Research Roundup for May 2023

By Dr Chris Mansell, Senior Scientific Writer at Terra Quantum

Shown below are summaries of a few interesting research papers in quantum technology that we have seen over the past month.

Hardware

Title: Scalable Multilayer Architecture of Assembled Single-Atom Qubit Arrays in a Three-Dimensional Talbot Tweezer Lattice
Organization: Technische Universität Darmstadt
In this paper, a new way of creating large, configurable, three-dimensional arrays of ultracold atoms is described and realised. The method employs the Talbot effect, where diffracted light has a certain intensity profile not only in the focal plane but in several other planes as well. Atoms can then be trapped in the local extrema of this profile (maxima for red-detuned light or minima for blue-detuning). The system, as currently implemented, has 17 planes capable of trapping atoms, with over 750 atom-trapping sites per plane. This presents an opportunity to investigate large instances of quantum computing protocols because the atoms can act as qubits that can be individually addressed and made to interact with one another by exciting them to Rydberg states.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.180601

Title: A Race Track Trapped-Ion Quantum Processor
Organizations: Quantinuum; Honeywell Aerospace
In this paper, a new ion-trap design is presented. It is based on the quantum charge-coupled device architecture but it looks like an oval race track. It includes several components that will enable it to scale up from its current size of 32 qubits. The researchers benchmarked the primitive operations and found that two-qubit logic gates were the dominant source of error. They also investigated how different applications depended on different aspects of the system, using the performance of Hamiltonian simulation to assess the two-qubit gate error, the QAOA algorithm to assess the qubit connectivity, and repetition codes and dynamical simulations to assess mid-circuit measurement and reset. Notably, the system achieved the highest quantum volume to date.
Link: https://arxiv.org/abs/2305.03828

Title: Loophole-free Bell inequality violation with superconducting circuits
Organizations: ETH Zurich; Quside Technologies S.L.; Institut de Ciencies Fotoniques; Institució Catalana de Recerca i Estudis Avançats; University of Paris-Saclay; Yale University; Université de Sherbrooke; Canadian Institute for Advanced Research
Until this landmark experiment, the only physical systems that had been used in loophole-free Bell tests were nitrogen–vacancy centres, optical photons and neutral atoms. Superconductors have now been added to this list. This is significant from a fundamental, foundational perspective because it is related to non-locality. The technological implications are thought-provoking too. Superconductors make up some of the world’s leading quantum processors and this experiment shows that they can be reliably connected using superconducting waveguides extending over tens of meters. This means they have the potential to be used in device-independent quantum key distribution protocols and for distributed quantum computation.
Link: https://www.nature.com/articles/s41586-023-05885-0

Title: Solving Graph Problems Using Gaussian Boson Sampling
Organizations: University of Science and Technology of China; Chinese Academy of Sciences; New Cornerstone Science Laboratory
Dense subgraph identification is a problem that occurs in computational biology and finance. Providing a speed-up over classical approaches to this problem is a possible application of Gaussian Boson Sampling, a non-universal model of quantum computation in which Gaussian squeezed states travel through a passive, linear interferometer. This latest research extends prior Gaussian Boson Sampling experiments to the regime where, to produce comparable results, today’s fastest supercomputers might need years to run exact classical algorithms. Perhaps faster classical algorithms can be developed but in the meantime, this is a very noteworthy development.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.190601

Title: Entangling microwaves with light
Organizations: Institute of Science and Technology Austria; Vienna Center for Quantum Science and Technology
In this paper, microwaves were deterministically entangled with light that has a wavelength used by the telecommunications industry. This is significant because it could enable superconducting quantum processors to be networked together. The result was achieved by placing a lithium niobate optical resonator inside an ultra-low noise superconducting aluminium microwave cavity. Measurements of the quadratures of the electromagnetic fields were shown to violate the Duan-Simon separability criterion, thus verifying that entanglement was successfully generated.
Link: https://www.science.org/doi/10.1126/science.adg3812

Title: Cryogenic sensor enabling broad-band and traceable power measurements
Organizations: Aalto University; Bluefors Oy; IQM; VTT Technical Research Centre of Finland; QTF Centre of Excellence
Circuit quantum electrodynamics, where superconductors interact with microwave photons, is one of the leading architectures for quantum computation. Improving how well we can measure the power of weak microwave signals would have very beneficial consequences for these setups. In this work, the researchers devised a precise way to do this in cryogenic environments over a broad range of frequencies. Compared to existing measurement devices, they improve a figure of merit called the noise equivalent power by two orders of magnitude and the thermal conductance of an important component by over five orders of magnitude. Their future experiments will involve using their device to calibrate microwave signals in experiments on superconducting qubits.
Link: https://pubs.aip.org/aip/rsi/article/94/5/054710/2892940/Cryogenic-sensor-enabling-broad-band-and-traceable

Software

Title: Quantum Lock: A Provable Quantum Communication Advantage
Organizations: University of Edinburgh; Sorbonne Université; Indian Institute of Technology
Integrated circuits are physically unique because small random variations arise during the manufacturing process.  A physically unclonable function (PUF) uses this fact to authenticate networked devices, which is important for applications related to the Internet of Things. However, there is growing skepticism about their security. In this paper, a hybrid PUF is envisioned where the output of a classical PUF is encoded into a quantum state so that the indistinguishability property of the non-orthogonal quantum states can enhance the security of the protocol.
Link: https://quantum-journal.org/papers/q-2023-05-23-1014/

Title: Toward perturbation theory methods on a quantum computer
Organizations: Purdue University; IBM
Perturbation theory allows physicists and chemists to use exact results from their understanding of simple quantum systems to approximate the eigenstates and eigenenergies of systems with additional interactions. In this paper, the authors devised quantum circuits that perform perturbation theory calculations. Applicable to a wide range of physical systems, they presented their methods by considering the two-site extended Hubbard model and implementing their circuits on a 27-qubit quantum computer from IBM. Since certain aspects of the calculation can be performed in superposition, the quantum circuit has the potential to outperform classical computation. The quantum circuit is especially efficient because it does not require any training or optimisation.
Link: https://www.science.org/doi/10.1126/sciadv.adg4576

Title: Learning Many-Body Hamiltonians with Heisenberg-Limited Scaling
Organizations: California Institute of Technology; University of California; Microsoft
Learning the parameters of an unknown Hamiltonian is an important task in quantum metrology, quantum computing and many-body physics. Algorithms that perform this task are often judged according to how their runtime scales with the precision to which they learn the parameters. In this paper, an algorithm for many-qubit, local Hamiltonians is presented where the runtime scales with the inverse of the precision. This is known as Heisenberg-limited scaling and is a significant improvement on prior algorithms. The algorithm is also robust to state preparation and measurement errors. Since the runtime is independent of the number of qubits, the algorithm can be scaled to large system sizes.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.200403

Title: Demonstration of Algorithmic Quantum Speedup
Organization: University of Southern California
Artificial intelligence once seemed to not extend much beyond board games. Now, it is having an incredible impact on the world. This goes to show how useful gamified situations, such as the one set up in this paper, can be. It is an adaptation of the Bernstein-Vazirani problem where, after one query of an oracle, the player tries to guess a hidden bit string. If they are incorrect, the hidden bit string is changed and the game repeats. When the oracle is quantum and there are no imperfections, the runtime scales with the size of the problem better than when the oracle is classical. This is an algorithmic quantum speedup and it does not rely on any conjectures or assumptions. By using dynamic decoupling on an IBM quantum computer, this speedup was demonstrated over the entire range of problem sizes that were investigated. 
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.210602

May 29, 2023