Quantum Sensors: Improved Optical Measurement via Specialized Quantum States

Figure 1

A Mach-Zehnder interferometer. Input at either of the input ports ( or ) has two possible ways of reaching the final beam splitter, . Along the upper path, a phase shift of is added. The second beam splitter makes it impossible to determine which path was followed simply from looking at the output at ports and . As a result, the amplitudes for the both possibilities must be present, leading to interference at the output ports.