1. What Is a Quantum Random Number Generator?
A QRNG produces randomness from a truly quantum process—one that is fundamentally unpredictable even in theory. Common physical sources:
a) Photon Path Splitting (Most Common)
A single photon hits a 50:50 beam splitter.
Output port A → bit = 0
Output port B → bit = 1
The outcome is intrinsically probabilistic.
b) Photon Arrival Time
A weak light source produces photons randomly in time; time-stamping their arrival yields random bits.
c) Vacuum Fluctuations (Homodyne QRNG)
Measures quantum noise in an electromagnetic field. More complex but produces high bit rates.
🧩 2. The Safest DIY Approach
The easiest and safest way to build a QRNG is using a single-photon–level LED source + beam splitter + two photodiodes.
This does not require lasers or hazardous optical power.
🔧 3. Hardware Components (Safe, Accessible)
Optics
50:50 non-polarizing beam splitter cube (small 1–2 cm)
Collimating lens (cheap plastic lens is fine)
Light Source
Weak LED (deep blue or near-UV works well)
Neutral density filters to reduce photon flux
(You do not need a coherent laser; incoherent light works if attenuated.)
Detectors
Two silicon photodiodes, e.g. BPW34 or similar
Optional: low-noise transimpedance amplifier (TIA) board
Electronics
Microcontroller or SBC:
Arduino
Raspberry Pi
ESP32
Comparator or ADC (many microcontrollers have built-in ones)
📐 4. System Architecture
[LED] → [Collimator] → [Beam Splitter] → (Left PD → 0)
→ (Right PD → 1)
Each photon detection event sets a bit.
⚙️ 5. Circuit Overview
Since photodiodes output tiny currents, you convert them to voltage:
PD → TIA (op-amp converts current to voltage)
TIA → Comparator (produces digital pulse)
Comparator → Microcontroller GPIO interrupt
Software: Interrupt handler records a “0” or “1”
You need two identical detector channels.
🧮 6. Software / Firmware
Minimal pseudocode for an Arduino-style device:
volatile uint8_t bit;
volatile bool hasBit = false;
void leftDetector() {
bit = 0;
hasBit = true;
}
void rightDetector() {
bit = 1;
hasBit = true;
}
void setup() {
pinMode(LEFT_PIN, INPUT);
pinMode(RIGHT_PIN, INPUT);
attachInterrupt(digitalPinToInterrupt(LEFT_PIN), leftDetector, RISING);
attachInterrupt(digitalPinToInterrupt(RIGHT_PIN), rightDetector, RISING);
Serial.begin(115200);
}
void loop() {
if (hasBit) {
Serial.write(bit);
hasBit = false;
}
}
🧪 7. Ensuring the Bits Are Truly Random
Bias Correction
Beam splitters and detectors are imperfect. Common fix:
Von Neumann Corrector
Read bits in pairs:
01 → output 0
10 → output 1
00 or 11 → discard
This removes first-order bias.
Statistical Tests
Use:
NIST SP 800-22 Test Suite
Dieharder
TestU01
Look for failures; adjust thresholding and filtering if necessary.
🚀 8. Expected Performance
Bitrate of DIY LED + PD QRNG: 10–100 kbps
With avalanche photodiodes (APDs): Mbps
With homodyne detectors: 100+ Mbps (advanced)
🧱 9. Building Without Custom Optics
If optical alignment is challenging, consider:
💡 Alternative: Quantum Shot-Noise QRNG Using a Photodiode Only
Shine a steady LED on a photodiode, amplify the signal, and digitize very small fluctuations caused by quantum shot noise.
This avoids alignment entirely and works well for educational purposes.
📦 10. Off-the-Shelf QRNG Modules
If you want quantum randomness without building optics:
ID Quantique Quantis USB
Quside QN400
ON Semiconductor J-Series APD breakout boards + open source QRNG code
These are plug-and-play.
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