A paper recently published in the journal Science demonstrated a new approach to realize controllable biased quantum randomness/fluctuations for the first time, unlocking the potential for ultra-precise field sensing and probabilistic computing.
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Importance of Vacuum Fluctuations for Controllable Randomness
Natural fluctuations of electromagnetic fields are one of the important insights of quantum field theory. Both magnetic and electric fields possess a nonzero variance even in the vacuum state, which leads to ubiquitous effects such as the Casimir effect, the Lamb shift, and spontaneous emission.
These vacuum fluctuations can be harnessed as a source of perfect randomness for several applications, such as to generate perfectly random photonic bits. However, several potential applications of quantum randomness in different fields, such as probabilistic computing, currently depend on controllable probability distributions and have not been realized on photonic platforms.
Probabilistic computing has received significant attention for accelerating the inference and optimization tasks while avoiding the implementation challenges of quantum computers.
Probabilistic computing systems leverage the inherent randomness of specific processes to perform computations and provide a number of possible outcomes, with every outcome having its associated probability.
Thus, these computing systems are suitable for simulating physical phenomena and tackling optimization problems where several solutions exist, and the exploration of different possibilities can result in a better solution.
A probabilistic bit (p-bit) is the central building block of probabilistic computing. The p-bit is a stochastic logical unit described using a controllable probability distribution. Vacuum fluctuations can be leveraged as a source of controllable randomness for photonic probabilistic computing as photonic analog platforms possess enhanced computational efficiency and greater speed.
However, most existing optical random bit generators depend on perfect symmetry to ensure unbiased outcomes from vacuum fluctuations. Additionally, the steady state is completely determined in externally seeded devices for nonlinear optical systems, which eliminates the perception of randomness.
Macroscopic injection-seeding mechanisms are used to enable single-mode operation and narrow bandwidth in lasers, optical parametric oscillators (OPOs), and microwave oscillators. The injection of extremely weak bias fields influences the turn-on time of lasers.
Thus, the simultaneous existence of complete determinism and perfect unbiased randomness regimes can provide a solution to harness electromagnetic fluctuations in a vacuum as a source of controllable biased quantum randomness.
A New Approach to Achieve Controllable Quantum Randomness
In this study, researchers proposed to inject extremely weak/vacuum level bias fields into a multistable optical system to realize a controllable biased quantum randomness source and demonstrated their concept in a biased OPO, where the random variable was the signal field phase that can take on values π and 0.
Bias fields with amplitudes similar to that of vacuum fluctuations were injected into the system to constantly interpolate between the perfect determinism and perfect unbiased randomness regimes. Controllable biased quantum randomness was realized for the first time in a nonlinear photonic system.
The two possible OPO output state probabilities were controlled by injecting less than one photon on average. The first photonic p-bit was generated with a parameter p that continuously tuned from zero to one through control over the bias phase and amplitude. This biased symmetry breaking was attributed to the interference between the vacuum fluctuations and the bias field, which led to biased randomness.
Probability measurements were performed in the system to translate the vacuum-level fluctuations into macroscopic observables that are easily manipulable to develop a new method to investigate the temporal profile of very weak electromagnetic pulses and vacuum field fluctuations. Moreover, the potential of the proposed approach was also displayed for sub-photon-level field sensing by reconstructing the temporal shape of fields below the single-photon level.
Conclusion and Future Outlook
The proposed framework offers a simple approach to realize p-bits in nonlinear driven-dissipative quantum systems by harnessing zero-point fluctuations as a noise source. The photonic p-bit generation system generated 10,000 bits per second, with each of them following an arbitrary binomial distribution.
Thus, the framework can be implemented in multistable systems, such as integrated nanolaser arrays, to realize p-bits with extreme bandwidth by leveraging spatial multiplexing, non-trivial p-bit topologies using Hamiltonians H0 with higher-order symmetries, and emulate complex many-body Hamiltonians by engineering couplings between p-bits.
To summarize, the findings of this study demonstrated the effectiveness of the proposed framework to realize controllable biased quantum randomness and its potential for applications such as weak field sensing and probabilistic computing.
Specifically, the findings can assist in realizing macroscopic configurable probability distributions based on biased quantum vacuum fluctuations. Moreover, quantum-enhanced probabilistic computing also creates the possibility of simulating complex dynamics in areas such as lattice quantum chromodynamics and combinatorial optimization.
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References and Further Reading
Roques-Carmes, C., Salamin, Y., Sloan, J., Choi, S., Velez, G., Koskas, E., Rivera, N., Kooi, S. E., Joannopoulos, J. D., Soljačić, M. (2023). Biasing the quantum vacuum to control macroscopic probability distributions. Science. https://doi.org/10.1126/science.adh4920
Study demonstrates control over quantum fluctuations, unlocking potential for ultra-precise field sensing [Online] Available at https://phys.org/news/2023-07-quantum-fluctuations-potential-ultra-precise-field.html (Accessed on 21 July 2023)
Gabriel, C., Wittmann, C., Sych, D., Dong, R., Mauerer, W., Andersen, U. L., Marquardt, C., Leuchs, G. (2010). A generator for unique quantum random numbers based on vacuum states. Nature Photonics, 4(10), 711-715. https://doi.org/10.1038/nphoton.2010.197