Nondegenerate Optical Parametric Amplifier Research Articles

Continuous variable quantum entanglement at 1.34 m

Continuous variable (CV) quantum entanglement is a fundamental resource of CV quantum communication and quantum computation. It is useful in a wide variety of applications, including quantum teleportation, quantum dense coding, quantum key distribution, and high-precision quantum measurement. In this paper, we generate CV quantum entanglement at a telecommunication wavelength of 1342 nm by using a nondegenerate optical parametric amplifier (NOPA) with a type-Ⅱ periodically poled KTiOPO4 (PPKTP) crystal. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1342 nm) Nd:YVO4/LiB3O5 laser is achieved with output powers of 1.5 W (671 nm) and 1.3 W (1342 nm). Then a mode cleaner (MC1) with a fineness of 400 and linewidth of 0.75 MHz and a mode cleaner MC2 with a fineness of 400 and linewidth of 0.75 MHz are used to filter the noises of laser at 1342 nm and 671 nm, respectively. By using MCs, the intensity noise of laser reaches a shot noise level (SNL) for analysis frequencies higher than 1.0 MHz, and the phase noise of laser reaches an SNL for analysis frequencies higher than 1.3 MHz. Utilizing this kind of low noise single-frequency 671 nm laser as a pump, a doubly-resonant optical parametric oscillator with a threshold of 325 mW is realised. When the low noise single-frequency 1342 nm laser is injected as a signal and the relative phase between the pump and injected signal is locked to , the NOPA is operated at deamplification. After optimizing the temperature of the type-Ⅱ PPKTP crystal and at a pump power of 260 mW, Einstein-Podolsky-Rosen (EPR)-entangled beams with quantum correlation of 3.0 dB for both the amplitude and phase quadratures are experimentally generated. The strength of EPR-entangled beams is relatively low. It is maybe due to the low nonlinear conversion efficiency and large absorption of the type-Ⅱ PPKTP crystal at 671 nm and 1342 nm. The generated CV quantum entanglement at 1.34 m has lower transmission loss and smaller phase diffusion effect in a silica fiber. The research contributes to a high quality quantum source for the CV quantum communication based on existing telecommunication fiber networks.

Measurement of audio signal by using low-frequency squeezed light

Measurement of audio signal plays a significant role in many applications, such as gravitational wave detection, bio-particle imaging and magnetometer. In this paper, low-frequency squeezed light is generated by a non-degenerate optical parametric amplifier. In order to avoid the effect of injected light on low-frequency squeezing, an auxiliary laser is used to lock the length of non-degenerate optical parametric amplifier and a method of locking quantum noise is employed to lock the phase between the local light and the squeezed light. By isolating the vibration noises at low-frequency and reducing back action of parasitic interference, the squeezing of (7.1±0.1) dB takes place at 19 kHz. Then the squeezed light is injected into the Mach-Zehnder interferometer to measure an audio signal which drives a piezoelectric transducer to generate a small phase variation between two arms of Mach-Zehnder interferometer. According to the low-frequency squeezing, we realize experimentally the measurement of phase signal at audio frequency which exceeds the shot-noise limit of (3.0±0.4) dB. The experiment provides technical supports for the generation of low-frequency squeezed light and the measurement of audio signal. Furthermore it can be extended to other quantum measurements, such as high-precision magnetometer and measurement of small-displacement.

Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters.

In this paper, an optical phase-locked loop assisted by sum-frequency and second-harmonic generation (SS-OPLL) for frequency nondegenerate optical parametric phase-sensitive amplifier repeaters is experimentally demonstrated. First, theoretical derivations show that carrier extraction from phase-conjugated twin waves (PCTWs) and reference light generation are achieved by sum-frequency generation; therefore, the SS-OPLL circuit enables optical phase locking between PCTWs and a pump wave by a simple architecture based on a balanced OPLL. Then, optical phase locking between 20-Gbit/s quadrature phase-shift keying PCTWs and an individual pump source is experimentally demonstrated. Experimental results indicate that phase errors were reduced during the SS-OPLL operation.

