the second order correlation g⁽²⁾(0) for these detector pairs. Count rates are measured over intervals of 0.5 s, and both graphs are updated at the corresponding rate (twice per second). The Delay Adjust tab shows the coincidence count rates as a function of the delay, allowing the necessary delay to be set for all future measurements.

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Figure 2: GRA Tab of Software

HBT/GRA Tab
In these tabs, the Hanbury-Brown-Twiss experiment and the Grangier-Roger-Aspect experiment can be performed. Both tabs have a similar structure, only differing in the quantities shown.

The software calculates the g⁽²⁾(0) values and displays them with all raw data required for statistical error analysis. The graph below plots the g⁽²⁾(0) value over time, showing how the correlation function stabilizes with increasing measurement duration.

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Figure 3: Malus' Law Tab of Software

Malus Tab
This tab is used for measuring the Malus’ law for single photons. You can perform Grangier-Roger-Aspect-like measurements for different angles of the polarizer in front of detector B and add the data points to the two plots on the right side of the tab. The results can be saved in a single file.

The upper plot shows the T&B coincidence count rates, which are the detected heralded single-photon events. The lower plot shows the g⁽²⁾(0) values calculated from the count rates. It is used to verify the non-classical properties of the light source for all of the above measurements. 

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Figure 4: Michelson Interferometer Tab of Software

Michelson Tab
In this tab, single-photon interference experiments and the quantum eraser experiment can be performed.

The graphs show the coincidence count rate of detectors T&B (upper graph) and the correlation g⁽²⁾(0) (lower graph) as a function of the stage position of the Michelson interferometer.

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Figure 4: Bell-Test Tab of Software

Bell-Test Tab
If the checkbox at startup is selected to indicate the presence of the EDU-QOPA1(/M) kit, the Bell test can be performed.

The four-by-four table on the right side displays the 16 settings for the polarizers in front of detectors T and A that must be measured to calculate the S-value of the Bell-type inequality of CHSH. Once the polarizers are set to the correct angles and the desired measurement time is set, start the measurement by clicking on the appropriate field of the table. Once all the fields are filled, the software calculates the S-value and displays it.

Quantum Optics Kit Webinar and Alignment Tutorial

Other Video Tutorials

In addition to the above video and the step-by-step alignment guides in the manual that are tailored to the kit, we recommend the following Video Insights. This selection covers tips and tricks about alignment skills which are useful when setting up the kit.

Thorlabs' Quantum Optics Educational Kit uses the L405P20 laser diode, which requires that all users be trained and follow all necessary safety protocols. This includes wearing the LG3 laser safety glasses shipped with the kit. More details about the laser classification system and Thorlabs laser safety products can be found on the Laser Safety tab.

Laser Safety and Classification

Safe practices and proper usage of safety equipment should be taken into consideration when operating lasers. The eye is susceptible to injury, even from very low levels of laser light. Thorlabs offers a range of laser safety accessories that can be used to reduce the risk of accidents or injuries. Laser emission in the visible and near infrared spectral ranges has the greatest potential for retinal injury, as the cornea and lens are transparent to those wavelengths, and the lens can focus the laser energy onto the retina. 

Safe Practices and Light Safety Accessories

• Laser safety eyewear must be worn whenever working with Class 3 or 4 lasers.
• Regardless of laser class, Thorlabs recommends the use of laser safety eyewear whenever working with laser beams with non-negligible powers, since metallic tools such as screwdrivers can accidentally redirect a beam.
• Laser goggles designed for specific wavelengths should be clearly available near laser setups to protect the wearer from unintentional laser reflections.
• Goggles are marked with the wavelength range over which protection is afforded and the minimum optical density within that range.
• Laser Safety Curtains and Laser Safety Fabric shield other parts of the lab from high energy lasers.
• Blackout Materials can prevent direct or reflected light from leaving the experimental setup area.
• Thorlabs' Enclosure Systems can be used to contain optical setups to isolate or minimize laser hazards.
• A fiber-pigtailed laser should always be turned off before connecting it to or disconnecting it from another fiber, especially when the laser is at power levels above 10 mW.
• All beams should be terminated at the edge of the table, and laboratory doors should be closed whenever a laser is in use.
• Do not place laser beams at eye level.
• Carry out experiments on an optical table such that all laser beams travel horizontally.
• Remove unnecessary reflective items such as reflective jewelry (e.g., rings, watches, etc.) while working near the beam path.
• Be aware that lenses and other optical devices may reflect a portion of the incident beam from the front or rear surface.
• Operate a laser at the minimum power necessary for any operation.
• If possible, reduce the output power of a laser during alignment procedures.
• Use beam shutters and filters to reduce the beam power.
• Post appropriate warning signs or labels near laser setups or rooms.
• Use a laser sign with a lightbox if operating Class 3R or 4 lasers (i.e., lasers requiring the use of a safety interlock).
• Do not use Laser Viewing Cards in place of a proper Beam Trap.

