Research Bits: April 26

Photonic quantum computers
Researchers from Stanford University propose a simpler design method for photonic quantum computers. The proposed design uses a laser to manipulate a single atom that, in turn, can modify the state of the photons via a phenomenon called “quantum teleportation.” The atom can be reset and reused for many quantum gates, eliminating the need to build multiple distinct physical gates, vastly reducing the complexity of building a quantum computer.

“Normally, if you wanted to build this type of quantum computer, you’d have to take potentially thousands of quantum emitters, make them all perfectly indistinguishable, and then integrate them into a giant photonic circuit,” said Ben Bartlett, a PhD candidate in applied physics at Stanford. “Whereas with this design, we only need a handful of relatively simple components, and the size of the machine doesn’t increase with the size of the quantum program you want to run.”

The design requires only a few pieces of equipment: a fiber optic cable, a beam splitter, a pair of optical switches, and an optical cavity.

“What we are proposing here is building upon the effort and the investment that people have put in for improving these components,” said Shanhui Fan, professor in the School of Engineering at Stanford. “They are not new components specifically for quantum computation.”

The researchers explained that the design consists of two main sections: a storage ring and a scattering unit. The storage ring, which functions similarly to memory in a regular computer, is a fiber optic loop holding multiple photons that travel around the ring. Analogous to bits that store information in a classical computer, in this system, each photon represents a quantum bit, or qubit. The photon’s direction of travel around the storage ring determines the value of the qubit, which like a bit, can be 0 or 1. Additionally, because photons can simultaneously exist in two states at once, an individual photon can flow in both directions at once, which represents a value that is a combination of 0 and 1 at the same time.

The researchers can manipulate a photon by directing it from the storage ring into the scattering unit, where it travels to a cavity containing a single atom. The photon then interacts with the atom, causing the two to become entangled, a quantum phenomenon whereby two particles can influence one another even across great distances. Then, the photon returns to the storage ring, and a laser alters the state of the atom. Because the atom and the photon are entangled, manipulating the atom also influences the state of its paired photon.

“By measuring the state of the atom, you can teleport operations onto the photons,” Bartlett said. “So we only need the one controllable atomic qubit and we can use it as a proxy to indirectly manipulate all of the other photonic qubits.”

The advantage of this system, the researchers say, is the ability to run many different quantum programs by controlling the way the atom and photons interact.

“For many photonic quantum computers, the gates are physical structures that photons pass through, so if you want to change the program that’s running, it often involves physically reconfiguring the hardware,” Bartlett said. “Whereas in this case, you don’t need to change the hardware – you just need to give the machine a different set of instructions.”

Reducing quantum errors
Researchers from University of California at Berkeley, Lawrence Berkeley National Lab, Massachusetts Institute of Technology, and Keysight Technologies found that randomized compiling (RC) can reduce error rates in quantum algorithms and lead to more accurate and stable quantum computations.

“We can perform quantum computations in this era of noisy intermediate-scale quantum (NISQ) computing, but these are very noisy, prone to errors from many different sources, and don’t last very long due to the decoherence – that is, information loss – of our qubits,” said Akel Hashim, a researcher at Lawrence Berkeley National Laboratory’s Advanced Quantum Testbed and graduate student at the University of California.

The researchers note that coherent errors are systematic and result from imperfect control of the qubits on a quantum processor, and can interfere constructively or destructively during a quantum algorithm. As a result, it is extremely difficult to predict their final impact on the performance of an algorithm.

The randomized compiling (RC) protocol does not try to fix or correct coherent errors. Instead, RC mitigates the problem by randomizing the direction in which coherent errors impact qubits, such that they behave as if they are a form of stochastic noise. RC achieves this goal by creating, measuring, and combining the results of many logically-equivalent quantum circuits, thus averaging out the impact that coherent errors can have on any single quantum circuit.

“We know that, on average, stochastic noise will occur consistently at the same average error rate, so we can reliably predict what the results will be from the average error rates. Stochastic noise will never impact our system worse than the average error rate – something that is not true for coherent errors, whose impact on algorithm performance can be orders of magnitude worse than their average error rates would suggest,” Hashim said.

Coherent errors in quantum algorithms can build upon themselves through constructive interference and often grow faster than stochastic noise. However, the experimental demonstration of RC showed that coherent errors in quantum algorithms can be controlled to grow at a much slower rate.

“RC is a universal protocol for gate-based quantum computing, which is agnostic to specific error models and hardware platforms,” Hashim said. “There are many applications and classes of algorithms out there that may benefit from the RC. Our collaborative research demonstrated that RC works to improve algorithms in the NISQ era, and we expect it will continue to be a useful protocol beyond NISQ. It is important to have this successful demonstration in our toolbox at AQT. We can now deploy it on other testbed user projects.”

Silicon carbide quantum systems
Researchers from the University of Stuttgart, University of California Davis, Linköping University, Fraunhofer Institute for Integrated Systems and Device Technology, Helmholtz-Zentrum Dresden-Rossendorf, and Leibniz-Institute of Surface Engineering explored ways to fabricate and integrate quantum photonic systems.

The researchers based the quantum system on the silicon vacancy center in silicon carbide, which is known to possess robust spin-optical properties. They fabricated nanophotonic waveguides around these color centers using gentle processing methods that keep the host material essentially free of damage.

“With our approach, we could demonstrate that the excellent spin-optical properties of our color centers are maintained after nanophotonic integration,” said Florian Kaiser, assistant professor at the University of Stuttgart. “Thanks to the robustness of our quantum devices, we gained enough headroom to perform quantum gates on multiple nuclear spin qubits. As these spins show very long coherence times, they are excellent for implementing small quantum computers.”

“In this project, we explored the peculiar triangular shape of photonic devices. While this geometry is of commercial appeal because it provides versatility needed for scalable production, little has been known about its utility for high performing quantum hardware. Our studies reveal that light emitted by the color center, which carries quantum information across the chip, can be efficiently propagated through a single optical mode. This is a key conclusion for viability of integration of color centers with other photonic devices, such as nanocavities, optical fiber and single-photon detectors, needed to realize full functionalities of quantum networking and computing,” said Marina Radulaski, assistant professor at the University of California Davis.

In addition, the silicon carbide platform is CMOS compatible. In future work, the researchers want to implement semiconductor circuitry to electrically initialize and readout the quantum states of the spin qubits.

“Maximizing electrical control – instead of traditional optical control via lasers – is an important step towards system simplification. The combination of efficient nanophotonics with electrical control will allow us to reliably integrate more quantum systems on one chip, which will result in significant performance gains,” said Kaiser. “In this sense, we are only at the dawn of quantum technologies with color centers in silicon carbide. Our successful nanophotonic integration is not only an exciting enabler for distributed quantum computing, but it can also boost the performance of compact quantum sensors.”