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Category: quantum physics – Page 40

https://www.eurekalert.org/news-releases/1065953

Researchers have explored a fascinating cooling phenomenon within halide perovskite-based “dots-in-crystal” materials, uncovering both their promise and challenges.

In a groundbreaking study, scientists from Chiba University investigated the potential of solid-state optical cooling through perovskite quantum dots. Central to their research was anti-Stokes photoluminescence, a rare process where materials emit photons with higher energy than those absorbed. This innovative approach could transform cooling technology, offering a path to more efficient, energy-saving solutions. Their work not only highlights the immense promise of this technique but also reveals key limitations that pave the way for further advancements in the field.

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PRESS RELEASE —-Toshiba Corporation (Toshiba) has confirmed a technology that they claimed promises to advance progress toward the development of higher-performance quantum computers through an investigation of a potential advance in quantum computing. Experiments conducted by a joint research group from Toshiba and RIKEN, one of Japan’s largest comprehensive research institutions, have successfully realized a Double-Transmon Coupler, a solution for superconducting quantum computers initially proposed by Toshiba. The researchers achieved a world-class fidelity of 99.90% for a two-qubit gate, which is at the heart of quantum computation. Fidelity is a standard performance indicator for quantum gates, quantifying how close an operation is to the ideal in a range from 0% to 100%, with higher percentages indicating greater accuracy in the quantum gate’s operation.

Originally proposed by Toshiba in a paper from September 2022, the Double-Transmon Coupler is a tunable coupler that holds the key to improving the performance of superconducting quantum computers. In successful experimental realization, Toshiba and RIKEN have confirmed its theoretical superiority over conventional tunable couplers in suppressing the long-standing problem of unnecessary residual coupling and enabling high-speed, high-fidelity two-qubit gates.

To improve the performance of two-qubit gates, the coherence time, the period for which the quantum superposition state can be maintained — critical in quantum computers — must be extended. Gates must also be executed quickly and the strength of residual coupling must be suppressed to reduce the errors it causes. The Toshiba-RIKEN team achieved a world-class coherence time for the transmon qubit, a short gate time of 48 ns, and reduced the residual coupling strength to as low as 6 kHz, thereby achieving a fidelity of 99.90%.

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Researchers from the RIKEN Center for Quantum Computing and Toshiba have succeeded in building a quantum computer gate based on a double-transmon coupler (DTC), which had been proposed theoretically by Hayato Goto, Senior Fellow at Toshiba, as a device that could significantly enhance the fidelity of quantum gates. Using this, they achieved a fidelity of 99.90 percent for a two-qubit device known as a CZ gate and 99.98 percent for a single-qubit gate. This breakthrough, which was carried out as part of the Q-LEAP project, not only boosts the performance of existing noisy intermediate-scale quantum (NISQ) devices but also helps pave the way for the realization of fault-tolerant quantum computation through effective quantum error correction.

The DTC is a new kind of tunable coupler composed of two fixed-frequency transmons—a type of qubit that is relatively insensitive to charge noise—coupled through a loop with an additional Josephson junction. Its architecture addresses one of the most pressing challenges in quantum computing: the development of hardware to entangle qubits in a high-fidelity manner. High gate fidelity is essential for minimizing errors and enhancing the reliability of quantum computations. The DTC scheme stands out by achieving both suppressed residual interaction and rapid high-fidelity two-qubit gate operations, even for highly detuned qubits. Though fidelity of 99.9 percent has been routinely achieved for single-qubit gates, error rates for two-qubit gates are typically 0.5 percent or more, mainly due to interactions between the qubits known as the ZZ interaction.

The key to the current work, published in Physical Review X, is the construction of qubits using state-of-the-art fabrication techniques and gate optimization using a type of machine learning known as reinforcement learning. These approaches allowed the researchers to translate the theoretical potential of the DTC into practical application. They used these approaches to balance two types of remaining errors—leakage error and decoherence error—that remained within the system, selecting a length of 48 nanoseconds as an optimal compromise between the two error sources. Thanks to this, they achieved fidelity levels among the highest reported in the field.