 

Laser Classification

Lasers are categorized into different classes according to their ability to cause eye and other damage. The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. The IEC document 60825-1 outlines the safety of laser products. A description of each class of laser is given below:

Class	Description	Warning Label
1	This class of laser is safe under all conditions of normal use, including use with optical instruments for intrabeam viewing. Lasers in this class do not emit radiation at levels that may cause injury during normal operation, and therefore the maximum permissible exposure (MPE) cannot be exceeded. Class 1 lasers can also include enclosed, high-power lasers where exposure to the radiation is not possible without opening or shutting down the laser.
1M	Class 1M lasers are safe except when used in conjunction with optical components such as telescopes and microscopes. Lasers belonging to this class emit large-diameter or divergent beams, and the MPE cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. However, if the beam is refocused, the hazard may be increased and the class may be changed accordingly.
2	Class 2 lasers, which are limited to 1 mW of visible continuous-wave radiation, are safe because the blink reflex will limit the exposure in the eye to 0.25 seconds. This category only applies to visible radiation (400 - 700 nm).
2M	Because of the blink reflex, this class of laser is classified as safe as long as the beam is not viewed through optical instruments. This laser class also applies to larger-diameter or diverging laser beams.
3R	Class 3R lasers produce visible and invisible light that is hazardous under direct and specular-reflection viewing conditions. Eye injuries may occur if you directly view the beam, especially when using optical instruments. Lasers in this class are considered safe as long as they are handled with restricted beam viewing. The MPE can be exceeded with this class of laser; however, this presents a low risk level to injury. Visible, continuous-wave lasers in this class are limited to 5 mW of output power.
3B	Class 3B lasers are hazardous to the eye if exposed directly. Diffuse reflections are usually not harmful, but may be when using higher-power Class 3B lasers. Safe handling of devices in this class includes wearing protective eyewear where direct viewing of the laser beam may occur. Lasers of this class must be equipped with a key switch and a safety interlock; moreover, laser safety signs should be used, such that the laser cannot be used without the safety light turning on. Laser products with power output near the upper range of Class 3B may also cause skin burns.
4	This class of laser may cause damage to the skin, and also to the eye, even from the viewing of diffuse reflections. These hazards may also apply to indirect or non-specular reflections of the beam, even from apparently matte surfaces. Great care must be taken when handling these lasers. They also represent a fire risk, because they may ignite combustible material. Class 4 lasers must be equipped with a key switch and a safety interlock.
All class 2 lasers (and higher) must display, in addition to the corresponding sign above, this triangular warning sign.

Dr. Rüdiger Scholz

Dr. Kim-Alessandro Weber

We are grateful for the various insights we have collected over the years from numerous committed educators who have taken on the challenge of experimentally teaching quantum optics to students.

The experimental realization in this kit was heavily influenced by our collaborators from the Leibniz University Hannover. We cordially thank Dr. Kim-Alessandro Weber and Dr. Rüdiger Scholz for their outstanding contributions to this kit. Between them, they have more than 40 years of experience in designing quantum optics and photon statistics experiments. This kit’s design borrows many ideas from the experiments they set up to teach both college students as well as teachers and students from high schools. We are grateful for their enthusiasm to share this experience with the rest of the teaching community by means of this kit. Finally, we thank them for testing and providing extensive feedback on the SPDMA single-photon detector incorporated into this kit during its development.

We cordially thank Paul Schlummer, Adrian Abazi, Carsten Schuck, and Wolfram Pernice from the University of Münster for supporting the development of this educational quantum optics setup. We acknowledge countless fruitful discussions, both on the physical as well as the teaching aspects of quantum systems. In particular, we are grateful for the thorough comparison to a type-II BBO system and their detailed feedback on our SPDMA single photon detector.

We also gratefully acknowledge the contributions of Prof. Dr. Jan-Peter Meyn who was one of the early adopters of real quantum optics experiments in the German teaching community (e.g., P. Bronner et al 2009 Eur. J. Phys. 30 1189). His expertise and the optical design of his setups helped spread knowledge among educators and was also an inspiration to certain design elements in the setups from Dr. Kim-Alessandro Weber and Dr. Rüdiger Scholz.

Do you have ideas for an experiment that you would like to see implemented in an educational kit? Contact us at techsupport@thorlabs.com; we'd love to hear from you.