Second-order interference of two independent photons with different spectra

The second-order interference of two independent photons with different spectra in a Shih–Alley/Hong–Ou–Mandel interferometer is studied in Feynman's path integral theory. There is a second-order interference pattern for photons with different spectra if the photons are indistinguishable for the employed detection system. The conditions to observe the second-order temporal beating with photons of different spectra are analyzed. The influence of the response time of the detection system on the observed second-order interference pattern is also discussed. It is a direct result of that measurement in quantum mechanics is dependent on the employed measuring apparatus. The results are helpful to understand the physics of two-photon interference in different schemes.     

Photon Coalescence in a Lossy Non-Hermitian Beam Splitter

We investigate photon coalescence in a lossy non-Hermitian system and study a dynamic device modeled by a beam splitter with an extra intrinsic phase term added in the transformation matrix, with which the device is a lossy non-Hermitian linear system. The two-photon interference behavior is altered accordingly since this extra intrinsic phase affects the unitary of transformation and the coalescence of the incoming photons. We calculate the coincidence between two single-photon pulses, considering the interferometric phase between two pulses and the extra intrinsic phase as the tunable parameters. The extra phase turns the famous Hong–Ou–Mandel dip into a bump, with the visibility dependent on both the interferometric phase and the extra phase.     

A laser feedback interferometer with an oscillating feedback mirror

A method is proposed to solve the problem of direction discrimination for laser feedback interferometers.By vibrating the feedback mirror with a small-amplitude and high-frequency sine wave,laser intensity is modulated accordingly.The modulation amplitude can be extracted using a phase sensitive detector(PSD).When the feedback mirror moves,the PSD output shows a quasi-sine waveform similar to a laser intensity interference fringe but with a phase difference of approximately ±π/2.If the movement direction of the feedback mirror changes,the phase difference sign reverses.Therefore,the laser feedback interferometer offers a potential application in displacement measurement with a resolution of 1/8 wavelength and in-time direction discrimination.Without using optical components such as polarization beam splitters and wave plates,the interferometer is very simple,easy to align,and less costly.     

A Novel Method to Calculate the Initial Phase Difference between the Two Fibre Arms of Laser Homodyne Iuterferometer

We suggest a novel method to calculate the initial phase difference between two fibre arms of a laser homodyne interferometer. Put the two fibre arms in a temperature controller, whose short term stability is 0.02° C （measured in an hour）, then measure the interference photocurrent and the photocurrents from the two fibre arms at a fixed temperature. With these three photocurrents we can calculate the value of the initial phase difference. We set up a simple laser homodyne interferometer to test the theory. The experimental results are repeatable and the measurement precision is about 0.04°. It is theoretically and experimentally proven that this method is potentially easy and practical.     

Quantum metrology with two-mode squeezed thermal state: Parity detection and phase sensitivity

Based on the Wigner-function method, we investigate the parity detection and phase sensitivity in a Mach–Zehnder interferometer(MZI) with two-mode squeezed thermal state(TMSTS). Using the classical transformation relation of the MZI, we derive the input–output Wigner functions and then obtain the explicit expressions of parity and phase sensitivity.The results from the numerical calculation show that supersensitivity can be reached only if the input TMSTS have a large number photons.     

A Novel Mach–Zehnder Interferometer Based on Hybrid Liquid Crystal–Photonic Crystal Fiber

We propose a novel all fiber Mach–Zehnder interferometer(MZI) based on photonic crystal fiber(PCF) filled with liquid crystal(LC). The interference between the core mode and the cladding modes of a PCF is utilized.To excite the cladding modes, a region is formed using fiber fusion splicer. Due to the fact that varying effective index difference between the core region and the LC-filled cladding region can cause different transmission spectra,we mainly study the MZIs with different LC-filled structures and different lengths of LC filling. The measured results demonstrate that quite clear interference spectra can be obtained. Through analysis spatial frequency spectrum and temperature spectrum of two MZIs with different LC-filled structures, we can obtain that the MZI with adjacent two LC-filled holes has clearer interference spectrum and higher temperature sensitivity. Thus we choose this MZI to measure the temperature sensitivity with different lengths of LC filling. When the length of LC filling is 2 cm, the temperature sensitivities can be enlarged to 1.59 nm/C. The interferometer shows a good temperature tunability and sensitivity, which can be a good candidate for a highly tunable optical filtering and temperature sensing applications.     

Enhancing Phase Sensitivity in Mach–Zehnder Interferometers for Arbitrary Input States

To enhance the phase sensitivity of Mach–Zehnder interferometers,we use a tunable phase shift before the light beams are injected into the interferometer.The analytical result of the optimal phase shift is obtained,which only depends on the initial input states.For a non-zero optimal phase shift,the phase sensitivity of the interferometers in the output ports is always enhanced.We can achieve this enhancement for most states,including entangled and mixed states.The optimal phase shift is exhibited in three examples.Compared to previous methods,this scheme provides a general way to improve phase sensitivity and could find wide applications in optical phase estimations.     

