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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.

Quantum Optics Educational Kit

• 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

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' 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.

The EDU-QOP1(/M) kit investigates the quantum properties of light, starting from the generation of (heralded) single photons and detection techniques up to experiments that demonstrate fundamental quantum mechanical concepts such as the superposition of states. Below is a brief overview of this kit; see 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.

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

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Figure 5: Definition for 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.

Quantum Description of Light

In quantum mechanics, light is described by 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).

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). Attenuation of the laser can only reduce the mean photon number while the underlying statistic remains Poissonian. Hence, an attenuated laser can never be used as a single photon source for quantum experiments.

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

Generation of Single Photons

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

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

The EDU-QOP1 kit uses a β-Barium borate (BBO) 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° for spatially separated detection of both photons.

Coincidence Detection

Three SPDMA single photon detectors are used in the kit, as seen in the 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.

These 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. This time period is called the coincidence window. If the condition is met, a coincidence 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.

For a pair source, the coincidences measured between both arms occur more often than for an uncorrelated light source. This can be characterized by the second order correlation function, g⁽²⁾, which compares the expected coincidences for an uncorrelated light source with the measured coincidences:

where R_A and R_B are the average count rates of detectors A and B, respectively, R_AB is the count rate of coincidences, and Δt is the time 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 coincidendence rates of detectors T and A and T and B, respectively. As a single photon pair is not able to 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 kit uses two iris apertures to 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. Using this visible indicator, the detectors can be reliably aligned at the correct positions and orientations. The laser light cone can further be used for demonstration purposes.

In the "Additional Experiments" section of the manual, there is 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 LCC1111-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. In that case, there is a one-to-one correspondence between logic gates in the quantum circuitry 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 based on two qubits, |x> and |y>. Figure 2 describes a Mach-Zehnder interferometer configuration, where |H> and |V> are polarization states that represent the |y> qubit, and |A> and |B> are path states that represent 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) described in the manual. The result is obtained from measuring the state of qubit x (M).

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Figure 2: Scheme for the 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.

Based on the EDU-QOP1(/M) kit, the optical implementation is set up in a Michelson interferometer configuration, due to the easier alignment and phase control of the interferometer. This means that 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 must be replaced with quarter-wave plates. Apart from that, the setup and interpretation for the Deutsch algorithm remains 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.

*This previous generation item is now offered as a special and can be ordered by contacting Tech Sales.

Quantum Optics 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.

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 a .xml file.

The software requires an installation of Kinesis^® and Swabian Instruments Time Tagger software packages (all installers 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 experiment.

Kinesis Software

Version 1.14.47

The Kinesis Software Package, which includes a GUI for control of Thorlabs' Kinesis and APT™ system controllers.

Also Available:

• Communications Protocol

EDU-QOP1 Software

Version 1.0.0.15

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

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Figure 1: Alignment Tab of Software

Alignment Tab
This tab, which is shown in Figure 1, is used to align the setup before taking measurements. The count rates of the three detectors T, A, and B are shown in the upper graph, while the lower graph can be selected to either show the coincidence count rates between T&A and T&B, respectively, 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).

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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 shown quantities.

The g⁽²⁾(0) values calculated by the software are displayed together with all raw data that is required for statistical error analysis. The graph below plots the g⁽²⁾(0) value over time, making it possible to see 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 the measurement of Malus’ law for single photons. In this tab, you can perform several Grangier-Roger-Aspect-like measurements for different angles of a polarizer in front of detector B and add those data points to the two plots on the right side of the tab. The collected results can then 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 and 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, the single-photon interference experiments as well as 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.

Video Tutorials

In addition to 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.