Intensity-difference Squeezing Research Articles

Near-resonant twin-beam generation from degenerate four-wave mixing in hot 133Cs vapor enabled by field-dressed energy levels.

Squeezed light near an atomic resonance is beneficial for efficient atom-light quantum interfaces. It is desirable but challenging to directly generate in atoms due to excess noise from spontaneous emission and reabsorption. Here, we report on the use of energy-level modulation to actively control atomic coherence and interference in degenerate four-wave mixing (DFWM) and then to enhance the DFWM gain process for the generation of near-resonant squeezed twin beams. With this technique, we obtain a -2.6 dB intensity-difference squeezing detuned 100 MHz from the D1 F = 4 to F' = 4 transition of 133Cs.

Multichannel Six‐Wave Mixing with Spatial Multimode and Squeezing via Atomic Coherence

AbstractMultimode properties, based on nonlinear parametric processes, have potential research value on entanglement imaging. This paper studies the spatial and frequency multimode from dressing parametric‐amplified six‐wave mixing (PA‐SWM) process with three entangled signals. The S1 signal contains fifth‐order nonlinearity, the S2 signal contains electromagnetically induced absorption and fifth‐order nonlinearity, and the S3 signal contains Autler–Townes splitting (AT), second‐ and fifth‐order nonlinearity. The frequency multimode is attributed to dressing energy level splitting in spontaneous SWM (SSWM) process. The spatial multimode is due to frequency multimode and beam splitting via Kerr nonlinearity. In the spatial domain, it finds the optimal conditions of intensity difference squeezing (IDS) and spatial imaging along with their competitive relationship. On the one hand, the light spots with single spatial mode are contributed to the maximum IDS but not conducive to entanglement imaging; on the other hand, the light spots with multiple spatial modes are instrumental in entanglement imaging but not appropriate for the IDS. This research has potential applications in squeezed light sources for quantum metrology, and correlated imaging of spatial signals.

Enhancement of quadripartite quantum correlation via phase-sensitive cascaded four-wave mixing process

Multipartite quantum correlation is important for both fundamental science and quantum information processing. Its enhancement is critical to improve the performance of physical systems. Therefore, it is of great practical significance to study the enhancement of multipartite quantum correlation. Here, we theoretically investigate a scheme for enhancing quadripartite quantum correlation by utilizing a phase-sensitive cascaded four-wave mixing (CFWM) process. We find that the intensity-difference squeezing (IDS) among the four output beams generated by the phase-insensitive CFWM process can be largely enhanced by introducing the phase-sensitive CFWM process. We also find that our phase-sensitive CFWM process can be used to generate intensity-sum squeezing (ISS), which cannot be generated by the phase-insensitive CFWM process. Moreover, we find that the maximum squeezing value of ISS is equal to that of IDS when the intensity gains of the three four-wave mixing processes are equal in the phase-sensitive cascaded scheme. In the end, we discuss the effects of the losses and the phase fluctuations on the squeezing values of our phase-sensitive CFWM process and the corresponding phase-insensitive CFWM process with the same intensity gains. It can be seen that the squeezing enhancement of our phase-sensitive CFWM process holds also in presence of the losses and the phase fluctuations. Our results pave the way for experimental implementation and may find applications in quantum metrology and quantum communication.

Enhanced intensity difference squeezing with a low gain off-resonant Four-Wave Mixing in potassium vapor

Although potassium stands out among alkalies by its smallest ground state hyperfine splitting, smaller than Doppler broadening, investigations of its four-wave mixing (FWM) quantum properties, like squeezing and entanglement, were neglected. Here we present measurements of quantum correlations in potassium vapor using far-detuned double Λ scheme for FWM. We obtained the relative intensity difference squeezing (IDS) of -6.1 dB, with the squeezing bandwidth between 0.9 to 4.5 MHz on the noise spectra. The squeezing maximum in potassium compares well with the best results obtained in Rb and Cs, and has larger value than previously reported IDS in potassium. This result agrees qualitatively well with the squeezing calculated using analytical expressions from the model of operators when with chosen parameters FWM generates both squeezing and probe at their maximums. The ultimate squeezing level in potassium, predicted by the model, when conjugate absorption in the cell and optical losses behind the cell are neglected, is −10.29 dB. This is among the largest predicting squeezing obtained by FWM in alkalies.

