We have demonstrated, for the first time, coherent terahertz (THz)-wave phase detection using nonlinear optical interference based on backward THz-wave parametric oscillation (BW-TPO). Slant-stripe-type periodically poled lithium niobate (PPLN) crystal were used as the BW-TPO gain medium to generate and detect the backward-propagating THz-wave at 0.3 THz. Phases of all pump, idler, and THz-wave were detected from the interference between the generation side idler and the detection signal. This is the first report of a comprehensive study of the phase information transfer by modulating and detecting all phases contributing to coherent THz-wave detection. This interference with suppressed phase noise achieved a THz-wave phase detection with a high visibility of 0.89. This achievement not only has sensing applications that simultaneously measure the real and imaginary parts of the refractive index as a stand-alone system but also reaches the limit of classical detection sensitivity when combined with highly sensitive THz-wave detection. Furthermore, this works paves way to the realization of ultra-sensitive THz-wave detection beyond the standard quantum limit by the non-classical states of squeezed light in our system.
We have demonstrated, for the first time, coherent terahertz (THz)-wave phase detection using nonlinear optical interference based on backward THz-wave parametric oscillation (BW-TPO). Slant-stripe-type periodically poled lithium niobate (PPLN) crystal were used as the BW-TPO gain medium to generate and detect the backward-propagating THz-wave at 0.3 THz. Phases of all pump, idler, and THz-wave were detected from the interference between the generation side idler and the detection signal. This is the first report of a comprehensive study of the phase information transfer by modulating and detecting all phases contributing to coherent THz-wave detection. This interference with suppressed phase noise achieved a THz-wave phase detection with a high visibility of 0.89. This achievement not only has sensing applications that simultaneously measure the real and imaginary parts of the refractive index as a stand-alone system but also reaches the limit of classical detection sensitivity when combined with highly sensitive THz-wave detection. Furthermore, this works paves way to the realization of ultra-sensitive THz-wave detection beyond the standard quantum limit by the non-classical states of squeezed light in our system.
Using nonlinear interferometers is highly attractive for accessing spectral regions in which the photon detection is technically challenging. Especially for terahertz radiation this is a long-lasting problem, as their photon energy is in the range of a few meV. Therefore, the properties of terahertz photons after interaction with a sample are transferred to visible photons, whose detection can be realized by widely available imaging sensors. We demonstrated this concept with terahertz photons propagating in free space, determining the thickness of polytetrafluoroethylene pla tes by only detecting visible photons. We utilized a nonlinear interferometer with one periodically poled lithium niobate crystal driven by a 660-nm pump source. In the crystal, pairs of visible signal and correlated terahertz photons are created and separated into different paths of the interferometer afterwards. The terahertz photons pass the sample and gain information that can be transferred to the visible photons. To detect the signal photons with an uncooled sCMOS camera, the pump photons are filtered out by narrowband volume Bragg gratings. As the signal photons are mainly generated by down - and up-conversion of thermal photons besidesspontaneous parametric down-conversion, the frequency-angular spectra show nonlinear interference in the Stokes and anti-Stokes regions. This interference can be used to assess information of coatings that are mainly transparent in the terahertz frequency range. Establishing on this first demonstration of a nonlinear interferomete r with terahertz photons, we improved the visibility by a factor of 3 by modifying the experimental setup, helping this concept on its way towards industrial applications.
Phase-sensitive detection is the essential projective measurement for measurement-based continuous-variable quantum information processing. The bandwidth of conventional electrical phase-sensitive detectors is up to several gigahertz, which would limit the speed of quantum computation. It is theoretically proposed to realize terahertz-order detection bandwidth by using all-optical phase-sensitive detection with an optical parametric amplifier (OPA). However, there have been experimental obstacles to achieve large parametric gain for continuous waves, which is required for use in quantum computation. Here, we adopt a fiber-coupled χ (2) OPA made of a periodically poled LiNbO 3 waveguide with high durability for intense continuous-wave pump light. Thanks to that, we manage to detect quadrature amplitudes of broadband continuous-wave squeezed light. 3 dB of squeezing is measured up to 3 THz of sideband frequency with an optical spectrum analyzer. Furthermore, we demonstrate the phase-locking and dispersion compensation of the broadband continuous-wave squeezed light, so that the phase of the squeezed light is maintained over 1 THz. The ultra-broadband continuous-wave detection method and dispersion compensation would help to realize all-optical quantum computation with over-THz clock frequency.
We achieved continuous-wave 8.3-dB squeezed light generation using a terahertz-order-broadband waveguide optical parametric amplifier by improving a measurement setup from our previous work [T. Kashiwazaki et al., Appl. Phys. Lett. 119, 251104 (2021)], where a low-loss periodically poled lithium niobate (PPLN) waveguide had shown 6.3-dB squeezing at a 6 THz frequency. First, to improve efficiency of the squeezed light detection, we reduced effective optical loss to about 12% by removing extra optics and changing the detection method into a low-loss balanced homodyne measurement. Second, to minimize phase-locking fluctuation, we constructed a frequency-optimized phase-locking system by comprehending its frequency responses. Finally, we found optimal experimental parameters of a measurement frequency and a pump power from their dependences for the squeezing levels. The measurement frequency was decided as 11 MHz to maximize a clearance between shot and circuit noises. Furthermore, pump power was optimized as 660 mW to get higher squeezing level while suppressing anti-squeezed-noise contamination due to an imperfection of phase locking. Note that this over-8-dB squeezing is achieved without any loss-correction and circuit-noise correction. Moreover, it is shown that the squeezing level soon after our PPLN waveguide is estimated at over 10 dB, which is thought to be mainly restricted by the waveguide loss. This broadband highly squeezed light opens the possibility to realize fault-tolerant ultra-fast optical quantum computers.
This work reports on the generation, modulation, and detection of temporally coherent pulses in periodically-poled lithium niobate (PPLN) configured as an optical parametric amplifier (OPA).The OPA was pumped at 1 µm, by a 10 kHz, 1 ns, Nd:YAG laser.The cw seeding sources for the OPA were two separate 1550 nm lasers.One source had a short coherence length of .3km, the other a long coherence length of 95 km.The generated pulse trains where phase modulated by using a set of mirrors on a single axis motorized stage.A 10 km fiber optic delay line was used to simulate free space propagation.A homodyne, coherent optical detection system, utilizing the in-phase and quadrature components of the circularly polarized local oscillator, was built to detect the amplitude and phase modulated pulse trains.We report the detection of the modulated signal for the long coherence length source and the loss of the modulated signal for the short coherence length source, demonstrating that the OPA process can transfer the temporal coherence (or coherence length) of the input signal to the pulse-amplified signal
We report generation of 10.1\pm0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al., Appl. Phys. Lett. 122, 234003 (2023)], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a novel phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with high-signal-to-noise ratio and suppressing degradation of squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12\% to 8\%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards realization of fault-tolerant ultra-fast universal optical quantum computation.
We report detection of 10.1 ± 0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report, where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al ., Appl. Phys. Lett. 122 , 234003 ( 2023 ) 10.1063/5.0144385 ], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with a high signal-to-noise ratio and suppressing the degradation of the squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12% to 8%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards the realization of fault-tolerant ultra-fast universal optical quantum computation.