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It’s hard to tell when you’re catching some rays at the beach, but light packs a punch. Not only does a beam of light carry energy, it can also carry momentum. This includes linear momentum, which is what makes a speeding train hard to stop, and orbital angular momentum, which is what the Earth carries as it revolves around the sun.

In a new paper, scientists seeking better methods for controlling the quantum interactions between light and matter have demonstrated a novel way to use light to give electrons a spinning kick. They reported the results of their experiment, which shows that a light beam can reliably transfer to itinerant electrons in graphene, on Nov. 26, 2024, in the journal Nature Photonics.

Having tight control over the way that light and matter interact is an essential requirement for applications like quantum computing or quantum sensing. In particular, scientists have been interested in coaxing electrons to respond to some of the more exotic shapes that light beams can assume.

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Researchers have discovered a way to recycle the tiny particles used to create supraparticle lasers, a technology that precisely controls light at a very small scale. The breakthrough could help manage these valuable materials in a more sustainable way.

Supraparticle lasers work by trapping light inside a tiny sphere made of special particles called quantum dots, which can absorb, emit, and amplify light very efficiently.

They are made by mixing quantum dots in a solution that helps them stick together in tiny bubbles. However, not all attempts succeed, and even successful lasers degrade over time. This leads to wasted materials, which can be expensive.

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Cooling systems are an integral part of many modern technologies, as heat tends to wear down materials and decrease performance in several ways. In many cases, however, cooling can be an inconvenient and energy-intensive process. Accordingly, scientists have been seeking innovative and efficient methods to cool substances down.

Solid-state optical cooling is a prominent example that leverages a very unique phenomenon called anti-Stokes (AS) emission. Usually, when materials absorb photons from incoming light, their electrons transition into an “excited” state.

Under ideal conditions, as electrons return to their original state, part of this excess energy is released as light, while the rest is converted into heat.

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Researchers at INRS have developed a synthetic photonic lattice capable of generating and manipulating quantum states of light, paving the way for promising advancements in applications ranging from quantum computing to secure quantum communication protocols.

A study co-directed by Professor Roberto Morandotti of Institut national de la recherche scientifique (INRS) in collaboration with teams from Germany, Italy, and Japan paves the way for innovative solutions that could enable the development of a system to process quantum information with both simplicity and power.

Their work, just published in the journal Nature Photonics, presents a method for manipulating the photonic states of light in a never-before-seen way, offering greater control over the evolution of photon propagation. This control makes it possible to improve the detection and number of photon coincidences, as well as the efficiency of the system.

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Conventional components perform incredibly inefficiently at these sub-freezing temperatures, the scientists said. They’re also very hard to maintain — as more and more qubits are added to a system, the more heat is emitted, which makes it more difficult and expensive to sustain these ultralow temperatures.

Because the new transistor — dubbed the “cryo-CMOS transistor” — is optimized to operate at temperatures under 1 K and emit near-zero heat, it offers plenty of advantages over traditional electronics, representatives of the Finnish company SemiQon, which developed the transistor, said in a statement.

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https://scirate.com/arxiv/2411.

Researchers present a #quantummachinelearning advantage of families of constant depth local quantum circuits over reasonably constrained log-log-depth classical circuits.

Quantum…

One of the core challenges of research in quantum computing is concerned with the question whether quantum advantages can be found for near-term quantum circuits that have implications for practical applications. Motivated by this mindset, in this work, we prove an unconditional quantum advantage in the probably approximately correct (PAC) distribution learning framework with shallow quantum circuit hypotheses. We identify a meaningful generative distribution learning problem where constant-depth quantum circuits using one and two qubit gates (QNC^0) are superior compared to constant-depth bounded fan-in classical circuits (NC^0) as a choice for hypothesis classes. We hence prove a PAC distribution learning separation for shallow quantum circuits over shallow classical circuits. We do so by building on recent results by Bene Watts and Parham on unconditional quantum advantages for sampling tasks with shallow circuits, which we technically uplift to a hyperplane learning problem, identifying non-local correlations as the origin of the quantum advantage.

Submitted 23 Nov 2024 to Quantum Physics [quant-ph]

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