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Researchers have built the fastest random-number generator ever made, using a simple laser. It exploits fluctuations in the intensity of light to generate randomnessa coveted resource in applications such as data encryption and scientific simulationsand could lead to devices that are small enough to fit on a single computer chip.

True randomness is surprisingly difficult to come by. Algorithms in conventional computers can produce sequences of numbers that seem random at first, but over time these tend to display patterns. This makes them at least partially predictable, and therefore vulnerable to being decoded.

To make encryption safer, researchers have turned to quantum mechanics, where the laws of physics guarantee that the results of certain measurementssuch as when a radioactive atom decaysare genuinely random.

A popular way to tap into quantum randomness is to exploit fluctuations in how photons are emitted by the materials used in lasers. Typical laser devices are designed to minimize these fluctuations to produce light of steady intensity: they make the light waves bounce around inside the material to force its atoms to emit more and more photons in sync with each other.

But for random-number generation, researchers aim for the opposite. We want the intensity to fluctuate randomly, so we can digitize the intensity to generate random numbers, says Hui Cao, an applied physicist at Yale University in New Haven, Connecticut.

Cao and her team made their laser materiala translucent semiconductorin the shape of a bow tie. Photons bounce between the curved walls of the bow tie multiple times, before coming out as a scattered beam. The researchers can then capture the light with an ultrafast camera. They recorded the light output of 254 independent pixels, which together produced random bits at a rate of around 250 terabits per second, or 250 terahertz. Thats several orders of magnitude faster than previous such devices, which recorded only one pixel at a time. Their results were reported inScienceon 25 February.

The invention represents a major leap in performance of random-number generators, says Krister Shalm, a physicist at the US National Institute of Standards and Technology in Boulder, Colorado.

The fastest existing computers have clock speeds measured in gigahertz, which is much too slow to fully exploit the full power of Caos device. The set-up could be made smaller by using simpler light detectors instead of a high-speed camera. This could eventually yield practical devices small enough to fit on a single computer chip, says Cao. These could have useful applications, such as encryption technology on mobile phones.

This article is reproduced with permission and was first published on March 2 2021.

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This Is the Fastest Random-Number Generator Ever Built - Scientific American

February 25, 2021• Physics 14, 27

A new analysis of heavy-ion collision experiments uncovers evidence that two colliding nuclei behave like a Josephson junctiona device in which Cooper pairs tunnel through a barrier between two superfluids.

The Josephson effect is a remarkable example of a macroscopic quantum phenomenon, in which, without an applied voltage, current flows between two superconductors separated by a thin film of normal material. In this structure, called a Josephson junction, the current is due to the quantum tunneling of paired, superconducting electrons (so-called Cooper pairs) [1]. For decades, nuclear physicists have hypothesized that similar effects can occur on much smaller scales, since atomic nuclei could be regarded as superfluids consisting of paired nucleons. Recent experiments have supported this hypothesis, delivering hints that two colliding nuclei could be described as a Josephson junction in which entangled neutron pairs play the role of Cooper pairs (Fig. 1) [2, 3]. Now, Gregory Potel from Lawrence Livermore National Laboratory in California and colleagues have put these ideas on firmer ground [4]. Analyzing tin-nickel collisions from previous experiments, they found that experimental observables offer compelling signatures that two nuclei indeed form, for a split second, a Josephson junction.

The orderly motion of gigantic ensembles of correlated electron pairs makes superconductors behave as a single objecta macroscopic quantum state called a condensate. The condensate is characterized by its density and phase, and the latter plays the same role as the orientation of magnetic moments in a ferromagnet: an isolated ferromagnet can be rotated at no energy cost, but two ferromagnets with different orientations affect each other. Similarly, according to quantum mechanics, the phase doesnt have implications for a single condensate. But if two condensates are sufficiently close, a Cooper-pair current, whose magnitude depends on the phase difference, may flow from one condensate to the other. A striking feature of this effect is that electric current may flow without a driving voltage.

There may be other systems in Nature where this effect occurs, and atomic nuclei, which can be regarded as superfluid ensembles of nucleons, are good candidates. This idea appeared among nuclear physicists as early as the 1970s [5]. In the 1980s and 1990s, several experiments indicated an enhanced probability of neutron-pair transfer between colliding nucleia possible manifestation of the Josephson effect. But the evidence for this interpretation wasnt compelling. There were doubts, in particular, about whether ensembles of nucleons are sufficiently large to be treated as a pair condensate. Superconductivity is an emergent phenomenon: It appears when dealing with a huge number of particles but vanishes when the system is broken down into smaller constituents. But can we consider a nucleus made of about 100 nucleons a huge ensemble of particles? Can we expect that two nuclei in close proximity exhibit a Josephson effect?

