# Quantum Technology Recap: Key Highlights from 2021 — Q1

2021 has been quite an excellent year for Quantum Technology enthusiasts. The year has been full of revolutionary breakthroughs and some setbacks in the field. Since there is a lot to cover, let’s revisit them quarterly. Here is a quick recap of Q1 of 2021 in the Quantum Science and Technology —

# A landmark in the efficient realization of fault-tolerant quantum computation

By the start of 2021, researchers from the University of Innsbruck, Austria, set a milestone in developing fault-tolerant Quantum Computers by generating entanglement between two encoded qubits and implementing logical state teleportation between them. This experiment is the first experimental realization of non-classical correlations between topologically encoded qubits. In a ten-qubit ion-trap quantum information processor, they used lattice surgery between two qubits protected via a topological error-correction code. Lattice surgery only requires operations along the boundary of the encoded qubits, not on their entire surface, which reduces the number of operations needed to create entanglement between two encoded qubits. This demonstration marks a step towards the efficient realization of fault-tolerant quantum computation.

# Microsoft retracts its paper on Majorana Qubits

Microsoft retracts its 2018’s famous paper on Majorana Zero-Modes(MZM), a type of localized quasiparticle suitable for topological quantum computers. Microsoft placed its bet to harness Majorana particles to build the quantum computer instead of more common approaches like superconducting circuits due to the reduction in random errors which are protected by basic principles of Topology. A team led by Leo Kouwenhoven, a physicist at the Delft University of Technology in the Netherlands, reported a quantized conductance plateau at a universal conductance value of 2e²/h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminum superconducting shell. However, a year later Sergey Frolov, a physicist at the University of Pittsburgh, and his collaborator Vincent Mourik of the University of New South Wales in Australia found they couldn’t reproduce the results and later found some odd inconsistencies with the raw data. Finally, in early 2021, the team retracted the paper stating they no longer claim the observation of a quantized Majorana conductance and apologized for insufficient scientific rigor in their original manuscript.

# Physicists show that the speed limit also exists for complex quantum operations

Physicists at the University of Bonn along with researchers from MIT, the universities of Hamburg, Cologne & Padua, and the Jülich Research Center showed that a lower speed limit applies to complex quantum processes for longer distances instead of Mandelstam-Tamm bound for simpler states. The speed limit is determined by both the energy uncertainty along with the number of intermediate states of the particles to successfully reach their destination without interference. The scientists used a cesium atom and an optical lattice made of two laser beams perfectly superimposed but directed against each other. The atom was placed in one of the valleys, created due to interference by the optical lattice, and then set the standing wave in motion which displaced the position of the valley itself. The goal was to get the atom to the target location in the shortest possible time without disrupting the atoms. The extensions of the study provide an application in packing as many computational operations as possible into the coherence time, boosting the sensitivity of quantum sensors, carrying out fundamental tests of quantum superposition states, and implementing fault-tolerant quantum memories.

# Quantum Computers used to simulate a toy universe with new physics rules

Aalto University researchers used an IBM quantum computer to simulate the evolution of a “toy universe” governed by non-Hermitian Hamiltonians. They observed some exciting results forbidden by regular Hermitian quantum mechanics. Quantum simulation of a single-qubit under a non-Hermitian Hamiltonian, resulted in the observation of the PT-symmetry breaking as the exceptional point is crossed and the associated loss of state distinguishability. This means operations applied to the qubits did not conserve quantum information, a fundamental behavior to standard quantum theory. In the case of a bipartite system with one of the qubits driven by a non-Hermitian Hamiltonian, researchers observed the violation of the entanglement monotonicity, i.e. altering the degree of entanglement by local operations on one of the particles, from standard quantum mechanics. This research will provide new insights into quantum thermodynamics and allow to test the unconventional mathematical ideas. It will also allow simulation and understanding of several novel optical or microwave-based devices that follow these rules.

# Breakthrough in building the large-scale modular quantum computers

Researchers at Pritzker School of Molecular Engineering (PME) at the University of Chicago demonstrated the deterministic generation and transmission of multi-qubit entanglement through a quantum network comprising of two superconducting quantum nodes connected by a meter-long superconducting coaxial cable, where each node includes three interconnected qubits. They directly connected the cable to one qubit in each node and transferred quantum states between the nodes with a process fidelity of 0.911 in a few nanoseconds. They also successfully generated a three-qubit Greenberger–Horne–Zeilinger (GHZ) state and transferred it to another node and generated a globally distributed two-node, six-qubit GHZ state with state fidelities above the threshold of 1/2 or genuine multipartite entanglement. This architecture showed that it can be used to coherently link together multiple superconducting quantum processors, providing a modular approach for building large-scale quantum computers.

