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Optics Express

Quantum-randomized polarization of laser pulses derived from zero-point diamond motion

Douglas J. Little, Ondřej Kitzler, Seyed Abedi, Akael Alias, Alexei Gilchrist, Richard Paul Mildren

Published Dec 22, 2020• 0 citations
Publisher

Abstract

Intrinsic randomness in quantum systems is a vital resource for cryptography and other quantum information protocols. To date, randomizing macroscopic polarization states requires randomness from an external source, which is then used to modulate the polarization e.g. for quantum key-distribution protocols. Here, we present a Raman-based device for directly generating laser pulses with quantum-randomized polarizations. We show that crystals of diamond lattice symmetry provide a unique operating point for which the Raman gain is isotropic, so that the spontaneous symmetry breaking initiated by the quantum-random zero-point motion determines the output polarization. Experimentally measured polarizations are demonstrated to be consistent with an independent and identical uniform distribution with an estimated quantum entropy rate of 3.8 bits/pulse.

DOI
10.1364/oe.410287

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Intrinsic randomness in quantum systems is a vital resource for cryptography and other quantum information protocols. To date, randomizing macroscopic polarization states requires randomness from an external source, which is then used to modulate the polarization e.g. for quantum key-distribution protocols. Here, we present a Raman-based device for directly generating laser pulses with quantum-randomized polarizations. We show that crystals of diamond lattice symmetry provide a unique operating point for which the Raman gain is isotropic, so that the spontaneous symmetry breaking initiated by the quantum-random zero-point motion determines the output polarization. Experimentally measured polarizations are demonstrated to be consistent with an independent and identical uniform distribution with an estimated quantum entropy rate of 3.8 bits/pulse.
We demonstrate the generation of coherent Stokes pulses with randomized linear-polarization in diamond, when the pump wave-vector and linear polarization were oriented along the [110][110][110] and [11ˉ0][1\bar{1}0][11ˉ0] axes respectively. In this configuration the excitation of multiple Raman modes produces isotropic gain, preventing the Stokes pulse from acquiring a deterministic orientation and is instead randomized by the zero-point motion of the crystal. Experimental polarization measurements were consistent with an independent, identical distribution with an estimated entropy rate of 6.67 bits per pulse.
Random number sequences are a critical resource in modern information processing systems, with applications in cryptography, numerical simulation, and data sampling. We introduce a quantum random number generator based on the measurement of pulse energy quantum fluctuations in Stokes light generated by spontaneously-initiated stimulated Raman scattering. Bright Stokes pulse energy fluctuations up to five times the mean energy are measured with fast photodiodes and converted to unbiased random binary strings. Since the pulse energy is a continuous variable, multiple bits can be extracted from a single measurement. Our approach can be generalized to a wide range of Raman active materials; here we demonstrate a prototype using the optical phonon line in bulk diamond.
Abstract Random number generators (RNGs) are an essential ingredient of modern cryptographic systems. Particular attention is paid today to a special class of RNGs – quantum RNGs, which are attracting close attention of researchers due to the explosive development of quantum key distribution systems, where the use of quantum randomness is a necessary security requirement. A large number of various quantum entropy sources have been proposed over the last 10-15 years. Most of them are based on the use of different quantum optics phenomena, particularly, on the effects occurring in semiconductor lasers. This choice is due to the relatively low cost and ease of use of such devices and the high random bit generation rate available with optical QRNGs. Motivated by these reasons, we study in the present research the QRNG based on variations of light polarization in a vertical-cavity surface-emitting laser (VCSEL). The scheme we propose allows creating an extremely compact and fast optical QRNG, consisting essentially of only a laser and a polarizing beam splitter. We revealed that it is possible to ensure the pulsed operation of a VCSEL in a bistable regime, which is characterized by random switch of light polarization. We show, however, that probabilistic properties of such laser pulses significantly depend on the internal laser parameters as well as on its operating mode. Understanding these properties is fundamentally important for the correct assessment of the quantum noise contribution and, consequently, for the subsequent post-processing of digitized random sequences.
Semiconductor lasers are normally used as single-photon sources in most commercial and research quantum key distribution (QKD) systems [1]. These lasers are also used for quantum random number generation (QRNG) that is a necessary security requirement for QKD. In device-dependent QRNGs unpredictability is guaranteed by comparing experimental measurements with simulation data obtained from a model of the physical entropy source [2]. The characterization of phase fluctuations in a gain-switched edge-emitting semiconductor laser for QRNG has been recently performed [2]. Very recently a QRNG based on the random excitation of the linearly polarized modes of a gain-switched vertical-cavity surface-emitting laser (VCSEL) has also been proposed [3]. In this work we characterize the polarization fluctuations found in this system by comparing experimental measurements with the results obtained from the simulation of a VCSEL's model that incorporates the device intrinsic parameters found using state-of-the-art experimental techniques [4].
Gain-switching of laser diodes is a well-established technique for generating optical pulses with random phases, where the quantum randomness arises naturally from spontaneous emission. However, the maximum switching rate is limited by the time required for phase diffusion: at high repetition rates, residual photons in the cavity seed subsequent pulses, leading to phase correlations, which degrade randomness. We present a method to overcome this limitation by employing an external source of spontaneous emission in conjunction with the laser. Our results show that this approach substantially suppresses interpulse phase correlations, restoring phase randomization at repetition rates as high as 10 GHz. This technique opens new opportunities for high-rate quantum key distribution and quantum random number generation.
Random number sequences are a critical resource in a wide variety of information systems, including applications in cryptography, simulation, and data sampling. We introduce a quantum random number generator based on the phase measurement of Stokes light generated by amplification of zero-point vacuum fluctuations using stimulated Raman scattering. This is an example of quantum noise amplification using the most noise-free process possible: near unitary quantum evolution. The use of phase offers robustness to classical pump noise and the ability to generate multiple bits per measurement. The Stokes light is generated with high intensity and as a result, fast detectors with high signal-to-noise ratios can be used for measurement, eliminating the need for single-photon sensitive devices. The demonstrated implementation uses optical phonons in bulk diamond.