The study by Potel and his colleagues provides strong arguments for affirmative answers to these questions. The researchers analyzed data from previous experiments in which tin-116 ( 116Sn) nuclei were collided with nickel-60 ( 60Ni) [2]. With energies between 140.60 and 167.95 MeV, these collisions are gentle: they allow the nuclei to overcome just enough of the Coulomb repulsion to get sufficiently close to exchange a few neutrons at most. Under such conditions, two reactions are possible: the transfer of one neutron and the transfer of two neutrons, producing 115Sn+61Ni and 114Sn+62Ni, respectively. The case of two-neutron transfer is particularly interesting, as it may carry signatures of the correlated pairing of neutrons in the nuclei.

The team devised a way to uncover the experimental evidence of Josephson flow. Their idea is that there can be a nuclear equivalent of the alternating current (ac) Josephson effect (Fig. 1). In this variant of the Josephson effect, a constant, or dc, voltage applied to a Josephson junction produces an ac current. This striking behavior arises because the voltage causes the phase difference between the two condensates to increase over time. Since phases that differ by multiples of 2 are equivalent, a linear phase growth produces an oscillating current. The researchers argue that for the nuclear case, a similar effect can occur because neutron pairs inside two colliding nuclei possess different energies. This energy difference plays the role of the dc voltage in the ac Josephson effect.

Therefore, similar oscillatory behavior is expected to occur during a nuclear collision: the back-and-forth tunneling of neutron pairs means that 116Sn+60Ni transforms into 114Sn+62Ni and then again into 116Sn+60Nia cyclical process whose frequency is determined by the energy difference of neutron pairs in initial and final nuclei. Because the collision lasts for only a short time, the team estimates that only about three such back-and-forth transfer cycles may occur in an experiment. However, even these few oscillations can lead to observable consequences. Since neutrons and protons interact strongly, oscillating neutron pairs cause protons to oscillate at the same frequency. Because of their charge, oscillating protons should emit electromagnetic radiation at this frequency. While electrons oscillating in a standard Josephson junction emit microwave photons [6], nuclei are expected to emit gamma-ray photons because of the much larger nuclear energy differences involved. The researchers calculate the expected radiation energy to be slightly less than 4 MeV, which matches the gamma-ray spectrum seen in previous experiments.

The results are thrilling for two reasons. First, they indicate that the principles of superconductivity valid for macroscopic phenomena in solids may be applicable to the much smaller (femtometer) nuclear scalesa truly spectacular conclusion. Second, the analysis shows that the pairing description is appropriate for a small number of particlesthe hundreds of nucleons making up the nuclei. It is worth pointing out, however, that this description contains a puzzling inconsistency. According to quantum mechanics, the phase and the number of particles in the condensate are related by the uncertainty principlemuch like the position and momentum of a quantum particle: if either quantity is well defined, the other isnt. But for the nuclear case, the number of nucleons is always exactly defined. Further theoretical work will need to resolve this inconsistency.

These findings whet our appetite for more work aimed at validating superfluid nuclear models by confronting theory with experiments. In particular, it would be crucial to show that such models can deliver accurate, quantitative predictions for analogous effects in nuclear collisions beyond those involving tin and nickel.

Piotr Magierski is Professor of Physics and Head of the Nuclear Physics Division at Warsaw University of Technology, Poland, and an Affiliate Professor at the University of Washington. He is a theoretical physicist whose research interests include superfluidity and superconductivity in systems far from equilibrium, such as nuclear fission and fusion reactions, nuclear matter in neutron stars, and ultracold atomic gases.

Two mirror nuclei, in which the numbers of neutrons and protons are interchanged, have markedly different shapesa finding that defies current nuclear theories. Read More

Particle physicists have detected a short-lived nucleus containing two strange quarks, whose properties could provide new insights into the behavior of other nuclear particles. Read More

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Physics - The Tiniest Superfluid Circuit in Nature - Physics

Quantum tunneling. Credit: Daria Sokol/MIPT Press Office

Scientists from MIPT, Moscow Pedagogical State University and the University of Manchester have created a highly sensitive terahertz detector based on the effect of quantum-mechanical tunneling in graphene. The sensitivity of the device is already superior to commercially available analogs based on semiconductors and superconductors, which opens up prospects for applications of the graphene detector in wireless communications, security systems, radio astronomy, and medical diagnostics. The research results are published in a high-rank journal Nature Communications.

Information transfer in wireless networks is based on the transformation of a high-frequency continuous electromagnetic wave into a discrete sequence of bits. This technique is known as signal modulation. To transfer the bits faster, one has to increase the modulation frequency. However, this requires a synchronous increase in carrier frequency. A common FM-radio transmits at frequencies of hundred megahertz, a Wi-Fi receiver uses signals of roughly five gigahertz in frequency, while the 5G mobile networks can transmit up to 20 gigahertz signals.

