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Optics Express

Pulse-resolved measurement of continuous-variable Einstein-Podolsky-Rosen entanglement with shaped local oscillators

Ami Shinjo, Yujiro Eto, Takuya Hirano

Published Jun 11, 2019• 8 citations
Publisher

Abstract

We report an experimental demonstration of the continuous-variable Einstein-Podolsky-Rosen (EPR) paradox and quantum steering using a pulsed light source and nonlinear optical waveguides. In this pulsed light source experiment, time-domain measurements were performed in which one measured value of the quadrature phase amplitude of the light field was independently obtained for each pulse. This independence is useful for application to quantum information processing and fundamental physics. To realize time-domain measurements, in-house built homodyne detectors were used to detect individual pulses. In addition, to improve the temporal-mode matching between the local oscillator (LO) and entangled pulse, the duration of the LO pulse was shortened by single-pass optical parametric amplification. The product of the conditional variances of the quadrature amplitudes was 0.82 ± 0.09 < 1, which satisfies the condition for realization of the EPR paradox and quantum steering.

DOI
10.1364/oe.27.017610

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We report an experimental demonstration of the continuous-variable Einstein-Podolsky-Rosen (EPR) paradox and quantum steering using a pulsed light source and nonlinear optical waveguides. In this pulsed light source experiment, time-domain measurements were performed in which one measured value of the quadrature phase amplitude of the light field was independently obtained for each pulse. This independence is useful for application to quantum information processing and fundamental physics. To realize time-domain measurements, in-house built homodyne detectors were used to detect individual pulses. In addition, to improve the temporal-mode matching between the local oscillator (LO) and entangled pulse, the duration of the LO pulse was shortened by single-pass optical parametric amplification. The product of the conditional variances of the quadrature amplitudes was 0.82 ± 0.09 < 1, which satisfies the condition for realization of the EPR paradox and quantum steering.
We demonstrated time-domain measurement and frequency domain measurement of continuous-variable (CV) entanglement with pulsed light source and waveguides. The experiment with pulsed light source has difficulty in temporal mode matching between the entangled beams and local oscillator (LO) beams. To break through this difficulty, we shorten the duration of LO pulse by optical parametric amplification (OPA) in a waveguide. We generated entanglement that satisfied EPR-Reid criterion in the time domain measurement.
Summary form only given. With the rapid development of spatially-resolving single-photon detectors, spatially structured multidimensional entangled states start to play a key role in modern quantum science. In particular, they find extensive applications in emerging fields such as quantum imaging, holography, computation or quantum-enhanced metrology. Moreover, spatially-multiplexed schemes hold a promise to increase the capacity of quantum channels, essential for quantum key distribution. An appealing perspective is to apply these ideas to light-atom interfaces, that are capable of storing and processing continuous-variable (CV) entanglement, critical for novel quantum cryptography, computation and imaging schemes.Here we generate a hybrid bipartite entangled state of a photon and an atomic spin-wave excitation [1], exhibiting Einstein-Podolsky-Rosen (EPR) correlations in real space of continuous position-momentum variables, as in the original EPR proposal [2]. We verify the entanglement by measuring the coincidence patterns corresponding to modulus-squared spatial wavefunctions of the state. By studying temporal evolution and decoherence we find that the EPR entanglement may be stored for several microseconds. Our approach turns out to be by far more robust than the only hitherto performed experiment, based on quadratures of squeezed light and a slow-light medium, where time-delayed EPR correlations were demonstrated [3]. We achieve two orders of magnitude longer delay time using quantum memory setup and significantly stronger violation of EPR inequality [4], with product of variances 3 times below the EPR bound for the full two-dimensional coincidence distribution.
For measurements describable by a discrete set of possibilities, numerous demonstrations of the violation of Bell's inequalities have been reported in optical physics based on observations of polarization correlations for photon pairs. By contrast for observables with a continuous spectrum, there appears to be no general inequalities corresponding to those of Bell and no experimental realization of the original Einstein-Podolsky-Rosen paradox. Following Reid and Drummond,1 we are attempting to demonstrate the EPR paradox in the context of the measurement of continuous quantum variables with a subthreshold optical parametric oscillator operated in a frequency degenerate but polarization nondegenerate mode. In this case, the EPR paradox is formulated in terms of two sets of quadrature phase amplitudes for the spatially separated signal and idler fields. As the parametric gain is increased toward threshold, the quadrature amplitudes of the signal and idler beams become increasingly faithful quantum copies of one another. Our measurements of quadrature amplitudes are carried out using independent balanced homodyne detectors at two sites. When operated for the detection of squeezing at one site, noise reductions of 4 dB relative to the vacuum state limit are observed, which is most encouraging since noise reductions >3 dB in the difference photocurrent from the two sites are sufficient for inference at a distance of a signal observable from a measurement of the corresponding idler variable.