[Paper Review] Controllable generation of highly nonclassical states from nearly pure squeezed vacua

Kentaro Wakui, H. Takahashi+2 more|CERN Bulletin|Sep 20, 2006

Quantum Information and Cryptography1 references120 citations

TL;DR

This paper presents a controllable scheme for generating highly nonclassical light states—such as single-photon and Schrödinger kitten states—by subtracting photons from a nearly pure squeezed vacuum generated via an optical parametric oscillator with periodically poled KTiOPO4. The method achieves the deepest negative Wigner function dips ever observed, with W(0,0) = −0.065, demonstrating high nonclassicality through precise control of squeezing via pump power.

ABSTRACT

We present controllable generation of various kinds of highly nonclassical states of light, including the single photon state and superposition states of mesoscopically distinct components. The high nonclassicality of the generated states is measured by the negativity of the Wigner function, which is largest ever observed to our knowledge. Our scheme is based on photon subtraction from a nearly pure squeezed vacuum, generated from an optical parametric oscillator with a periodically-poled KTiOPO$_4$ crystal as a nonlinear medium. This is an important step to realize basic elements of universal quantum gates, and to serve as a highly nonclassical input probe for spectroscopy and the study of quantum memory.

Motivation & Objective

• To develop a controllable method for generating highly nonclassical photonic states, including single-photon and mesoscopically distinct superposition states (Schrödinger kittens).
• To overcome limitations in prior photon subtraction schemes by using a nearly pure squeezed vacuum input, enabling high signal-to-noise ratio in trigger photon detection.
• To achieve the deepest negative Wigner function dips to date, serving as a benchmark for nonclassicality in continuous-variable quantum states.
• To enable a continuous transition from the single-photon regime to the mesoscopic superposition regime by tuning the squeezing level.

Proposed method

• Utilizes a continuous-wave optical parametric oscillator (OPO) with a periodically poled KTiOPO4 (PPKTP) crystal to generate a nearly pure squeezed vacuum state.
• Employs a tapping beam splitter (TBS) to extract a small fraction of the squeezed beam as a trigger for photon counting.
• Uses a commercial Si-APD with three filtering cavities to detect trigger photons, with the detection modeled as an on/off positive operator-valued measure (POVM).
• Conditions the output state in path A on the detection of trigger photons, resulting in a photon-subtracted squeezed state.
• Applies a mode function expansion to define the signal mode, with optimal mode functions determined by minimizing square error in the state reconstruction.
• Constructs the Wigner function using covariance matrices and the on/off detector model, incorporating effective transmittance, loss, and noise parameters to correct for experimental imperfections.

Experimental results

Research questions

• RQ1 Can photon subtraction from a nearly pure squeezed vacuum generate highly nonclassical states with deeper negative Wigner function dips than previously achieved?
• RQ2 How does the squeezing level, controlled by pump power, influence the transition between single-photon and mesoscopic superposition states?
• RQ3 To what extent can experimental imperfections such as loss and detector noise be mitigated to preserve the negativity of the Wigner function?
• RQ4 Can the scheme achieve a high-fidelity, controllable generation of nonclassical states suitable for quantum information processing and quantum metrology?

Key findings

• The scheme achieved the deepest negative Wigner function dip ever observed, with W(0,0) = −0.065, significantly deeper than previous results such as −0.04 for single-photon states and −0.026 for Schrödinger kittens.
• The high nonclassicality is enabled by using a nearly pure squeezed vacuum input, which allows efficient and high signal-to-noise ratio detection of trigger photons from a small beam fraction (1–10%).
• The Wigner function negativity is robust against experimental imperfections due to careful modeling of loss and detector noise, with a phenomenological model for squeezing-level-dependent loss (τs(z) = τs0 − κz²) used to explain degradation at higher z.
• The system enables continuous tuning from the single-photon regime to the mesoscopic superposition regime by adjusting the pump power, thereby controlling the squeezing level.
• Theoretical modeling of the Wigner function using the on/off detector POVM and covariance matrix formalism accurately reproduces the experimental data, validating the state reconstruction.