Transmission estimation at the quantum Cramér-Rao bound with macroscopic quantum light

The field of quantum metrology seeks to apply quantum techniques and/or resources to classical sensing approaches with the goal of enhancing the precision in the estimation of a parameter beyond what can be achieved with classical resources. Theoretically, the fundamental minimum uncertainty in the estimation of a parameter for a given probing state is bounded by the quantum Cramér-Rao bound. From a practical perspective, it is necessary to find physical measurements that can saturate this fundamental limit and to show experimentally that it is possible to perform measurements with the required precision to do so. Here we perform experiments that saturate the quantum Cramér-Rao bound for transmission estimation over a wide range of transmissions when probing the system under study with a continuous wave bright two-mode squeezed state. To properly take into account the imperfections in the generation of the quantum state, we extend our previous theoretical results to incorporate the measured properties of the generated quantum state. For our largest transmission level of 84%, we show a 62% reduction over the optimal classical protocol in the variance in transmission estimation when probing with a bright two-mode squeezed state with −8 dB of intensity-difference squeezing. Given that transmission estimation is an integral part of many sensing protocols, such as plasmonic sensing, spectroscopy, calibration of the quantum efficiency of detectors, etc., the results presented promise to have a significant impact on a number of applications in various fields of research.

Generation of tripartite squeezed state by cascaded four-wave mixing in single hot rubidium atomic system

We report the quantum correlated triple beams via cascaded four-wave mixing (CFWM) amplified in single hot atomic vapor. Experimentally, we show that strong quantum correlation of three light beams, among them any two’s quantum correlation is characterized by the maximum value of intensity-difference squeezing (IDS) about −7.8 ± 0.3 dB. We found there is IDS between two idler beams, because two pairs of Einstein–Podolsky–Rosen injections potentially exist in our system. Besides, CFWM can emit three-mode beams at three different frequencies, in which these beams can be well separated in the spatial domain. Moreover, much difference with other methods, the injecting probe field can manipulate the gain and IDS of output three-mode light beams, which is resulting from competition relationship between cascaded two four-wave mixing processes. More interestingly, Autler–Townes splitting of gain peaks of output signals due to dressing effect of pumping fields, will lead to the evolution of measured two- and three-mode IDS from single-mode to multi-mode at frequency domain. This result will provide a multimode quantum resource which can potentially realize multimode entanglement and quantum networks.

Experimental generation of multimode quantum correlations between a conical probe and a conical conjugate based on a four-wave mixing process

Multimode quantum correlations and entanglements has drawn much attention recently due to its importance for both fundamental science and the future development of quantum information processing and quantum communication. Here, by using a four-wave mixing process with a conical pump beam and a conical probe beam, we experimentally generate the multimode quantum correlations between a conical probe beam and a conical conjugate beam, and we also observe about –2.6-dB intensity-difference squeezing between these two conical beams. In addition, we find the optimal value of each parameter in this scheme for further applications. Besides, the multi-spatial-mode nature of the generated quantum correlated beams is shown by comparing the variation tendencies of the intensity-difference noise of the probe and conjugate beams under global attenuation and local cutting attenuation. Our scheme may find potential applications in quantum information processing and quantum communication.

