Research Roundup for June 2021

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.

Hardware

Title: Hardware-Efficient, Fault-Tolerant Quantum Computation with Rydberg Atoms Organizations: Harvard University; QuEra Computing Rydberg states are highly-excited atomic states with many useful properties for quantum computing. However, they have a finite lifetime and can leak out of the computational states in a correlated way. Standard fault-tolerance methods cannot be applied but in this paper, an error model that accounted for the multi-level nature of atoms and their dipole selection rules was constructed. The number of physical qubits and logic gates required for fault-tolerant operation could then be reduced by at least an order of magnitude. https://arxiv.org/abs/2105.13501

Title: Quantum-enhanced nonlinear microscopy Organizations: University of Queensland; Universität Rostock Light microscopes are limited by the shot noise of the photons. Increasing the intensity reduces this noise but it can unduly influence or harm biological specimens. Quantum techniques have been used to improve the performance of microscopes operating with extremely low intensities but this work addresses a more practically relevant intensity regime and achieves a 35% improved signal-to-noise ratio (SNR)over a state-of-the-art coherent Raman microscope. This quantum-enhanced microscope could be used to study a wide range of biological processes and as the technology matures, the SNR could improve by an order of magnitude. https://www.nature.com/articles/s41586-021-03528-w

Title: 600-km repeater-like quantum communications with dual-band stabilization Organizations: Toshiba; University of Leeds; Cambridge University; University of York; Corning Incorporated Twin-field quantum key distribution aims to extend the transmission range of quantum key distribution. One obstacle, however, is that the phases of the encoded pulses drift as they travel through optical fibers. The authors of this paper devised a dual band stabilization scheme that increased the secure key rate at long distances by two orders of magnitude. Consequently, their scheme could be close to being useful in realistic settings. https://www.nature.com/articles/s41566-021-00811-0

Title: Telecom-heralded entanglement between multimode solid-state quantum memories Organizations: The Barcelona Institute of Science and Technology; Institució Catalana de Recerca i Estudis Avançats Quantum repeaters help to distribute quantum information over large distances. Different aspects of their operation had been achieved in separate experiments: quantum memories had been entangled and photons had been stored for a long time with high ratios and efficiencies. These researchers put it all together in a way that is compatible with current telecom networks. This is a promising step towards the quantum internet. https://www.nature.com/articles/s41586-021-03481-8

Title: Quantum-Enhanced Data Classification with a Variational Entangled Sensor Network Organization: University of Arizona Quantum entanglement can improve the sensitivity of a measurement beyond what is possible classically. The output of a sensor could be processed in order to classify the system that was measured, e.g. identify what type of molecule it is. In an experiment that combined quantum sensing and machine learning, entangled radio-frequency signals were produced by a quantum circuit in a way reduced measurement noise and allowed a classical machine learning algorithm to classify the features embedded in the signals. This work has the potential to dramatically enhance the power of large sensor networks using only current or near term technology. https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021047

Title: Realization of High-Fidelity CZ and ZZ-Free iSWAP Gates with a Tunable Coupler Organization: Massachusetts Institute of Technology Tunable couplers, which dynamically control the interaction between superconducting qubits, help resolve many scalability issues. This work employed a tunable coupler and control pulses with optimal pulse shapes to achieve very high fidelity gates. There is room for improvement in the fidelity because these results did not make use of dynamical decoupling or refocusing techniques. Since tunable couplers can reduce crosstalk between qubits, the gates demonstrated in this research will only become more important as devices with larger numbers of qubits are engineered. https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021058

Title: Compact Ion-Trap Quantum Computing Demonstrator Organizations: Universität Innsbruck; Alpine Quantum Technologies; ETH Zürich; Fraunhofer IOF; Russian Quantum Center; IQOQI An ion trap quantum computer usually takes up 30 to 50 square meters of floor space in a lab. The reported design is substantially more compact: the ion trap device fits inside two server racks, each with a volume of 1.7 cubic meters. There is no loss in performance. For example, the researchers could create a fully entangled, 24-particle quantum state. This design means the parts are easily replaceable and it may require less maintenance. https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.2.020343

