Emulation of the Six-State Quantum Key Distribution Protocol with Pulsed Lasers

TL;DR

This work addresses how to study multi-basis quantum key distribution without true quantum hardware by implementing a compact tabletop emulation of the six-state QKD protocol with a pulsed laser and bulk polarization optics. A software-augmented setup reproduces the six-state preparation and measurement across three bases, with a simulated intercept--resend eavesdropper to benchmark security statistics. The study reports per-cell and aggregate fractions that closely match theoretical benchmarks, e.g., $P_{ ext{sift}}=\frac{1}{3}$ in secure runs and $P(\text{undisturbed\&compromised})=\frac{1}{9}$ under IR, demonstrating high fidelity of the emulation. The results establish a cost-effective, educational platform for exploring multi-basis quantum communication and benchmarking protocols in non-specialized laboratories.

Abstract

Quantum cryptography remains a topic of enduring scientific and educational interest. Here, we present a clear and accessible framework for exploring the six-state quantum key distribution protocol, an enhanced three-basis extension of the BB84 scheme that combines optical experiments with computational analysis. Designed for testing quantum communication protocols through emulation, this approach provides a robust and cost-effective platform that highlights the fundamental principles of multi-basis encoding and demonstrates how experimental measurements connect directly to theoretical expectations in a controlled tabletop setting.

Figures (5)

• Figure 1: Core fractions of all transmissions. Right: Alice--Bob (no Eve), undisturbed (sifted) = 1/3 vs. disturbed 2/3. Left: Alice--Eve--Bob (IR), undisturbed-and-compromised 1/9 vs. disturbed 8/9. Horizontal lines at $1/3$ and $1/9$ indicate the theoretical benchmarks.
• Figure 2: Experimental setup for the six-state QKD implementation using a pulsed laser source. The beam from the laser passes sequentially through Alice’s polarization state preparation stage—comprising a half-wave plate (HWP) and a quarter-wave plate (QWP)—which generates one of the six polarization states $\{H, V, D, A, R, L\}$. The prepared beam is then analyzed by Bob’s measurement module, consisting of a QWP, an HWP, and a polarizing beam splitter (PBS) that projects the incoming state onto the selected measurement basis. The two output ports of the PBS are connected to photodetectors (PD), corresponding to logical bit values 0 and 1.
• Figure 3: Experimental setup for the six-state QKD system with an intercept--resend eavesdropper. The optical bench consists of three modules: Alice, Eve, and Bob. Alice prepares one of the six polarization states using a laser, half-wave plate (HWP), and quarter-wave plate (QWP). Eve intercepts the transmitted beam and performs a full measurement--resend operation using her own QWP, HWP, and polarizing beam splitter (PBS) connected to two photodetectors (PDs). A secondary laser, followed by an HWP and QWP, re-encodes the state that Eve resends to Bob. Bob’s receiver module (right) analyzes the incoming beam using a QWP, an HWP, and a PBS with two PDs corresponding to logical bit values 0 and 1.
• Figure 4: Experimental correlation matrices for the six-state QKD protocol. (A) Alice--Bob configuration without eavesdropping, showing detection probabilities for each combination of Alice’s prepared state and Bob’s measurement basis. The bright diagonal blocks correspond to correctly correlated basis pairs ($HV$, $DA$, and $RL$), each exhibiting a mean detection probability of approximately $5.6\%$ of all transmitted pulses. (B) Alice--Eve--Bob configuration under an intercept--resend (IR) attack, where Eve performs a full measurement and resends the state to Bob. Here, the correlated regions are reduced to approximately $1.85\%$ per correlated cell, consistent with the expected theoretical reduction by a factor of three due to the added measurement stage. Color intensity represents the percentage of all pulses detected per state--basis combination, as indicated by the color scale on the right.
• Figure 5: Core fractions of all transmissions. Left: Alice--Bob (no Eve), showing an experimentally measured undisturbed (sifted) fraction of approximately 31.3%, compared to the theoretical expectation of $1/3 \approx 33.3%$; the remaining $\approx68.7%$ correspond to disturbed or mismatched events. Right: Alice--Eve--Bob (IR) configuration, where the experimentally measured undisturbed-and-compromised fraction is 10.4%, close to the theoretical prediction of $1/9 \approx 11.1%$; the remaining $\approx89.6%$ represent disturbed transmissions. Horizontal black dashed and dotted lines mark the theoretical reference levels at $1/3$ and $1/9$, respectively, for direct comparison with the experimental data.