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Category: quantum physics – Page 517

Yakir Aharonov and David Bohm proposed the effect that now bears their name in 1959, arguing that while classical potentials have no physical reality apart from the fields they represent, the same is not true in the quantum world. To make their case, the pair proposed a thought experiment in which an electron beam in a superposition of two wave packets is exposed to a time-varying electrical potential (but no field) when passing through a pair of metal tubes. They argued that the potential would introduce a phase difference between the wave packets and therefore lead to a measurable physical effect – a set of interference fringes – when the wave packets are recombined.

Seeking a gravitational counterpart

In the latest research, Mark Kasevich and colleagues at Stanford University show that the same effect also holds true for gravity. The platform for their experiment is an atom interferometer, which uses a series of laser pulses to split, guide and recombine atomic wave packets. The interference from these wave packets then reveals any change in the relative phase experienced along the two arms.

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By using quantum key distribution (QKD), quantum cryptographers can share information via theoretic secure keys between remote peers through physics-based protocols. The laws of quantum physics dictate that photons carrying signals cannot be amplified or relayed through classical optical methods to maintain quantum security. The resulting transmission loss of the channel can limit its achievable distance to form a huge barrier to build large-scale quantum secure networks. In a new report now published in Nature Photonics, Shuang Wang and a research team in quantum information, cryptology and quantum physics in China developed an experimental QKD system to tolerate a channel loss beyond 140 dB across a secure distance of 833.8 km to set a new record for fiber-based quantum key distribution. Using the optimized four-phase twin-field protocol and high quality setup, they achieved secure key rates that were more than two orders of magnitude greater than previous records across similar distances. The results form a breakthrough to build reliable and terrestrial quantum networks across a scale of 1,000 km.

Quantum cryptography and twin-field quantum key distribution (QKD)

Quantum key distribution is based on fundamental laws of physics to distribute secret bits for information-theoretic secure communication, regardless of the unlimited computational power of a potential eavesdropper. The process has attracted widespread attention in the past three decades relative to the development of a global quantum internet, and matured to real-world deployment through optical-fiber networks. Despite this, wider applications of QKD are limited due to channel loss, limiting increase in the key rate and range of QKD. For example, photons are carriers of quantum keys in a QKD setup, and they can be prepared at the single-photon level to be scattered and absorbed by the transmission channel. The photons, however, cannot be amplified, and therefore the receiver can only detect them with very low probability. When transmitted via a direct fiber-based link from the transmitter to the receiver, the key rate can therefore decrease with transmission distance.

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Rebooting a quantum computer is a tricky process that can damage its parts, but now two RIKEN physicists have proposed a fast and controllable way to hit reset.

Conventional computers process information stored as bits that take a value of zero or one. The potential power of quantum computers lies in their ability to process ‘qubits’ that can take a value of zero or one—or be some fuzzy mix of both simultaneously.

“However, to reuse the same circuit for multiple operations, you have to force the qubits back to zero fast,” says Jaw Shen Tsai, a quantum physicist at the RIKEN Center for Quantum Computing. But that is easier said than done.

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Perovskites are hybrid compounds made from metal halides and organic constituents. They show great potential in a range of applications, e.g. LED lights, lasers, and photodetectors, but their major contribution is in solar cells, where they are poised to overtake the market from their silicon counterparts.

One of the obstacles facing the commercialization of solar is that their power-conversion efficiency and operational stability drop as they scale up, making it a challenge to maintain in a complete solar cell.

The problem is partly with the cell’s electron-transport , which ensures that the electrons produced when the cell absorbs light will transfer efficiently to the device’s electrode. In perovskite solar cells, the electron-transport layer is made with mesoporous titanium dioxide, which shows low electron mobility, and is also susceptible to adverse, photocatalytic events under ultraviolet light.

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Over 240 years ago, famous mathematician Leonhard Euler came up with a question: if six army regiments each have six officers of six different ranks, can they be arranged in a square formation such that no row or column repeats either a rank or regiment?

After searching in vain for a solution, Euler declared the problem impossible – and over a century later, the French mathematician Gaston Tarry proved him right. Then, 60 years after that, when the advent of computers removed the need for laboriously testing every possible combination by hand, the mathematicians Parker, Bose, and Shrikhande proved an even stronger result: not only is the six-by-six square impossible, but it’s the only size of square other than two-by-two that doesn’t have a solution at all.

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Most quantum computers are based on superconductors or trapped ions, but an alternative approach using ordinary atoms may have advantages.

Back in 2016, we told you about the iBubble, an underwater drone that autonomously follows and films scuba divers. Well, it now has a more capable industrial-use big brother, known as the Seasam.

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