Title: Correlated charge noise and relaxation errors in superconducting qubits Organizations: University of Wisconsin-Madison; Fermilab; University of Chicago; Stanford University; INFN Sezione di Roma; Sorbonne Université; Google; Lawrence Livermore National Laboratory When gamma rays and cosmic-ray muons hit a superconducting quantum computer, they give rise to damaging correlated errors. With the goal of characterising this noise, the scientists performed experiments on a multiqubit circuit and combined this with detailed numerical modelling. Their improved understanding of the processes at play will inform the design of future superconducting qubits. https://www.nature.com/articles/s41586-021-03557-5

Title: Moving beyond the transmon: Noise-protected superconducting quantum circuits Organizations: University of Copenhagen; University of Colorado; Université de Sherbrooke; Massachusetts Institute of Technology; Northwestern University; Canadian Institute for Advanced Research; Princeton University; University of Chicago Several types of superconducting qubits have been invented in the last twenty years and as a whole, their coherence times has increased by orders of magnitude. The progress seems to be slowing for transmon qubits but fluxonium qubits are still rapidly improving. This review article discusses the possibility that even higher levels of noise protection can be built into new qubit designs. Finding faster and more robust way to implement logic gates on such qubits remains an outstanding challenge for the field. https://arxiv.org/abs/2106.10296

Software

Title: Connecting geometry and performance of two-qubit parameterized quantum circuits Organizations: Zapata Computing; Harvard University; Agency for Science, Technology and Research Previous work on parameterized quantum circuits has related to their performance to their expressibility or their cost function landscape. Less research has been done on the connection between performance and entanglement. This research provides various insights by taking a geometric perspective on entanglement. It only investigated two-qubit circuits, so the next step is to consider larger numbers of qubits. https://arxiv.org/abs/2106.02593

Title: Linear growth of quantum circuit complexity Organizations: Freie Universität Berlin; Helmholtz-Zentrum Berlin; University of Cologne; Harvard-Smithsonian Center for Astrophysics; Harvard University; Massachusetts Institute of Technology; University of Maryland The circuit complexity of a unitary is the minimum number of two-qubit gates required to implement that unitary. It is difficult to calculate - even finding a lower bound is a long-standing open problem. The authors of this paper prove that when the given unitary is initially made by a number of random gates, the lower bound of its complexity grows linearly with this number until it plateaus at a value that is exponential in the amount of qubits. This work has implications for theoretical physics but at a more practical level, it will inform gate-synthesis algorithms. https://arxiv.org/abs/2106.05305

Title: Optimal provable robustness of quantum classification via quantum hypothesis testing Organizations: ETH Zürich; Shanghai Jiao Tong University; University of Michigan-Shanghai Jiao Tong University Joint Institute; University of Illinois Some classical machine learning algorithms are vulnerable to adversarial attacks. Less is known about the robustness of quantum classifiers, whether this be to deliberately manipulated inputs or simply to noise. The researchers propose the use quantum hypothesis testing as a tool for assessing how robust any given quantum classifier is. In future work, it could be used to investigate if there is a trade-off between robustness and expressiveness. https://www.nature.com/articles/s41534-021-00410-5

Title: Quantum Portfolio Optimization with Investment Bands and Target Volatility Organizations: Multiverse Computing; Donostia International Physics Center; Ikerbasque Foundation for Science When creating an investment portfolio, questions about how much to invest into different assets can be framed in terms of how much risk you are willing to accept. Given some quantification of this, the task is to maximize the expected return. An important line of research is whether quantum computers can help when the large number of available choices make the optimization problem almost intractable for a classical computer. The authors reframe the problem so that the highest order polynomial that needs to be considered is a quadratic. They implement their protocol on a D-Wave quantum annealer for the full S&P 500, continuing the recent progress in the lively field of quantum finance. https://arxiv.org/abs/2106.06735

June 26, 2021