# Researchers developed World’s First Distributed Quantum Computer Prototype

Researchers at Max Planck Institute of Quantum Optics developed the world’s first-ever distributed quantum computer prototype by executing quantum-logic gates between two quantum modules. They connected two qubits, the memory, and processing units, with a 60m long optical fiber in two different labs. While the connection of distant qubits for entanglements was previously achieved, this is the first time the connection can lead to quantum computations. They used an ancillary photon that they successively reflected from two remote qubit modules, followed by a heralding photon detection, which triggered a final qubit rotation. They used the gate for remote entanglement creation of all four Bell states. The researchers hoped to extend their nonlocal quantum-logic gate both to multiple qubits and many modules for a tailor-made multi-qubit computing register.

# Honeywell Sets New Record For Quantum Computing Performance

Honeywell Quantum Solutions’ System Model H1 achieved a quantum volume of 512, the highest measured on a commercial quantum computer to date. It is the third time in nine months Honeywell has set a record for quantum volume on one of its systems. The average single-qubit gate fidelity for this milestone was 99.991(8)%, the average two-qubit gate fidelity was 99.76(3)% with fully-connected qubits, and measurement fidelity was 99.75(3)%. The System Model H1 successfully passed the quantum volume 512 benchmark, outputting heavy outcomes 73.32% of the time, which is above the 2/3 threshold with 99.54% confidence.

# An important landmark in the development of scalable Quantum Processors

It was shown by the researchers from the group of Menno Veldhorst at QuTech that semiconductor technology can be used to build a two-dimensional array of qubits to function as a quantum processor. They demonstrated a four-qubit quantum processor in a two-by-two array based on hole spins in germanium quantum dots and obtained controllable coupling along with both directions. They were able to perfectly implement quantum logic with only electricity and could freely program one, two, three, and four-qubit operations, resulting in a compact and highly connected circuit. This processor didn’t require any large additional structure next to each qubit making them almost identical to the transistors in a computer chip. This result is a major step towards quantum error correction and quantum simulation using quantum dots.

# The new quantum algorithm surpasses the QPE norm

A new Quantum Algorithm developed by researchers at Osaka City University is capable of directly calculating the energy gap between two electronic states having different spin quantum numbers without inspecting the total energy of the individual electronic states extending the algorithm to the direct calculation of vertical ionization energies. This quantum algorithm, called Bayesian eXchange coupling parameter calculator with Broken-symmetry wave functions (BxB), guarantees an exponential speedup, like quantum phase estimation (QPE)-based full-CI, with much 1/10th of its costs within 0.1 eV of precision as well for small atoms and molecules. This algorithm is easier to implement than QPE and has the potential to perform high-precision energy calculations for large molecules that cannot be treated in real-time with conventional computers.

# Researchers developed a theorem to find barren plateau for scaled-up variational algorithms

A team at Los Alamos National Laboratory devised two theorems for Variational Quantum Algorithms(VQAs) to find if any given VQA will encounter barren plateaus issues for scalable architectures. This work solves a key problem of useability for quantum machine learning where most algorithms suffer from barren plateaus, dead ends on optimization problems. The first result states that defining the cost function in terms of global observables leads to exponentially vanishing gradients (i.e., barren plateaus) even when the parametrized quantum circuit is shallow. With this theorem, several VQAs in the literature must revise their proposed costs. Their second result states that defining cost function with local observables leads to at worst a polynomially vanishing gradient, so long as the depth of the parametrized quantum circuit is O(log(n)). Here they assumed that the parametrized quantum circuit is an alternating layered ansatz composed of blocks forming local 2-designs. Their results establish a connection between locality and trainability and will allow researchers to find the scalability issues while developing algorithms.

With all these developments in the first Quater of 2021, the year has been amazing in the field of Quantum Technology. Stay in tune for the upcoming recaps of 2021 for the remaining developments.

P.S. I know it’s a bit late to publish this list, but it’s better to be late than never. Also, Covid infection delayed this list but I’ll try to publish the next soon :D