By Dr. Chris Mansell
Shown below are summaries of a few interesting research papers in quantum computing and communications that have been published over the past month.
Title: Qubits made by advanced semiconductor manufacturing
Organizations: Intel, QuTech
This paper reports on quantum dots that are hosted at a 28Si/28SiO2 interface and fabricated in a 300 mm semiconductor manufacturing facility using all-optical lithography and fully industrial processing. This breakthrough can be a crucial step towards scaling to the thousands of qubits that are needed for a practical quantum computer. With this approach, nanoscale gate patterns with excellent yield have been achieved. In the multi-electron regime, the quantum dots allow good tunnel barrier control—a crucial feature for fault-tolerant two-qubit gates. Single-spin qubit operation using magnetic resonance in the few-electron regime reveals relaxation times of over 1 second at 1 Tesla and coherence times of over 3 milliseconds.
Title: Entanglement of Spin-Pair Qubits with Intrinsic Dephasing Times Exceeding a Minute
Organizations: QuTech; Delft University of Technology; Harvard University; Element Six
In solid-state experiments, single spins have been used as qubits and pairs of spins have been regarded as a source of noise. In this paper, the scientists rethought these roles: they treated pairs of nuclear spins as the qubits and then used single nitrogen vacancy spins to sense and control them. This resulted in the qubits having a dephasing time of almost two minutes, which is the longest reported for any individually controllable qubit. Three underlying mechanisms for this record-breaking performance were clearly identified. Furthermore, they could well apply to other spin pairs that are naturally abundant in a variety of solids and so while the experiment took place in diamond at 3.7 kelvin, it could be the beginning of a wider search for even better materials.
Title: A hole spin qubit in a fin field-effect transistor above 4 kelvin
Organizations: University of Basel; IBM
Claims about advancements in technology can be hard to evaluate. For example, will silicon qubits actually be able to use the same silicon manufacturing processes that ordinary computers use? Will the shrinking of today’s ubiquitous FinFET transistors ever lead manufacturers to be truly concerned about quantum effects? A new paper sheds light on these issues by fabricating a spin qubit in a silicon FinFET transistor and demonstrating extremely high-fidelity single-qubit logic gates. This is such an impressive level of compatibility with semiconductor industry standards that it may even be plausible for such qubits to be on the same chip as their classical control electronics. Furthermore, the device did not have to be cooled to as low temperatures as previous silicon qubit platforms, which means that simpler and more scalable cryogenic apparatus can be used. Before attempting to scale up, steps like improving the readout techniques and implementing two-qubit gates will have to be taken.
Title: Single electrons on solid neon as a solid-state qubit platform
Organizations: Argonne National Laboratory; Lawrence Berkeley National Laboratory; The NSF AI Institute for Artificial Intelligence and Fundamental Interactions; Massachusetts Institute of Technology; University of Chicago; National High Magnetic Field Laboratory; Florida State University; Washington University in St. Louis
In 1999, there was a proposal that electrons confined slightly above the surface of superfluid helium could function as qubits and since then experiments have gone some way towards realising this potential. Now, a novel approach replacing the helium with solid neon in a vacuum has been demonstrated. Even without any optimisation, the resulting qubits have a relaxation time of 15 microseconds and a coherence time of 200 nanoseconds, which is near the state of the art for a charge qubit. The paper suggests there is considerable room for improvement. For example, purifying the neon could increase the coherence time to over one second and improved trap designs could speed up single-qubit logic gates to below 12 nanoseconds. After making these upgrades, it may be possible to scale up using the quantum charge-coupled device technique that was originally devised for trapped ions.
Title (1): Dual-Element, Two-Dimensional Atom Array with Continuous-Mode Operation
Organization (1): University of Chicago
Link (1): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.011040
Title (2): Defect-Free Arbitrary-Geometry Assembly of Mixed-Species Atom Arrays
Organizations (2): Wuhan Institute of Physics and Mathematics; University of Chinese Academy of Sciences; Zhengzhou University
Link (2): https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.083202
Several quantum computing platforms have experimented with using one type of qubit for computation and another type for memory. Two works have now demonstrated this is possible for ultracold atoms. In paper (1), an array of 512 optical tweezers each trap either a rubidium or a cesium atom and in paper (2), two different isotopes of rubidium are individually trapped in a similar manner. This has the potential to reduce some issues with atomic quantum computing platforms, such as crosstalk, and could allow for some improved functionality, such as the ability to measure atoms in a way that does not lead to the measured atom being lost from its trap.
Title: Random Quantum Circuits Anticoncentrate in Log Depth
Organizations: Caltech; Perimeter Institute for Theoretical Physics; AWS
Imagine only having half of a superpower, like being able to leap tall buildings in a single bound but not being able to survive the impact when you come crashing down on the ground from several stories in the air. This is a bit like being a quantum computer. You can store the answer to a complicated question in your exponentially large quantum state but when you get measured, only a few randomly sampled bits are revealed. Given the probabilistic nature of the readout process, one can ask how the distribution of measurement outcomes is different in different circumstances. In particular, are there any important features which distinguish a structured quantum circuit from a random one? In this paper, the authors show that random quantum circuits can output fairly uniform results even when the circuit depth is surprisingly shallow. This has several implications for the interpretation of quantum supremacy experiments.
Title: Optimal quantum dataset for learning a unitary transformation
Organization: Baidu Research
In quantum mechanics, a typical experiment involves preparing an initial state, letting it evolve unitarily and then, perhaps by quantum tomography, deducing the final state. An important question is how many different initial states it takes to learn what the unitary operator is. The authors of this paper prove that when the unitary acts on n qubits and there are no ancillary qubits to help out, the answer is 2^n if the initial states have to be pure and 2 if they can be mixed. These results will make it easier to implement both oracle compiling and Hamiltonian simulation. Future work could involve considering the same question but for a quantum channel rather than a unitary.
Title: Is quantum advantage the right goal for quantum machine learning?
What is a quantum machine learning (QML) researcher to do? The QML algorithms that can be analysed according to our current understanding of complexity theory only make up a tiny proportion of all possible QML algorithms. Since this seems to be the case with classical machine learning (ML) too, the QML researcher decides to look into the theory devised especially for classical ML but they find that it is rapidly evolving. When they try to benchmark their best attempts at a particular problem, it appears that the problem is too small or too artificial and contrived to say anything about a genuine practical application. Perhaps the resolution to this conundrum is to stop trying to outperform classical ML and instead expand the arena of what counts as productive QML research. These are some of the interesting points made by two prominent theorists in their latest preprint.
Title: Breaking Rainbow Takes a Weekend on a Laptop
Post-quantum cryptography aims to develop cryptographic tools that are resistant to both quantum and classical attacks. The National Institute of Standards and Technology is currently trying to find some secure and practical algorithms that could go into widespread use. A signature scheme named Rainbow is currently in consideration, but it may not be for much longer if the results from this new paper can be replicated. It claims to break the protocol in just two days on an ordinary laptop. It only does this for the protocol’s lowest security settings, which can be straightforwardly increased. However, this would make it less practical for everyday use. Whether further improvements could be made to the attack and whether the Rainbow protocol would be resilient to them is currently unclear.
March 21, 2022