Cunjin Liu

Quantum metrology with parametric amplifier-based photon correlation interferometers

Conventional interferometers usually utilize beam splitters for wave splitting and recombination. These interferometers are widely used for precision measurement. Their sensitivity for phase measurement is limited by the shot noise, which can be suppressed with squeezed states of light. Here we study a new type of interferometer in which the beam splitting and recombination elements are parametric amplifiers. We observe an improvement of 4.1±0.3 dB in signal-to-noise ratio compared with a conventional interferometer under the same operating condition, which is a 1.6-fold enhancement in rms phase measurement sensitivity beyond the shot noise limit. The improvement is due to signal enhancement. Combined with the squeezed state technique for shot noise suppression, this interferometer promises further improvement in sensitivity. Furthermore, because nonlinear processes are involved in this interferometer, we can couple a variety of different waves and form new types of hybrid interferometers, opening a door for many applications in metrology.

Realization of a nonlinear interferometer with parametric amplifiers

We construct an interferometer with parametric amplifiers as beam splitters. Because of the gain in the parametric amplifiers, the maximum output intensity of the interferometer can be much bigger than the input intensity as well as the intensity inside the interferometer (the phase sensing intensity). We find that the fringe intensity depends quadratically on the intensity of the phase sensing field at high gain. This type of nonlinear interferometer has better sensitivity than the traditional linear interferometer made of beam splitters with the same phase sensing intensity.

Realization of low frequency and controllable bandwidth squeezing based on a four-wave-mixing amplifier in rubidium vapor.

We experimentally demonstrate the creation of two correlated beams generated by a nondegenerate four-wave-mixing amplifier at λ=795 nm in hot rubidium vapor. We achieve intensity difference squeezing at frequencies as low as 1.5 kHz which is so far the lowest frequency to observe squeezing in an atomic system. The squeezing spans from 5.5 to 16.5 MHz with a maximum squeezing of -5 dB at 1 MHz. We can control the squeezing bandwidth by changing the pump power. Both low frequency and controllable bandwidth squeezing show great potential in sensitivity detection and precise control of the atom optics measurement.

Compact diode-laser-pumped quantum light source based on four-wave mixing in hot rubidium vapor

Using a nondegenerate four-wave mixing process in hot rubidium vapor, we demonstrate a compact diode-laser-pumped system for the generation of intensity-difference squeezing down to 8 kHz with a maximum squeezing of -7 dB. To the best of our knowledge, this is the first demonstration of kilohertz-level intensity-difference squeezing using a semiconductor laser as the pump source. This scheme is of interest for experiments involving atomic ensembles, quantum communications, and precision measurements. The diode-laser-pumped system would extend the range of possible applications for squeezing due to its low cost, ease of operation, and ease of integration.

Experimental investigation of the visibility dependence in a nonlinear interferometer using parametric amplifiers

Two four-wave mixing processes have been employed to experimentally construct a nonlinear interferometer [Jing et al., Appl. Phys. Lett. 99, 011110 (2011)], which has a better phase sensitivity than the traditional linear interferometer. For its applications in quantum measurement, interference fringe visibility can significantly affect the quantum detection efficiency. In this letter, we study how various parameters, such as the pump power, the one-photon detuning, and the two-photon detuning, influence the visibility of nonlinear interferometer. We find that the visibility greater than 0.9 can be achieved for large range of system parameters.

Optical logic gates using coherent feedback

We experimentally demonstrate optical logic “or” and “nor” gates via coherent feedback. Based on a four-wave mixing process in hot rubidium vapor, two feedback beams are capable of fulfilling an optical “nor” gate for the feedback-suppressed state and an optical “or” gate for the feedback-boosted state simultaneously. The logic gates exhibit transition times faster than previously demonstrated in rubidium vapor. Coherent photon conversion between the two logic states, due to the atomic coherence, is observed in the coherent feedback process.

Ultralow-light-level all-optical transistor in rubidium vapor

An all-optical transistor (AOT) is a device in which one light beam can efficiently manipulate another. It is the foundational component of an all-optical communication network. An AOT that can operate at ultralow light levels is especially attractive for its potential application in the quantum information field. Here, we demonstrate an AOT driven by a weak light beam with an energy density of 2.5 × 10−5 photons/(λ2/2π) (corresponding to 6  yJ/(λ2/2π) and about 800 total photons) using the double-Λ four-wave mixing process in hot rubidium vapor. This makes it a promising candidate for ultralow-light-level optical communication and quantum information science.

Manipulation of Quantum Correlations and Squeezing Enhancement in a Cascaded Four-Wave Mixing System

We experimentally demonstrate that quantum correlations from a cascaded four wave mixing process can be manipulated by controlling the relative phase inside this system. It results in a squeezing enhancement of about 4dB.

Realization of Nonlinear Interferometer using the Four Wave Mixing in Hot Rubidium Vapor

We experimentally realized a nonlinear interferometer which has a visibility close to 1 and can result in an enhancement of phase sensitivity with a factor of 2G2 compared to the linear interferometer.