Computer Simulation of Photonic Quantum Networks

Greetings, my name is Alex Kolar and I’m a rising junior studying Computer Engineering at Northwestern University. This summer, I’ve been working with researchers Rajkumar Kettimuthu and Martin Suchara to develop and implement the Simulator of QUantum Network Communications (SeQUeNCe) to simulate the operation of large-scale quantum networks.

Quantum information networks operate in a similar manner to classical networks, transmitting information and communications between distant entities, but with the added ability to utilize principles of quantum mechanics in data encoding and transmission. These principles include supersposition, by which a single unit of quantum information (qubit) can exist in a probabilistic mixture of different states, and entanglement, by which multiple qubits are affected by actions on one qubit. Many applications are thus available for quantum networks including distribution of secure keys for encryption and reduction of complexity for distributed computing problems.

The simulator, in its current implementation, models the behavior of such networks at the single-photon level with picosecond resolution. This will allow us to test the behavior of complicated networks that cannot be tested without significant time and monetary investment or even within the confines of current optical technology.

As a test of the simulator, we reproduced the results of an existing physical experiment (see references) to generate secure encryption keys. The simulated optical hardware is shown in Figure 1, where Alice generates the bits for the key and transfers these to Bob.

Figure 1: Quantum Key Distribution (QKD) setup

After constructing the keys, we measured the percentage of bits in the keys that differed between Alice and Bob (Figure 2) and compared these to the results of the experiment (Figure 3). Our measured error (blue squares) corresponds closely to the error predicted in the original experiment. We were also able to add a “phase error” to error rate (red diamonds) such that our results matched the experimental results.

Figure 2: Simulated QKD error rate
Figure 3: Experimental error rate results

In continuing this simulation, we also were able to generate timing data on the rate of key formation (Figure 4) and latency of key formation (the time until the first key is generated – Figure 5). For the latency, this value was calculated with differing qubit generation frequency, showcasing the ability of the simulator to quickly produce results with varying parameters for its elements.

Figure 4: Simulated rate of key generation
Figure 5: Simulated Latency of key generation for varying qubit generation frequencies

As shown, SeQUeNCe allows for quick and accurate modeling of quantum communication networks. In the future, this will allow us to test increasingly complicated networks and develop new protocols for their real-world operation.


C. Gobby, Z. Yuan, and A. Shields, “Quantum key distribution over 122 km of standard telecom fiber,” Applied Physics Letters, vol. 84, no. 19, pp. 3762–3764, 2004.