This is far from the limit, and a further increase in carrier frequency admits a proportional increase in data transfer rates. Unfortunately, picking up signals with hundred gigahertz frequencies and higher is an increasingly challenging problem.

A typical receiver used in wireless communications consists of a transistor-based amplifier of weak signals and a demodulator that rectifies the sequence of bits from the modulated signal. This scheme originated in the age of radio and television, and becomes inefficient at frequencies of hundreds of gigahertz desirable for mobile systems. The fact is that most of the existing transistors arent fast enough to recharge at such a high frequency.

An evolutionary way to solve this problem is just to increase the maximum operation frequency of a transistor. Most specialists in the area of nanoelectronics work hard in this direction. A revolutionary way to solve the problem was theoretically proposed in the beginning of 1990s by physicists Michael Dyakonov and Michael Shur, and realized, among others, by the group of authors in 2018. It implies abandoning active amplification by transistor, and abandoning a separate demodulator. Whats left in the circuit is a single transistor, but its role is now different. It transforms a modulated signal into bit sequence or voice signal by itself, due to non-linear relation between its current and voltage drop.

In the present work, the authors have proved that the detection of a terahertz signal is very efficient in the so-called tunneling field-effect transistor. To understand its work, one can just recall the principle of an electromechanical relay, where the passage of current through control contacts leads to a mechanical connection between two conductors and, hence, to the emergence of current. In a tunneling transistor, applying voltage to the control contact (termed as gate) leads to alignment of the energy levels of the source and channel. This also leads to the flow of current. A distinctive feature of a tunneling transistor is its very strong sensitivity to control voltage. Even a small detuning of energy levels is enough to interrupt the subtle process of quantum mechanical tunneling. Similarly, a small voltage at the control gate is able to connect the levels and initiate the tunneling current.

The idea of a strong reaction of a tunneling transistor to low voltages is known for about fifteen years, says Dr. Dmitry Svintsov, one of the authors of the study, head of theLaboratory of 2D Materials for Optoelectronicsat the MIPT center for Photonics and 2D materials. But its been known only in the community of low-power electronics. No one realized before us that the same property of a tunneling transistor can be applied in the technology of terahertz detectors. Georgy Alymov (co-author of the study) and I were lucky to work in both areas. We realized then: if the transistor is opened and closed at a low power of the control signal, then it should also be good in picking up weak signals from the ambient surrounding.

The created device is based on bilayer graphene, a unique material in which the position of energy levels (more strictly, the band structure) can be controlled using an electric voltage. This allowed the authors to switch between classical transport and quantum tunneling transport within a single device, with just a change in the polarities of the voltage at the control contacts. This possibility is of extreme importance for an accurate comparison of the detecting ability of a classical and quantum tunneling transistor.

The experiment showed that the sensitivity of the device in the tunneling mode is few orders of magnitude higher than that in the classical transport mode. The minimum signal distinguishable by the detector against the noisy background already competes with that of commercially available superconducting and semiconductor bolometers. However, this is not the limit the sensitivity of the detector can be further increased in cleaner devices with a low concentration of residual impurities. The developed detection theory, tested by the experiment, shows that the sensitivity of the optimal detector can be a hundred times higher.

The current characteristics give rise to great hopes for the creation of fast and sensitive detectors for wireless communications, says the author of the work, Dr. Denis Bandurin. And this area is not limited to graphene and is not limited to tunnel transistors. We expect that, with the same success, a remarkable detector can be created, for example, based on an electrically controlled phase transition. Graphene turned out to be just a good launching pad here, just a door, behind which is a whole world of exciting new research.

The results presented in this paper are an example of a successful collaboration between several research groups. The authors note that it is this format of work that allows them to obtain world-class scientific results. For example, earlier, the same team of scientists demonstrated how waves in the electron sea of graphene can contribute to the development of terahertz technology. In an era of rapidly evolving technology, it is becoming increasingly difficult to achieve competitive results. comments Dr. Georgy Fedorov, deputy head of the Laboratory of Nanocarbon Materials, MIPT, Only by combining the efforts and expertise of several groups can we successfully realize the most difficult tasks and achieve the most ambitious goals, which we will continue to do.

Reference: Tunnel field-effect transistors for sensitive terahertz detection by I. Gayduchenko, S. G. Xu, G. Alymov, M. Moskotin, I. Tretyakov, T. Taniguchi, K. Watanabe, G. Goltsman, A. K. Geim, G. Fedorov, D. Svintsov and D. A. Bandurin, 22 January 2021, Nature Communications.DOI: 10.1038/s41467-020-20721-z

The work was supported by Russian Science Foundation (grant # 16-19-10557) and Russian Foundation for Basic Research (grant # 18-29-20116 mk).

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Quantum Tunneling in Graphene Advances the Age of High Speed Terahertz Wireless Communications - SciTechDaily