Quantum Cryptography Analogy Demonstration Kit

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Binary information encoded in the polarization state of light is detected by a sensor module (indicated by a blue LED). 

Quantum Cryptography Analogy Demonstration Kit

• Designed for Educational, Demonstration, and Classroom Use
• Complete Photonics Kit Includes All Hardware Plus Extensive Manual and Teaching Materials
• 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 all the necessary components and 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: English and German language manuals/teaching information are available for this product. 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 need a different language, cord style, or power supply. As with all products on our website, taxes are not included in the price shown below.

This kit is designed to demonstrate the fundamentals of quantum cryptography and the BB84 encryption protocol through a series of classroom experiments. In these experiments, students will learn how to encode messages in binary using the polarization state of light and then encrypt them using the BB84 protocol.

The BB84 Protocol
The BB84 protocol is a communication scheme that combines an encryption technique known as the one-time pad with a quantum key distribution method. Message encryption using the one-time pad method is accomplished by adding a binary encryption key (consisting of 0s and 1s) to a message represented in binary. Because there is no methodology or pattern to the key or message, this method of encryption ensures that the encrypted message can be transmitted through public channels.

However, the fundamental challenge is ensuring that only Alice and Bob have knowledge of the encryption key and preventing an eavesdropper (designated as Eve) from intercepting the encryption key. The BB84 encryption protocol uses the idea of quantum key distribution (QKD) to ensure that only Alice and Bob have knowledge of the encryption key. QKD employs principles of quantum physics, such as the encoding of a single bit of information within a photon of light, to ensure that the information cannot be copied. Any interception attempt by Eve will inevitably change the state of the photon. Thus, the BB84 protocol offers the ability to send truly secure encrypted transmissions.

Modeling the BB84 Protocol Using the Polarization of Light
The EDU-QCRY1(/M) kit models the BB84 method using the polarization states of light (as illustrated in the diagram below). Data in the form of a bit is encoded into a pulsed laser beam by altering the polarization state. More specifically, the bit is encoded using a basis polarization pair (represented as a + or x) which converts the bit into a specific polarization angle. The + basis is represented by 0° and 90° polarizations while the x basis is represented by -45° and 45° polarizations. One of the polarizations in each pair represents a "0" bit while the other represents a "1" bit. Alice performs a transmission by emitting a pulsed beam of light and setting the polarization state with a half-wave plate to encode the bit and basis. The receiver (Bob) is equipped with a half-wave plate and beamsplitter to interpret the transmission as either a "0" or "1". The bits and bases are randomly chosen in each transmission; however by comparing bases, Alice and Bob can generate a unique encryption key only known to each other. 

In this experiment, the eavesdropper (Eve) is represented by a module that can receive a transmission from Alice and then attempt to send the same signal to Bob. While Eve is free to do this since the "transmission" is public, students will learn how this will cause errors that reveal the presence of an eavesdropper. Because detection of an eavesdropper occurs during transmission of the encryption key, once Alice and Bob have verified that there is no eavesdropper, the message can be sent without risk of interception. 

This schematic illustrates the Alice, Eve, and Bob modules of the EDU-QCRY1(/M) Quantum Cryptography Kit. The polarization state of light is changed by the polarization rotators to encode a "1" or "0" in a specific basis for transmission. The light is received by another polarization rotator, which allows the beamsplitter to send the light to the appropriate sensor for detection.

Single Photons Versus Classical Light
Genuine safety from an eavesdropper is only guaranteed if the BB84 protocol is carried out using single photons, which obey the “no cloning theorem”. In general, this theorem states a perfect copy of an unknown quantum state cannot be made without altering the state. As a result, Eve cannot copy a photon from Alice without altering it and cannot send an unaltered photon to Bob while keeping a copy for analysis.

Please note that the EDU-QCRY1(/M) analogy kit uses a pulsed laser source, i.e. classical light. While the sequence of the protocol is completely identical to the true quantum encryption system, it cannot be used as a perfect encryption system. Eve is able to eavesdrop unnoticed by separating a portion of the transmitted light for analysis while the remainder is sent to Bob.

This tab illustrates an example exercise that can be done by students using the EDU-QCRY1(/M) Quantum Cryptography Analogy Demonstration Kit. For full details on exercises and worksheets for students, please refer to the manual.

This example illustrates the generation of an encryption key between Alice and Bob, and the transmission a 10-bit, two-letter encrypted message using the BB84 protocol. 

Generating an Encryption Key
First, Alice will randomly choose a sequence of bits and a basis (either + or ×) for the key transmission. The combination of bit and basis determines the polarization of the pulse of light transmitted by Alice (see the table to the right for possible combinations). Table 1 shows a sequence of 18 randomly chosen bits and bases, and the corresponding polarization angle. For further details on how to determine the polarization angle, please refer to the manual.

At the same time, Bob chooses a random set of bases to use in receiving the transmission. These bases are chosen independently from Alice and will not always match. After transmission, Alice and Bob compare bases that they used. In cases where the basis matches with the one chosen by Alice (highlighted in green in Table 2), Bob will obtain the same bit as chosen by Alice. However, if the basis does not match, Bob will randomly receive a 0 or 1. Because the result is random, these bits are thrown out; the remaining green-highlighted bits become the encryption key between Alice and Bob.