Phase transition filter from controllable hybrid line shape of correlation and squeezing of Pr3+: YPO4 and Eu3+: YPO4

We report the symmetry-dependent intensity–noise correlation and the intensity-difference squeezing of spontaneous parametric four-wave mixing (SP-FWM) and fluorescence (FL) hybrid signals from the different phases [hexagonal (H) + tetragonal (T), T + H] of Pr3+: YPO4 and Eu3+: YPO4 crystals. The competition between FL and SP-FWM in hybrid signals determines the amplitude and hybrid line shape of the two-mode and three-mode correlation. The correlation amplitude switches from positive to negative due to strong double dressing caused by a high transition probability and a dipole moment of the (more H + less T)-phase of Pr3+: YPO4. This switching phenomenon is used to realize a metal–oxide semiconductor field-effect transistor-based logic inverter. Furthermore, the line shape of correlation changes from broad to sharp due to weak dressing, transition probability, and a low dipole moment of the (more T + less H)-phase of Pr3+: YPO4. Based on these results, we proposed the model of a tunable correlation filter. Gate position, frequency detuning, and laser power control the amplitude and hybrid line shape. Our experimental results provide an advanced technique of bandwidth contrast of about 0.9%. Such notch filters have potential applications in quantum communications.

Nonlinear interferometric surface-plasmon-resonance sensor.

A nonlinear interferometer can be constructed by replacing the beam splitter in the Mach-Zehnder interferometer with four-wave mixing (FWM) process. Meanwhile, the conventional surface plasmon resonance (SPR) sensors can be extensively used to infer the information of refractive index of the sample to be measured via either angle demodulation technique or intensity demodulation technique. Combined with a single FWM process, a quantum SPR sensor has been realized, whose noise floor is reduced below standard quantum limit with sensitivity unobtainable with classical SPR sensor. Therefore, in this work we have theoretically proposed a nonlinear interferometric SPR sensor, in which a conventional SPR sensor is placed inside nonlinear interferometer, which is called as I-type nonlinear interferometric SPR sensor. We demonstrate that near resonance angle I-type nonlinear interferometric SPR sensor has the following advantages: its degree of intensity-difference squeezing, estimation precision ratio, and signal-noise-ratio are improved by the factors of 4.6 dB, 2.3 dB, and 4.6 dB respectively than that obtained with a quantum SPR sensor based on a single FWM process. In addition, the theoretical principle of this work can also be expanded to other types of sensing, such as bending, pressure, and temperature sensors based on a nonlinear interferometer.

Characterization of quantum squeezing generated from the phase-sensitive and phase-insensitive amplifiers in the ultra-low average input photon number regime.

We give the general expressions of intensity-difference squeezing (IDS) generated from two types of optical parametric amplifiers [i.e. phase-sensitive amplifier (PSA) and phase-insensitive amplifier (PIA)] based on the four-wave mixing process, which clearly shows the IDS transition between the ultra-low average input photon number regime and the ultra-high average input photon number regime. We find that both the IDS of the PSA and the IDS of the PIA get enhanced with the decrease of the average input photon number especially in the ultra-low average input photon number regime. This result is substantially different from the result in the ultra-high average input photon number regime where the IDS does not vary with the average input photon number. Moreover, under the same intensity gain, we find that the optimal IDS of the PSA is better than the IDS of the PIA in the ultra-low average input photon number regime. Our theoretical work predicts the presence of strong quantum correlation in the ultra-low average input photon number regime, which may have potential applications for probing photon-sensitive biological samples.

Quantum Noise Correlations of an Optical Parametric Oscillator Based on a Nondegenerate Four Wave Mixing Process in Hot Alkali Atoms.

We present the first measurement of two-mode squeezing between the twin beams produced by a doubly resonant optical parameter oscillator (OPO) in an above threshold operation based on parametric amplification by nondegenerate four wave mixing with rubidium (^{85}Rb). We demonstrate a maximum intensity difference squeezing of -2.7 dB (-3.5 dB corrected for losses) with a pump power of 285mW and an output power of 12mW for each beam, operating close to the D1 line of Rb atoms. The use of open cavities combined with the high gain media can provide a strong level of noise compression and the access to new operation regimes that could not be explored by crystal based OPOs. The spectral bandwidth of the squeezed light is broadened by the cavity dynamics, and the squeezing level is robust for strong pump powers. Stable operation was obtained up to 4 times above the threshold. Moreover, operation of the OPO close to the atomic resonances of alkali atoms allows a natural integration into quantum networks, including structures such as quantum memories.

