Schrödinger cat states without hybrid measurement
Schrödinger's cat was proposed to illustrate the absurdity of quantum mechanics. Naturally there has been an effort made in the last few years to generate such states in a laboratory to observe this aspect of quantum mechanics first hand.
Rather than having a cat that is alive or dead in a box, our cat states are prepared using laser light that is has a phase of either 0 or π in phase space. When you measure the state, the wavefunction of the light collapses and the phase becomes apparent.
To make the cat states we start with a
squeezed source of light and split it into 2 parts (top figure). A small amount is sent to a conditioning detector that measures the amplitude and phase of the light. This data is recorded. Simultaneously we record measurements of the remaining part of the squeezed beam. Using the data in the conditioning signal, we can uncover the hidden cats in the "photonsubtracted" squeezed light.
To be sure that you have really prepared a cat state, you have to prepare and measure a huge number of cats and look at the statistics of the measured phase. The key signature of a cat state is a negative value in the phase space probability distribution. Classical probability functions only allow values between 0 and 1. Quantum mechanical quasiprobability distributions can take on negative values, as shown in the bottom figure.
Publications:
Photonnumber discrimination without a photon counter and its application to reconstructing nonGaussian states
H. M. Chrzanowski, J. Bernu, B. M. Sparkes, B. Hage, A. P. Lund, T. C. Ralph, P. K. Lam, and T. Symul

The homodyne detectors (green box on the right) are used to measure a part of the squeezed beam that is split off using a beamsplitter. The results of these measurements are used to discover the Schrödinger cat states hidden in the remaining part of the beam that is detected in the homodyne system shown in the purple box.


A cat state conditioned on having a single photon in the conditioning beam.

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