In this example, the encryption key would be the sequence: "0 1 1 0 0 1 0 0 0 1".

Sending an Encrypted Message Using an Encryption Key
Now we will illustrate how to send a two-letter message "QM" using the encryption key above. First, each letter is converted into a 5-bit binary representation, as shown in Table 3 (see Chapter 9 of the manual for a reference table with binary representations for the full alphabet). While this example uses binary format to send alphabetical letters, any kind of information could theoretically be encoded into a binary sequence.

To encrypt the message, Alice performs binary addition on the message and encryption key (see the table below for addition rules). The resulting 10-bit message can then sent from Alice to Bob through public channels and using a previously agreed upon basis (i.e., Alice and Bob both choose to use either a + or × basis). After transmission, Bob can then decode the message by using binary addition on the encrypted message key using the encryption key. Because the results of binary addition are reversible, adding in this fashion returns the original binary message.

This method for sending secure messages, however, assumes that no eavesdropper has knowledge of the encryption key. Because the transmissions described above are carried out via public channels, Eve can try to intercept the encryption key by intercepting the transmission between Alice and Bob. However, doing so will cause errors in the transmission. In the BB84 protocol, after comparing bases (Table 2), Alice and Bob will compare a test region of the encryption key. The errors introduced by Eve can be found by Alice and Bob when comparing the test region, alerting them to the presence of the eavesdropper. For a more detailed example, see Section 6.2 of the manual.

Quantum Cryptography Analogy Demonstration Kit Components

Thorlabs' Quantum Cryptography Analogy Demonstration 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 are listed next to their imperial counterpart and measurements are indicated in parenthesis.

User-Generated Content

Thorlabs' Educational Kits allow users to adapt the content to their own teaching needs. With that in mind, we enjoy hearing feedback from our customers with details about how they use the kits in their own classrooms. Use the Contact Us button to the right if you would like to submit your own user-generated content.

Second Wavelength and Decoy States

We cordially thank Brit Riggs and Professor Bertrand Cambou, Northern Arizona University, for sharing their following modification of the EDU-QCRY1 kit to demonstrate the use of a second wavelength and decoy states as an improvement upon the BB84 protocol.

User-Modified EDU-QCRY1 Kit
*Image courtesy of Brit Riggs and Professor Bertrand Cambou of Northern Arizona University.

Theory
BB84 is a well-known quantum key distribution (QKD) protocol that allows two parties to securely generate matching cryptographic keys by encoding information in single-photon polarization states. As an extension of the BB84 protocol, a second single-photon source with different wavelength may be introduced which doubles the number of usable polarization states. The largest advantage of a two-wavelength system over a single-wavelength system is that the two parties (Alice and Bob) no longer need to sacrifice a portion of their key to check for the presence of eavesdroppers (Eve). A subset of the additional polarization states can be used as "decoy states". These states are intentionally unused by Alice, but may be generated by Eve when intercepting Alice's transmission. If a statistically significant number of decoy states are detected by Bob, then he knows an eavesdropper must be present. For more information on decoy states and two-wavelength encoding, please see the synopsis here. Additionally, the setup presented here was featured in the following publication: "Multi-Wavelength Quantum Key Distribution Emulation with Physical Unclonable Function," B. Riggs et al., Cryptography 6(3), 36 (2022).

Like our standard EDU-QCRY1(/M), this custom quantum cryptography teaching kit uses pulsed lasers in order to emulate the behavior of a true quantum system. Therefore, while it is not a quantum safe communication setup, it is still ideally suited to teach students about quantum cryptography.

This schematic illustrates the Alice, Eve, and Bob modules that have been modified to incorporate a second wavelength. There are four polarization states across two bases per wavelength which may be used to encode information. Dichroic mirrors are used to combine and separate the different wavelengths for transmission and measurement respectively. (HWP: Half-Wave Plate, PBS: Polarizing Beamsplitter)

Key Additional Components
The following partial list of components may be used to incorporate a second wavelength into the EDU-QCRY1(/M) kit:

• 2X CPS520 520 nm Laser Diode Module
• 2X Laser Electronics Module
• 4X Sensor Modules
• 2X Sensor Electronics Module
• 2X PBS201 20 mm Polarizing Beamsplitter Cube, 420 - 680 nm
• 4X WPH10E-514 Ø1" Polymer Zero-Order Half-Wave Plate, 514 nm
• 4X DMLP605 Ø1" Longpass Dichroic Mirror, 605 nm Cut-On

For a complete list of components, please contact Tech Support.

The EDU-QCRY1(/M) was developed in cooperation with several educators and organizations specializing in teaching quantum physics:

• Jörn Schneider, Leibniz-Gymnasium Dormagen
• Jan-Peter Meyn and Andreas Vetter, University Erlangen-Nuremberg (www.quantumlab.de)
• Quantum Physics in School 2014 Meeting, Munich (www.heisenberg-gesellschaft.de)

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 your ideas.