Switching of enhancement and suppression in dressed Eu3+: YPO4 and Pr3+:YPO4

We investigated the evolution of spectral intensity of the composite signal (Stokes (ES) + fluorescence (FL)) by varying time delay in different phases (tetragonal (T), hexagonal (H), (T + H) and (H + T)) of Eu3+:YPO4 and Pr3+:YPO4. In a composite signal, there exists a competition between FL and ES that can be modulated through dressing effect. We observed that dressing effect has strong dependence on dipole moment, transition probability and phase symmetry of atomic like media. The dressing suppression dip in H- and T-phase YPO4 is relatively weaker than mixed (H + T)-phase YPO, while the dressing effect is weaker in Eu3+:YPO4 as compared to Pr3+:YPO4. By adjusting the time delay, FL signal can be switched from suppression dip to enhancement peak and vice versa. Further, we explored intensity-difference squeezing and intensity-noise correlation between composite signal (FL + ES) and anti-Stokes under nonlinear phase modulated by time delay. Based on our spectral intensity results, we realized the model of dressing based switch.

Enhancement of quantum correlations using correlation injection scheme in a cascaded four-wave mixing processes.

Quantum correlations and entanglement shared among multiple quantum beams are important for both fundamental science and the development of quantum technologies. The enhancement for them is necessary and important to implement the specific quantum tasks and goals. Here, we report a correlation injection scheme (CIS) which is an effective method to enhance the quantum correlations and entanglement in the symmetrical cascaded four-wave mixing processes, and the properties of quantum correlations and entanglement can be characterized by the values of the degree of intensity-difference squeezing (DS) and the smallest symplectic eigenvalues, respectively. Our results show that the CIS can enhance the quantum correlations and entanglement under certain conditions, while for other conditions it can only decrease the values of the DS and the smallest symplectic eigenvalues to the level of standard quantum limit, respectively. We believe that our scheme is experimentally accessible and will contribute to a deeper understanding of the manipulations of the quantum correlations and entanglement in various quantum networks.

Enhanced squeezing of four-wave mixing by phase-sensitive dressed effect in a hot atomic system

In this paper, we demonstrate that the intensity difference squeezing (IDS) of parametric amplified four-wave mixing (PA-FWM) can be modulated by the phase-sensitive dressed effect in a hot rubidium (Rb) atomic system. The introduction of phase-sensitive dressed effect can enhance the degree of IDS of PA-FWM to −8.1 dB, as compared to that in the phase-insensitive dressed PA-FWM of −6.6 dB. The phase-sensitive dressing field can not only enhance the degree of IDS of PA-FWM by the dressed effect, but can also be treated as the seed beam of the PA-six wave mixing process. So, double-injected three-mode IDS obviously has a bigger degree of IDS compared with the single-injection case. One advantage of the current scheme for enhancing the generated IDS is that the experimental setup is simple and mechanically stable. These outcomes can be used in the fabrication of quantum devices and the realization of quantum metrology.

Experimental characterization of multiple quantum correlated beams in two-beam pumped cascaded four-wave mixing process.

We experimentally explore the relationships between the number of multiple quantum correlated beams generated by two-beam pumped cascaded four-wave mixing (CFWM) process and the system parameters, such as the angle between the two pump beams, one-photon detuning and two-photon detuning. We find that all of three system parameters can influence the number of multiple quantum correlated beams. Under the optimal system parameters, we can observe the emission of up to 14 quantum correlated beams with the intensity-difference squeezing of -6.29 ± 0.20 dB (-7.93 ± 0.64 dB after accounting for losses) from such CFWM scheme. Our results may find potential applications in building multi-user quantum network and multi-parameter quantum metrology.

