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Quantum Cryptography Demonstration Kit


  • Designed for Education, Demonstration, and Classroom Use
  • Easy-to-Use Kits Include Components Plus Educational Materials
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Binary information encoded in the polarization state of light is detected by a sensor module (indicated by a blue LED). 
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Quantum Cryptography Analogy Demonstration Kit

  • Designed for Educational, Demonstration, and Classroom Use
  • Complete Kit Includes All Hardware Plus Extensive Manual and Teaching Materials
  • Easy to Assemble and Use
  • Choose from Kits Containing Imperial or Metric Components

Experiment

  • Learn How Information Can Be Encrypted and Sent Using the Polarization of Light
  • Generate an Encryption Key that Allows for Private Communication
  • Encrypt, Transmit, and Decrypt a Secret Message
  • Examine How an Eavesdropper Causes Errors in Transmission that Can Be Detected
  • Demonstrate the Steps of the BB84 Encryption Protocol

The EDU-QCRY1(/M) Quantum Cryptography Analogy Demonstration Kit contains components to model a data transmission setup using the BB84 encryption protocol. This encryption method allows a sender and receiver to generate an encryption key that only they know and eavesdroppers to be detected. In this analogy experiment, the polarization of transmitted light carries bits of information which are manipulated using half-wave plates and polarizing beamsplitters. The kit includes the lasers, half-wave plates, polarizing beamsplitters, and detectors required to model the sender (Alice), the receiver (Bob) and the eavesdropper (Eve). 

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 kit includes all the necessary components and a manual that contains both detailed setup instructions and extensive teaching materials. These 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 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. 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-QCRY(/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.

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.

Basis Bit Polarization
+ 0
1 90°
× 0 -45°
1 45°

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 bases (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.

Table 1: Transmitting (Alice)
Index 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Alice's Basis (Random) × × + × + + + × × + × × × + + × + ×
Alice's Bit (Random) 1 0 0 1 1 0 0 1 1 1 0 1 0 0 0 1 0 1
Polarization Angle 45° -45° 45° 90° 45° 45° 90° -45° 45° -45° 45° 45°

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.

Table 2: Receiving (Bob)
Index 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Bob's Basis (Random) + × × × + + + × + × × + + × + + + ×
Bob's Bit (From Alice) 1 0 1 1 1 0 0 1 1 1 0 1 0 1 0 0 0 1

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.

Table 3: Sending an Encrypted Message
Lettera Q M
Binary Message 1 0 0 0 0 0 1 1 0 0
Encryption Key 0 1 1 0 0 1 0 0 0 1
Encrypted Messageb 1 1 1 0 0 1 1 1 0 1
  • In this example, a letter is expressed in terms of a 5-bit sequence. A reference table for the entire alphabet is included in Chapter 9 of the manual.
  • The encrypted message is generated by binary addition of the message and key.
Binary Addition Rules
0 + 0 = 0
0 + 1 = 1
1 + 0 = 1
1 + 1 = 0

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 Demonstration Kit Components

Thorlabs' Quantum Cryptography 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.

Kit Components
EDU-QCRY1
Item #
EDU-QCRY1/M
Item #
Description Qty.
MB8 MB2020/M Aluminum Breadboard
8" x 8" (20 cm x 20 cm)
1
MB810 MB2025/M Aluminum Breadboard
8" x 10" (20 cm x 25 cm)
1
MB1218 MB3045/M Aluminum Breadboard
12" x 18" (30 cm x 45 cm)
1
BA1 BA1/M Post Holder Base 10
PH2 PH50/M Post Holder, 2" (50 mm) Long 5
PH1.5 PH40/M Post Holder, 1.5" (40 mm) Long 6
UPH2 UPH50/M Universal Post Holder, 2" (50 mm) Long 2
TR1.5 - Optical Post, 1.5" Long 6
- TR30/M Optical Post, 30 mm Long 4
- TR40/M Optical Post, 40 mm Long 2
TR2 TR50/M Optical Post, 2" (50 mm) Long 7
RSP1X225-ALICEa RSP1X225/M-ALICEa Ø1" Indexing Rotation Mount, 22.5° Steps 2
RSP1X225-BOBa RSP1X225/M-BOBa Ø1" Indexing Rotation Mount, 22.5° Steps 2
PM3 PM3/M Clamping Arm 2
  • These parts are based on the RSP1X225, but feature a custom dial face designed for use in these kits.
Kit Components
EDU-QCRY1 Item # EDU-QCRY1/M Item # Description Qty.
BA1S BA1S/M Swivel Post Holder Base 1
AT1 AT1/M Alignment Tool 1
CL3 CL3/M Compact Variable Height Clamp 2
N/A N/A Kinematic Platform Mounta 2
RDF1 Rubber Damping Feet (4 Pack) 3
WPH10E-633 Zero-Order Half-Wave Plate 2
KM100 Ø1" Kinematic Mount 2
PBS201 Polarizing Beamsplitter Cube 2
AD11NT Ø1" Adapter for Ø11 mm Components 2
CPS635R-C2b 635 nm Laser Diode Module 2
BBH1 Breadboard Handles 1
SPW606 SM1 (1.034"-40) Spanner Wrench 1
N/A Sensor Module 4
N/A Sensor Electronics Module 2
N/A Laser Electronics Module 2
  • These parts are a custom version of the KM100PM(/M).
  • This part is a class 2 version of the CPS635R.

Imperial Kit: Included Hardware and Screws

Item # Description Qty. Item # Description Qty.
BD-3/16L 3/16" Balldriver 1 - 1/16" Hex Key 1
- 1/8" Hex Key 1 - 3/32" Hex Key 1
W25S050 1/4" Washer 19 SH25S038 1/4"-20 Cap Screw,
3/8" Long
11
SH25S050 1/4"-20 Cap Screw,
1/2" Long
12 SH25S063 1/4"-20 Cap Screw,
5/8" Long
17
- 1/4"-20 Cap Screw,
1.25" Long
2 SH25S200 1/4"-20 Cap Screw,
2" Long
2
AS4M8E M4 to 8-32 Thread Adapter 4 - - -

Metric Kit: Included Hardware and Screws

Item # Description Qty. Item # Description Qty.
BD-5ML 6 mm Balldriver 1 - 1.5 mm Hex Key 1
- 2 mm Hex Key 1 - 3 mm Hex Key 1
W25S050 M6 Washer 19 SH6MS10 M6 Cap Screw,
10 mm Long
11
SH6MS12 M6 Cap Screw,
12 mm Long
12 SH6MS16 M6 Cap Screw,
16 mm Long
17
- M6 Cap Screw,
30 mm Long
2 - M6 Cap Screw,
45 mm Long
2

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.


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EDU-QCRY1 Support Documentation
EDU-QCRY1Quantum Cryptography Analogy Demonstration Kit, Imperial
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+1 Qty Docs Part Number - Metric Price Available / Ships
EDU-QCRY1/M Support Documentation
EDU-QCRY1/MQuantum Cryptography Analogy Demonstration Kit, Metric
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