Utilizing weak pump depletion to stabilize squeezed vacuum states

One of the most important resources for a large number of fields in the quantum domain is the squeezed vacuum state. These states are essential for high profile research areas such as continuous variable quantum computing, quantum communication and quantum key distribution. Optical squeezed vacuum states often generated using an optical parametric amplifier (OPA) also have applications in fields requiring precise measurements such as quantum parameter estimation and gravitational wave detection. For these reasons the generation and stabilization of squeezed vacuum states has been an active field for a number of years. Often such applications require a stable squeezing angle, thus requiring an error signal and a controller to lock the angle to some reference, e.g. in an OPA the phase of the pump beam. All existing methods of generating the error signal, e.g. tilt locking, Pound-Drever-Hall etc. interact with the squeezed state in some way. As squeezed states are extremely sensitive to loss, such schemes potentially degrade the state. We proposed and demonstrated a pump-phase locking technique that uses a signal that can be measured without interaction with the squeezed state. This technique makes use of an unavoidable phenomenon known as weak pump depletion that is usually neglected. We show theoretically that the squeezing phase angle is imprinted on both the squeezed state and the normally discarded pump beam. We also show that this phase angle can be extracted from the pump beam and used to create an error signal that can be used to lock the squeezing angle. This is experimentally demonstrated and compared with a conventional locking technique for a squeezing (anti-squeezing) level of 1.96 ± .01 (3.78 ± 0.02) dB, without state degradation. These results are published in [T. Denker, D. Schütte, M. H. Wimmer, T. A. Wheatley, E. H.  Huntington, and M. Heurs, Optics Express, 16517 (2015)].

 Figure: Figure 1 OE_figureTAW2015.pdf

Figure 1. Squeezing spectrum around 197.4MHz with squeezing level (red) and antisqueezing level (green) versus scanned frequency. Left: OPO stabilization via dither locking. Right: OPO stabilization via WPD locking [Denker et al].