Two- and three-mode intensity-difference squeezing in cascaded four-wave mixing inside an optical cavity

We report a theoretical study about the two- and three-mode intensity-difference squeezing inside an atom-like optical cavity. A cascaded parametric amplification four-wave mixing process is considered, and the cavity dressing and ‘dressing-like’ effects with different nonlinear coefficients affecting the degree of squeezing of the fluctuation spectra are researched. The altering of the nonlinear coupling coefficient changes the relationship between the cavity dressing and ‘dressing-like’ effects, which determines the squeezing degree and results in a new type of multi-dip squeezing and double-peak anti-squeezing.

Logic NOT gates of the intensity–noise correlation and squeezing in a diamond nitrogen-vacancy center

Color defects in a diamond nitrogen-vacancy (NV) center represent an important physical system for emergent quantum technologies such as logic gates and registers at room temperature. We realized an optical logic NOT gate by inducing a phase shift in a diamond NV center under nonlinear Kerr effect of spontaneous parametric four-wave mixing (SP-FWM). Also, we discussed the intensity–noise correlation and intensity-difference squeezing of SP-FWM and composite signals (SP-FWM+fluorescence) under self-phase modulation (SPM) and cross-phase modulation (XPM) induced by the nonlinear Kerr effect in V-type and double V-type atomic systems. Our experimental results provide an advanced technique of switching contrast by approximately 90%. Different parameters, such as the power of input laser beam and boxcar gate position, were also configured to control the SPM and XPM, which determine the switching phenomenon for correlation and squeezing.

Interference-Induced Quantum Squeezing Enhancement in a Two-beam Phase-Sensitive Amplifier.

We experimentally demonstrate a method for realizing quantum squeezing enhancement which is induced by the interference in a two-beam phase-sensitive amplifier (PSA) based on a four-wave mixing process. Compared to the normal phase-insensitive amplifier with an intensity-difference squeezing (IDS) of 8.97±0.24 dB or 8.76±0.26 dB, the IDS of our two-beam PSA is enhanced to 10.13±0.21 dB under the same experimental situation. Furthermore, we study how various parameters influence the quantum squeezing enhancement of the PSA. These results clearly show that the physical mechanism inducing the IDS enhancement of the two-beam PSA is its intrinsic interference nature. Our results may find potential applications in improving the fidelity of quantum information processing and the precision of quantum metrology.

Modulation gain and squeezing by dressed state in hot atomic system

We report that the optical gain and intensity difference squeezing (IDS) of four-wave mixing can be controlled using an external dressing field in a hot rubidium atomic system. The intensity of the parametric amplified four-wave mixing (PA-FWM) signal in the probe and corresponding conjugate channels can be enhanced using the dressed effect, resulting in an increase of the optical gain. So, the IDS in the dressed PA-FWM has a higher degree of −7.1 dB, as compared to that in the PA-FWM of −2.5 dB. This scheme of enhancing the optical gain and the generated IDS requires a simple experimental setup that is mechanically stable. These outcomes can be used in the fabrication of quantum devices and the realization of quantum metrology.

Long-distance distribution of the telecom band intensity difference squeezing generated in a fiber optical parametric amplifier.

Using an all-fiber source of pulsed twin beams, with the record intensity difference squeezing of 6.1±0.15 dB for fiber optical parametric amplifiers, we experimentally distribute the pulsed continuous variable (CV) quantum state over a long distance. After separating the signal and idler twin beams by 26km single-mode fibers, the measured intensity difference noise is still below the shot noise limit by 2dB. The result shows that the distribution process is only affected by the vacuum noise originated from the fiber transmission loss. The extra noise contributed by the acoustic wave Brillouin scattering and Raman scattering in 26km transmission fibers has not been observed. The investigation is beneficial for studying long-distance CV quantum communication.