An alternative phase modulator half-wave voltage measurement based on photonic link

We measure the half-wave voltage of LiNbO 3 phase modulators in a 26-GHz wideband frequency range, and then analyze it by the phase modulation photonic peak gain linked to the interferometric demodulation. The optical interferometer is constructed with two 50:50 couplers and two fiber arms with a 1-m difference in fiber length. The photonic link gain peaks are frequency dependent, as indicated by the differential time delay in the interferometer. The procedure described is a new and practical half-wave voltage measurement method that can accurately predict the nonlinear frequency characteristics of the phase modulator. Moreover, the method can be applied to various types of LiNbO 3 phase modulators.     

Interference from Two-Photon Sources in Silica-on-Silicon Circuits at Telecom Wavelength

The integrated photonic chip is a promising way to realize future quantum technology.Here we demonstrate a two-photon interference in the standard telecommunication band on a silica-on-silicon integrated photonic chip.Two identical photons in the 1.55 μm band,which are indistinguishable in spatial,frequency and polarization,are generated by type-Ⅰ collinear spontaneous parametric down-conversion via bismuth borate.The silica-on-silicon integrated chip,which has an insertion loss less than 1 dB,is a Mach-Zehnder interferometer with a thermo-optic phase shifter.A high visibility of 100% in the classical interference and 99.2% in the two-photon interference is achieved,indicating that the two-photon interference with high interference visibility on the chip is attained successfully.     

Coalescence and Anti-Coalescence Interference of Two-Photon Wavepacket in a Beam Splitter

We study theoretically the interference of a two-photon wavepacket in a beam splitter.We find that the spectrum symmetry for the two-photon wavepacket dominates the perfect coalescence and anti-coalescence interference.The coalescence interference is unrelated to photon entanglement.Only the anti-coalescence interference has evidence of photon entanglement.We prove that the two-photon wavepacket with an anti-symmetric spectrum is transparent to pass the 50/50 beam splitter,showing perfect anti-coalescence interference.     

Absorption interferometer of two-sided cavity

We propose a scheme in which an arbitrary incidence can be made perfectly reflected/transmitted with a phase modulator. We analyze the variation of intracavity field as well as output field with closed-loop phase φ1 of the control fields and relative phase φ2 of the probe beams. With two phases, medium absorption and light interference can be controlled so that photon escape from the cavity can be manipulated, thus an intensity switching based on phase modulation can be realized. And the condition for perfect transmitter or reflector is obtained. Then based on the transmission/reflection analysis,the total absorption of this system can be investigated. Therefore our scheme can be used as an absorption interferometer to explore the optical absorption in some complicated system. The state delay of the output light intensity, which is dependent on φ1 or φ2, can be applied in the realization of quantum phase gate and subtle wave filter. And based on this scheme, we implement the state transfer between perfect transmitter/reflector and non-perfect coherent photon absorber via relative-phase modulation.     

Phase estimation of phase shifts in two arms for an SU(1,1) interferometer with coherent and squeezed vacuum states

We theoretically study the quantum Fisher information(QFI) of the SU(1,1) interferometer with phase shifts in two arms by coherent ? squeezed vacuum state input, and give the comparison with the result of phase shift only in one arm.Different from the traditional Mach–Zehnder interferometer, the QFI of single-arm case for an SU(1,1) interferometer can be slightly higher or lower than that of two-arm case, which depends on the intensities of the two arms of the interferometer.For coherent ? squeezed vacuum state input with a fixed mean photon number, the optimal sensitivity is achieved with a squeezed vacuum input in one mode and the vacuum input in the other.     

Measurement of absolute phase Shift on reflection of thin films using white-light spectral interferometry

A novel method to measure the absolute phase shift on reflection of thin film is presented utilizing a white-light interferometer in spectral domain. By applying Fourier transformation to the recorded spectral interference signal, we retrieve the spectral phase function Ф, which is induced by three parts： the path length difference in air L, the effective thickness of slightly dispersive cube beam splitter Teff and the nonlinear phase function due to multi-reflection of the thin film structure. We utilize the fact that the overall optical path difference （OPD） is linearly dependent on the refractive index of the beam splitter to determine both L and Teff. The spectral phase shift on reflection of thin film structure can be obtained by subtracting these two parts from Ф. We show theoretically and experimentally that our new method can provide a simple and fast solution in calculating the absolute spectral phase function of optical thin films, while still maintaining high accuracy.     