The continuous-variable entanglement in a phase-mismatched frequency non-degenerated optical parametric amplifier

We present a new analytical solution for Fokker–Planck equation of phase-mismatched frequency non-degenerate optical parametric amplifier (FNOPA). We investigate the squeezing and continuous variable (CV) entanglement characteristic of a FNOPA working at phase-mismatched condition. We numerically calculate the dependence of the CV entanglement on the relative loss parameter ξ and the phase-mismatched parameter . The smaller can bring a long-time entanglement. This method can also be applied to multipartite entanglement situation.

Entanglement in a linear coherent feedback chain of nondegenerate optical parametric amplifiers

This paper is concerned with linear quantum networks of $N$ nondegenerate optical parametric amplifiers (NOPAs), with $N$ up to 6, which are interconnected in a coherent feedback chain. Each network connects two communicating parties (Alice and Bob) over two transmission channels. In previous work we have shown that a dual-NOPA coherent feedback network generates better Einstein-Podolsky-Rosen (EPR) entanglement (i.e., more two-mode squeezing) between its two outgoing Gaussian fields than a single NOPA, when the same total pump power is consumed and the systems undergo the same transmission losses over the same distance. This paper aims to analyze stability, EPR entanglement between two outgoing fields of interest, and bipartite entanglement of two-mode Gaussian states of cavity modes of the $N$-NOPA networks under the effect of transmission and amplification losses, as well as the effect of time delays on the outgoing fields. It is numerically shown that, in the absence of losses and delays, the network with more NOPAs in the chain requires less total pump power to generate the same degree of EPR entanglement. Moreover, we report on the internal entanglement synchronization that occurs in the steady state between certain pairs of Gaussian oscillator modes inside the NOPA cavities of the networks.

Scheme for efficient extraction of low-frequency signal beyond the quantum limit by frequency-shift detection.

Low-frequency (Hz~kHz) squeezing is very important in many schemes of quantum precision measurement. But it is more difficult than that at megahertz-frequency because of the introduction of laser low-frequency technical noise. In this paper, we propose a scheme to obtain a low-frequency signal beyond the quantum limit from the frequency comb in a non-degenerate frequency and degenerate polarization optical parametric amplifier (NOPA) operating below threshold with type I phase matching by frequency-shift detection. Low-frequency squeezing immune to laser technical noise is obtained by a detection system with a local beam of two-frequency intense laser. Furthermore, the low-frequency squeezing can be used for phase measurement in Mach-Zehnder interferometer, and the signal-to-noise ratio (SNR) can be enhanced greatly.

Quantum Coherent Feedback Control for Generation System of Optical Entangled State.

The non-measurement based coherent feedback control (CFC) is a control method without introducing any backaction noise into the controlled system, thus is specially suitable to manipulate various quantum optical systems for preparing nonclassical states of light. By simply tuning the transmissivity of an optical controller in a CFC loop attached to a non-degenerate optical parametric amplifier (NOPA), the quantum entanglement degree of the output optical entangled state of the system is improved. At the same time, the threshold pump power of the NOPA is reduced also. The experimental results are in reasonable agreement with the theoretical expectation.

Classical imaging with undetected photons.

Barreto Lemos et al. [Nature 512, 409–412 (2014)] reported an experiment in which a non-degenerate parametric downconverter and a non-degenerate optical parametric amplifier—used as a wavelength-converting phase conjugator—were employed to image object transparencies in a manner akin to ghost imaging. Their experiment, however, relied on single-photon detection, rather than the photon-coincidence measurements employed in ghost imaging with a parametric downconverter source. More importantly, their system formed images despite the photons that passed through the object never being detected. Barreto Lemos et al. interpreted their experiment as a quantum imager, as assuredly it is, owing to its downconverter’s emitting entangled signal and idler beams. We show, however, that virtually all the features of their setup can be realized in a quantum-mimetic fashion using classical-state light, specifically a pair of bright pseudothermal beams possessing a phase-sensitive cross correlation. Owing to its much higher signal-to-noise ratio, our bright-source classical imager could greatly reduce image-acquisition time compared to that of Barreto Lemos et al.‘s quantum system, while retaining the latter’s ability to image with undetected photons.

