Observation of superluminal and negative group velocities in a Mach–Zehnder interferometer

We demonstrate superluminal and negative group velocity regimes in a linear passive Mach–Zehnder interferometer. This phenomenon occurs in a narrow frequency region around the interferometer’s transmission minima. Experiments are performed in the radio frequency range by using coaxial cables and $1×2$ wave splitters. Group velocities of $2c$ and tunneling with a maximum fractional advancement of 0.12 were measured for electromagnetic sinusoidal wave packets of $2μs$ width. These results agree with theoretical predictions using the interferometer’s transmission phase function. This system is proposed as a simpler alternative to photonic crystals and active or microstructured multiple-beam interferometers for sustaining anomalous group velocities.