Novel homodyne frequency-shifting interference pattern locking system

We present a novel homodyne frequency-shifting interference pattern locking system to enhance the exposure contrast of interference lithography and scanning beam interference lithography(SBIL). The novel interference pattern locking system employs a special homodyne redundant phase measurement interferometer(HRPMI) as the sensor and an acousto-opto modulator(AOM) as the actuator. The HRPMI offers the highly accurate value as well as the direction recognition of the interference pattern drift from four quadrature interference signals. The AOM provides a very fine resolution with a high speed for phase modulation. A compact and concise system with a short optical path can be achieved with this new scheme and a small power laser head in tens of microwatts is sufficient for exposure and phase locking, which results in a relatively low-cost system compared with the heterodyne system. More importantly, the accuracy of the system is at a high level as well as having robustness to environmental fluctuation. The experiment results show that the short-time(4 s) accuracy of the system is 0.0481 rad e3σT at present. Moreover, the phase of the interference pattern can also be set arbitrarily to any value with a high accuracy in a relatively large range, which indicates that the system can also be extended to the SBIL application.     

Generation and Quantum Interference of Entangled Electron-Hole Pairs in a Hanbury Brown and Twiss Interferometer

The Hanbury Brown-Twiss interferometer is proposed to serve as a detector of the crossed Andreev reflection and to generate entangled electron-hole pairs. It is shown that the non-local electron and hole induced by the crossed Andreev reflection are entangled. Quantum interference of the entangled electron-hole pairs gives rise to a new measurable effect of the phase difference between two superconductors in the cross correlation. The present theoretical predictions can be experimentally realized within the present-day microelectronic technology.     

Subwavelength asymmetric Au–VO_2 nanodisk dimer for switchable directional scattering

We propose an asymmetric Au–VO_2 nanodisk dimer for realizing a switchable directional scattering. Specifically, the directional scattering can be triggered on/off through controlling the phase transition of the VO_2 nanodisk from metallic to semiconductor state. More strikingly, an obvious directional scattering with the directivity of ～40 dB is achieved under the metallic state of VO_2 nanodisk. This tunable directional scattering is further explained with an interference model where the Au and VO_2 nanodisks are treated as two weakly interacting electric dipoles. The phase transition controlled scattering patterns of asymmetric Au–VO_2 nanodisk dimer are then well interpreted from the phase difference between these two dipoles.     

Stable multi-wavelength thulium-doped fiber laser based on all-fiber Mach–Zehnder interferometer

We propose and experimentally demonstrate a multi-wavelength thulium-doped fiber（TDF） laser based on all-fiber Mach–Zehnder interferometer（MZI） at 1.9 mm. Here a segment of 4 m single-mode TDF is pumped by 1568 nm fiber laser for 2 mm band optical gain. The MZI includes two cascaded 3 d B coupler. A segment of 3.5 m long un-pumped polarization-maintaining TDF and polarization controller（PC） are joined in the ring cavity to suppress the mode competition. Multi-wavelength lasers at 1.9 mm with wavelength number from one to four are obtained by adjusting the PC and the stability of output power of multi-wavelength fiber laser is analyzed.     

Resonator fiber optic gyros with light time-division input and multiplexing output in clockwise and counterclockwise directions

A resonator fiber optic gyro with the light time-division input and multiplexing output(TDM-RFOG)in the clockwise(CW)and counterclockwise(CCW)directions is proposed.The light time-division input in the CW and CCW directions can effectively suppress the backscattering induced noise.The TDM-RFOG is implemented with the 2×2 Mach–Zehnder interferometer(MZI)optical switch.The response time of the fiber ring resonator is analyzed,and it is demonstrated by experiments that light time-division input in the CW and CCW directions can reduce the backscattering induced noise.The suppression effectiveness of backscattering induced noise in the TDM-RFOG is determined by the extinction ratio of the optical switch,so a closed loop is designed to adjust the phase shift difference between the two arms of the MZI optical switch to control the extinction ratio.The method using two arms of the MZI optical switch with twin 90°polarization-axis rotated splices is proposed to make the extinction ratio along both slow and fast axes greater than-70 d B.     

All-Optical Modulation with a Surface Plasmon Mach-Zehnder Interferometer

We use two parallel nano-slits in a silver film to form a surface plasmon Mach Zehnder interferometer （MZI）, based on the interference of two surface plasmon waves propagating along the two surfaces of the silver film. Coating the silver film with a photoinduced birefringence polymer film, we achieve optical modulation of the MZI output by changing the refractive index of the polymer film with a pump beam. An on/off ratio 0f 2. 7 is obtained for a probe wavelength of 865 nm.     

Electronic transport through a quantum ring coupled to ferromagnetic leads

We study the spin-dependent transport through a one-dimensional quantum ring with taking both the Rashba spin--orbit coupling (RSOC) and ferromagnetic leads into consideration. The linear conductance is obtained by the Green's function method. We find that due to the quantum interference effect arising from the RSOC-induced spin precession phase and the difference in travelling phase between the two arms of the ring, the conductance becomes spin-polarized even in the antiparallel magnetic configuration of the two leads, which is different from the case in single conduction channel system. The linear conductance, the spin polarization and the tunnel magnetoresistance are periodic functions of the two phases, and can be efficiently tuned by the structure parameters.