Continuous-variable entanglement measurement using an unbalanced Mach–Zehnder interferometer

We propose a simple scheme using an unbalanced Mach-Zehnder interferometer to directly measure the entangled beams with correlation of amplitude quadratures and anticorrelation of phase quadratures. In the experiment, we use two fibers with lengths of 2 and 50m to construct the interferometer. The correlation variances of amplitude difference and phase sum of the entangled beams from a nondegenerate optical parametric amplifier are simultaneously measured to be 1.79 and 1.62dB below the shot noise limit, respectively. Such a simple and convenient method has potential applications in quantum measurements.

Feedback-optimized extraordinary optical transmission of continuous-variable entangled states

We report on the feedback-optimized extraordinary optical transmission of continuous-variable entangled states through a hexagonal metal-hole array. The continuous-variable entanglements from a nondegenerate optical parametric amplifier are first demonstrated to survive after a photon-plasmon-photon conversion process. By controlling the reflected light from the metal-hole array, a significant enhancement of quantum correlations has then been experimentally achieved, compared to the case without such coherent feedback control. This result presents a useful technique to efficiently recover the substantial reflective losses in plasmonic circuits for quantum information processing.

Experimental generation of 84 dB entangled state with an optical cavity involving a wedged type-II nonlinear crystal

Entangled state of light is one of the essential quantum resources in quantum information science and technology. Especially, when the fundamental principle experiments have been achieved in labs and the applications of continuous variable quantum information in the real world are considered, it is crucial to design and construct the generation devices of entangled states with high entanglement and compact configuration. We have designed and built an efficient and compact light source of entangled state, which is a non-degenerate optical parametric amplifier (NOPA) with the triple resonance of the pump and two subharmonic modes. A wedged type-II KTP crystal inside the NOPA is used for implementing frequency-down-conversion of the pump field to generate the optical entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. The EPR entangled state of light with quantum correlations of 8.4 dB for both amplitude and phase quadratures are experimentally produced by a single NOPA under the pump power of 75 mW.

Coherent feedback enabled distributed generation of entanglement between propagating Gaussian fields

In this paper, we show how Einstein---Podolsky---Rosen-like entanglement between a pair of spatially separated propagating continuous-mode Gaussian fields can be generated via a coherent feedback loop that connects two spatially distant nondegenerate optical parametric amplifiers (NOPAs) over two transmission channels. In particular, the scheme generates entanglement in a distributed manner using spatially distributed resources. It is shown that similar to a single NOPA, the coherent feedback scheme has parameters that determine the achievable frequency-dependent two-mode squeezing and entanglement bandwidth between the pair of continuous-mode fields. It is also shown that in ideal scenarios, the feedback connection is able to yield an increase in the quality of the entanglement while consuming less power, compared to conventional distribution of entanglement using a single NOPA and a two-cascaded NOPA system. Furthermore, in contrast to the two conventional systems, under the same pump power, the coherent feedback system provides more entanglement in the presence of transmission losses, which indicates that the feedback scheme increases tolerance to transmission losses.

Nondegenerate parametric generation of 2.2-mJ, few-cycle 2.05-μm pulses using a mixed phase matching scheme

We describe the production of 2.2-mJ, ∼6 optical-cycle-long mid-infrared laser pulses with a carrier wavelength of 2.05 μm in a two-stage β-BaB2O4 nondegenerate optical parametric amplifier design with a mixed phase matching scheme, which is pumped by a standard Ti:sapphire chirped-pulse amplification system. It is demonstrated that relatively high pulse energies, short pulse durations, high stability, and excellent beam profiles can be obtained using this simple approach, even without the use of optical parametric chirped-pulse amplification.

Quantum limits for cascaded optical parametric amplifiers

We demonstrate the enhanced quantum noise reduction of the output field from an optical parametric amplifier (OPA) by employing the cascaded optical parametric amplifiers (COPA), in which the output field of one OPA is the input field of the subsequent one. The output field noise formulas of the cascaded degenerate and nondegenerate OPAs (CDOPA and CNOPA) are obtained. The analytical results show that a CDOPA and a CNOPA can generate squeezing beams and entangled beams with a wider bandwidth under a low pump parameter, respectively. The cavity output efficiency and the intracavity loss are the main factors for improving the degrees of squeezing and entanglement. Our work will provide improved schemes for designing COPAs to efficiently produce high-quality squeezed and entangled states, which have potential applications in quantum-information processing.

