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Optics Express

5-bit all-optical quantum random number generator based on a time-multiplexed optical parametric oscillator

Shaojie Li, Xiaoxuan Zhu, Jintao Fan, Kai Wen, Minglie Hu

Published Oct 25, 2023• 7 citations

Abstract

Random numbers are of critical importance in many applications, including secure communication, photonics computing and cryptography. Due to the non-deterministic nature of the quantum processes, a degenerate optical parametric oscillator (DOPO) constitutes a solution to produce true randomness. Nevertheless, one of the existing challenges for DOPO in this field is bit sequence scalability. Here, we experimentally report on the generation of 5-bit random number streams in a time-multiplexed femtosecond DOPO system. A multi-pass cell is added to elongate the OPO cavity to scale up the bit sequences. To this end, for a ∼15 m long all free space OPO cavity, resonating 5 signal pulses with a repetition rate of 50 MHz is demonstrated. The above-threshold binary phase nature originates from vacuum fluctuations of a DOPO ensuring the randomness of the system. The phase state of the output is characterized by the interference pattern between the output pulses and the fundamental pump pulses. Different bit sequences are presented here by turning on and off the OPO. Conditional probability is performed to verify the randomness of the output for 1200 bits. Our scheme provides a new direction for an all-optical random number generator.

DOI: 10.1364/oe.503779

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Random numbers play an important role in many fields, such as computation, simulation and cryptography. The randomness inherent in quantum mechanics is a perfect source of entropy for a true random number generator. Nevertheless, many QRNG schemes require post-processing work to ensure randomness. On the other hand, the bit number of the QRNG is limited, which hamper its further applications. Here, we demonstrate a novel 5-bit all free space optical quantum random number generator based on a time-division-multiplexed degenerate optical parametric oscillator (DOPO), which does not need a complicated experimental environment and tedious post-processing work. By inserting a multi-pass cell into the optical parametric oscillator, the equivalent cavity length is calculated to be as long as 15 m, which corresponds to fifth of the separation times of pump pulses. As a result, 5 identical DOPO pulses are generated simultaneously. The randomness comes from phase responses of degenerate OPOs. Under threshold, the photons produced by optical parametric down conversion have random initial phase, while above threshold, stable oscillations occurs in one of the two possible phase states. Random number is reliably verified by measuring the relative phase states between DOPO output pulses and the original pump pulse using a fast response photodiode. All spatial structures have the advantages of higher stability and are less sensitive to temperature fluctuation. The bit number can be readily multiplied by increasing the cavity length of DOPO or the repetition rate of the pump laser source.

True random number generators (RNGs) are desirable for applications ranging from cryptogra-phy to computer simulations. Quantum phenomena prove to be attractive for physical RNGs due to their fundamental randomness and immunity to attack [1] – [5]. Optical parametric down conver-sion is an essential element in most quantum optical experiments including optical squeezing [9], and generation of entangled photons [10]. In an optical parametric oscillator (OPO), photons generated through spontaneous down conversion of the pump initiate the oscillation in the absence of other in-puts [11, 12]. This quantum process is the dominant effect during the oscillation build-up, leading to selection of one of the two possible phase states above threshold in a degenerate OPO [13]. Building on this, we demonstrate a novel all-optical quantum RNG in which the photodetection is not a part of the random process, and no post processing is required for the generated bit sequence. We implement a synchronously pumped twin degenerate OPO, which comprises two identical independent OPOs in a single cavity, and measure the relative phase states of the OPO outputs above threshold as a bit value. We show that the outcome is statistically random with 99 % confidence. With the use of micro- and nanoscale OPO resonators, this technique offers a promise for simple, robust, and high-speed on-chip all-optical quantum random number generators.

We demonstrate a novel all-optical quantum random number generator (RNG) based on above-threshold binary phase state selection in a degenerate optical parametric oscillator (OPO). Photodetection is not a part of the random process, and no post processing is required for the generated bit sequence. We show that the outcome is statistically random with 99% confidence, and verify that the randomness is due to the phase of initiating photons generated through spontaneous parametric down conversion of the pump, with negligible contribution of classical noise sources. With the use of micro- and nanoscale OPO resonators, this technique offers a promise for simple, robust, and high-speed on-chip all-optical quantum RNGs.

We focus on an integrated all-optical quantum random number generator (QRNG) based on the bi-phase states of degenerate optical parametric oscillators (DOPOs), implemented using a silicon nitride microresonator driven by dual pumps. In our numerical analysis, we present a comparative study of phase bifurcation in DOPOs initialized under four distinct noise schemes. As a representative case, we simulate random number generation in the Kerr-based microresonator at a rate of 10 MHz for each noise model by using stochastic differential equations.