Alessandro S. Villar

Extra phase noise from thermal fluctuations in non-linear optical crystals

Recent experiments with Optical Parametric Oscillators have shown [1–3] that an unpredicted extra noise on the phase of the fields reduces the level of squeezing or quantum correlations. In some cases, it can hinder the attempts to observe entanglement in those systems [4]. It is probably the main reason why signal and idler entanglement above threshold remained unobserved by almost 20 years after its prediction [5].

Analyzing the Gaussian character of the spectral quantum state of light via quantum noise measurements

Gaussian quantum states hold special importance in the continuous variable (CV) regime. In quantum information science, the understanding and characterization of central resources such as entanglement may strongly rely on the knowledge of the Gaussian or non-Gaussian character of the quantum state. However, the quantum measurement associated with the spectral photocurrent of light modes consists of a mixture of quadrature observables. Within the framework of two recent papers [Phys. Rev. A 88, 052113 (2013) and Phys. Rev. Lett. 111, 200402 (2013)], we address here how the statistics of the spectral photocurrent relates to the character of the Wigner function describing those modes. We show that a Gaussian state can be misidentified as non-Gaussian and vice-versa, a conclusion that forces the adoption of tacit textit{a priori} assumptions to perform quantum state reconstruction. We experimentally analyze the light beams generated by the optical parametric oscillator (OPO) operating above threshold to show that the data strongly supports the generation of Gaussian states of the field, validating the use of necessary and sufficient criteria to characterize entanglement in this system.

Coherent Manipulation of Motional States of a Single Trapped Ion

The ability to precisely manipulate an isolated quantum system opens new avenues in the simulation of more complicated quantum systems, using the tools of quantum computation. While the paradigm of quantum computation nowadays considers a collection of two-level systems known as qubits, it is also possible to generalize the basic logic quantum units to multi-level or even continuous systems.

Triple Quantum Correlations from an Above-Threshold Optical Parametric Oscillator

We measured triple quantum correlations between the bright beams in an optical parametric oscillator, operating above threshold. Owing to extra noise in the system, still unaccounted for, tripartite entanglement is yet to be demonstrated.

Entanglement and noise in the above-threshold optical parametric oscillator

We review the measurement of entanglement in the above-threshold optical parametric oscillator, an experimental challenge that lasted 17 years. The challenge was in part technical, owing to the difficulty of measuring phase noise, but also related to unexpected noise features that remain unexplained. We describe the experimental strategies used to measure phase noise and to circumvent the extra noise. The theoretical prediction of a higher order of entanglement is presented, as well as measurements of bright three-color quantum correlations.

Quantum Key Distribution with Bright Twin Beams

We have implemented quantum key distribution with bright twin beams produced by an optical parametric oscillator. Switching between amplitude and phase quadrature measurements is done by self-homodyne detection without local oscillators.

Bright Entangled Beams from an Above-Threshold Optical Parametric Oscillator

We generated bright entangled beams in an optical parametric oscillator, operating above threshold. Tripartite pump-signal-idler entanglement, all with different frequencies, was also predicted and three-color quantum correlations already measured.