Demonstration of eight-partite two-diamond shape cluster state for continuous variables

Multipartite entangled state is the basic resource for implementing quantum information networks and quantum computation. In this paper, we present the experimental demonstration of the eight-partite two-diamond shape cluster states for continuous variables, which consist of eight spatially separated and entangled optical modes. Eight resource squeezed states of light with classical coherence are produced by four nondegenerate optical parametric amplifiers and then they are transformed to the eight-partite two-diamond shape cluster states by a specially designed linear optical network. Since the spatially separated multipartite entangled state can be prepared off-line, it can be conveniently applied in the future quantum technology.

Generating multiplexed entanglement frequency comb in a nondegenerate optical parametric amplifier

In this paper, we propose a scheme to produce a multiplexed entanglement frequency comb in a type II phase-matching nondegenerate optical parametric amplifier (NOPA) operating below threshold. The entanglement of the signal and idler frequency combs in the wide frequency range, which is limited by the phase-matching bandwidth of the NOPA, is investigated by the inseparability criterion. Furthermore, N Einstein–Podolsky–Rosen pairs can be created from the two combs without any reduction in the correlations by using frequency-dependent beam splitters.

Experimental study on the imaging of the squeezed-state light with a virtual object

A stable amplitude squeezed-state light was generated by utilizing the nondegenerate optical parametric amplifier based on periodically poled KTiOPO4 crystal at 1064 nm. We observed −4.93 dB of squeezing with a local oscillator phase locked in homodyne measurement. The imaging experiments of the resolution target have shown that the imaging resolution, which was based on squeezed-state light, is 1.41 times as much as the resolution that was acquired using coherent light as light source. In addition, the squeezed-state light was applied for imaging of the virtual object. It was found that the characters on the board were much more easily discerned with squeezed light as a light source than with coherent light. This result paves the way for further improvements of imaging resolution by use of the squeezed-state light.

Two-beam pumped infrared femtosecond nondegenerate optical parametric amplifier

We demonstrate a two-beam pumped infrared femtosecond (fs, 10−15s) broadband nondegenerate optical parametric amplifier (OPA). Compared with one-beam pumped femtosecond OPA, the output pulse energy is promoted relatively, and the broadband amplified pulse spectrum and pulse duration from the two-beam pumped infrared femtosecond OPA are similar. Though the two femtosecond pump beams are coherent and interferential with each other, the near field of the output beam is smooth and close to the Gaussian figuration. The results demonstrated that two pump sources can be used in the femtosecond OPA instead of one pump source for a combined higher pump energy and relative higher OPA amplified pulse energy with broadband femtosecond pulse duration.

Influence of pumping laser bandwidth on the quantum fluctuation chracteristic of a non-degenerate optical parametric amplifier

Usually the quantum fluctuation characteristic of a non-degenerate optical parametric amplifier is analysed under the assumption of monochromatic pumping. However, in experiments, a driving beam with finite bandwidth is used to obtain the non-degenerate signal and idler beam amplifications. On account of that, we derive an analytical solution for the non-degenerate optical parametric amplification system with finite bandwidth laser pumping, and evaluate the associated quantum fluctuation. Finally, the application of the V1 criterion to bipartite entanglement is discussed.

Generating a Schrödinger-cat-like state via a coherent superposition of photonic operations

We propose an optical scheme to generate a superposition of coherent states with enhanced size adopting an interferometric setting at the single-photon level currently available in the laboratory. Our scheme employs a nondegenerate optical parametric amplifier together with two beam splitters so that the detection of single photons at the output conditionally implements the desired superposition of second-order photonic operations. We analyze our proposed scheme by considering realistic on-off photodetectors with nonideal efficiency in heralding the success of conditional events. A high-quality performance of our scheme is demonstrated in view of various criteria such as quantum fidelity, mean output energy, and measure of quantum interference.