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Category: quantum physics – Page 510

German nanotechnology specialist attocube says its attoDRY800 cryostat enables quantum scientists to “reclaim the optical table” and focus on their research not the experimental set-up.

Twin-track innovations in cryogenic cooling and optical table design are “creating the space” for fundamental scientific breakthroughs in quantum communications, allowing researchers to optimize the performance of secure, long-distance quantum key distribution (QKD) using engineered single-photon-emitting light sources.

In a proof-of-concept study last year, Tobias Heindel and colleagues in the Institute of Solid State Physics at the Technische Universität (TU) Berlin, Germany, implemented a basic QKD testbed in their laboratory. The experimental set-up uses a semiconductor quantum-dot emitter to send single-photon pulses along an optical fibre to a four-port receiver that analyses the polarization state of the transmitted qubits.

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“Conditional witnessing” technique makes many-body entangled states easier to measure.

Quantum error correction – a crucial ingredient in bringing quantum computers into the mainstream – relies on sharing entanglement between many particles at once. Thanks to researchers in the UK, Spain and Germany, measuring those entangled states just got a lot easier. The new measurement procedure, which the researchers term “conditional witnessing”, is more robust to noise than previous techniques and minimizes the number of measurements required, making it a valuable method for testing imperfect real-life quantum systems.

Quantum computers run their algorithms on quantum bits, or qubits. These physical two-level quantum systems play an analogous role to classical bits, except that instead of being restricted to just “0” or “1” states, a single qubit can be in any combination of the two. This extra information capacity, combined with the ability to manipulate quantum entanglement between qubits (thus allowing multiple calculations to be performed simultaneously), is a key advantage of quantum computers.

The problem with qubits

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Physics World

Quantum mechanics describes this frustration by suggesting that the orientation of the spins is not rigid. Instead, it constantly changes direction in a fluid-like way to produce an entangled ensemble of spin-ups and spin-downs. Thanks to this behaviour, a spin liquid will remain in a liquid state even at temperatures near absolute zero, where most materials usually freeze solid.

The holon and the spinon

To describe this behaviour in mathematical terms, the late Nobel laureate Philip W Anderson, who predicted the existence of spin liquids in 1973, proposed that in the quantum regime, an electron might in fact be composed of two distinct particles. The first, known as a “holon”, would bear the electron’s negative charge, while the second “spinon” particle would carry its spin. Anderson later suggested that this spin-charge separation might provide a microscopic mechanism to explain the high superconducting transition temperatures (Tc) that were observed in copper oxides, or cuprates, beginning in the late 1980s.

Continue reading “Electrons dual nature appears in a quantum spin liquid” | >

When particles are cooled down to temperatures just above absolute zero, they form a BEC – a state of matter in which all the particles occupy the same quantum state and thus act in unison, like a superfluid. A BEC made up of tens of thousands of particles therefore behaves as if it were just one single giant quantum particle.

An international team of researchers led by Carlos Anton-Solanas and Christian Schneider from the University of Oldenburg, Germany; Sven Höfling of the University of Würzburg, Germany; Sefaattin Tongay at Arizona State University, US; and Alexey Kavokin of Westlake University in China, has now generated a BEC from quasiparticles known as exciton-polaritons in atomically thin crystals. These quasiparticles form when excited electrons in solids couple strongly with photons.

“Devices that can control these novel light-matter states hold the promise of a technological leap in comparison with current electronic circuits,” explains Anton-Solanas, who is in the quantum materials group at Oldenburg’s Institute of Physics. “Such optoelectronic circuits, which operate using light instead of electric current, could be better and faster at processing information than today’s processors.”

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Using an ultrafast transmission electron microscope, researchers from the Technion – Israel Institute of Technology have, for the first time, recorded the propagation of combined sound and light waves in atomically thin materials.

The experiments were performed in the Robert and Ruth Magid Electron Beam Quantum Dynamics Laboratory headed by Professor Ido Kaminer, of the Andrew and Erna Viterbi Faculty of Electrical & Computer Engineering and the Solid State Institute.

Single-layer materials, alternatively known as 2D materials, are in themselves novel materials, solids consisting of a single layer of atoms. Graphene, the first 2D material discovered, was isolated for the first time in 2004, an achievement that garnered the 2010 Nobel Prize. Now, for the first time, Technion scientists show how pulses of light move inside these materials. Their findings, “Spatiotemporal Imaging of 2D Polariton Wavepacket Dynamics Using Free Electrons,” were published in Science following great interest by many scientists.

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A secure quantum internet is one step closer thanks to a quantum memory made from a crystal, which could form a crucial part of a device able to transmit entangled photons over a distance of 5 kilometres. Crucially, it is entirely compatible with existing communication networks, making it suitable for real-world use.

There has long been a vision of a quantum version of the internet, which would allow quantum computers to communicate across long distances by exchanging particles of light called photons that have been linked together with quantum entanglement, allowing them to transmit quantum states.

The problem is that photons get lost when they are transmitted through long lengths of fibre-optic cable. For normal photons, this isn’t an issue, because networking equipment can simply measure and retransmit them after a certain distance, which is how normal fibre data connections work. But for entangled photons, any attempt to measure or amplify them changes their state.

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Just as microelectronics transformed the modern world through the creation of the integrated circuit, which is now at the heart of most electronic devices, quantum photonics needs an equivalent platform to fulfil its application potential. In this special focus issue of Nature Photonics, we report on the progress in making this a reality with the developments in integrated quantum photonics (IQP).

In a Review Article, Jianwei Wang and colleagues provide a general overview and introduction to IQP circuits and summarize the present development of quantum hardware based on IQP chips. They remark that the challenge for measurement-based quantum computation may shift from the need for deterministic gates to constructing a generic entangled cluster-state, on which any quantum computation could be mapped by a sequence of measurements.

IQP circuits are also a desirable platform for chip-based quantum communications. However, fully integrated chip-based quantum communication has not yet been realized, largely because of the integration difficulties between silicon wafers that feature optical waveguides and other passive components and light sources and photodetectors that are made from different semiconductors. Key components such as transmitters and receivers for quantum key distribution and quantum random number generators are instead individually fabricated.

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The creation, transfer, and stabilization of localized excitations are studied in a donor–acceptor Frenkel exciton model in an atomistic treatment of reduced-size double quantum dots (QDs) of various sizes. The explicit time-dependent dynamics simulations carried out by hybrid time-dependent density functional theory/configuration interaction show that laser-controlled hole trapping in stacked, coupled germanium/silicon quantum dots can be achieved by a UV/IR pump–dump pulse sequence. The first UV excitation creates an exciton localized on the topmost QD and after some coherent transfer time, an IR pulse dumps and localizes an exciton in the bottom QD. While hole trapping is observed in each excitation step, we show that the stability of the localized electron depends on its multiexcitonic character.

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