99.9904%！ Quantum bit readout fidelity breaks the world record

Last December, IonQ announced that it would use barium ions as qubits in a new generation of ion trap quantum computing systems, saying that barium ions would bring lower error rates or higher fidelity.

On March 3, IonQ announced the readout fidelity of its new barium-based quantum computer, or State Preparation and Measurement (SPAM) fidelity, from 99.5% to an industry-leading 99.96%.[1]

Coincidentally, Honeywell subsidiary Quantinuum's ion trap system uses barium-ion qubits to increase SPAM fidelity to 99.9904% – the highest of any quantum technology to date. The relevant papers have been published [2].

99.96% of IonQ

IonQ says barium qubits reduce SPAM errors by a factor of 13, in other words, from 50 errors per 10,000 calculations to 4 errors per 10,000 calculations. As a result, fidelity increased from 99.5% to 99.96%.

As we wrote in the article "what are the performance indicators of quantum computers?" what do they represent? as mentioned in the book, there are three main errors in quantum computers: imperfect state preparation at the beginning of the algorithm; imperfect quantum logic gates when running the algorithm; and imperfect measurements when reading out results. for quantum computers to scale while maintaining accuracy, all three sources of error must be reduced.

With each additional qubit in a system, state measurement errors increase, which means that as systems scale up, improving state measurement fidelity is increasingly important for computers to provide accurate results to users.

Assuming that the quantum logic gate is perfect, an average 99% SPAM fidelity limits the system's #AQ (i.e., algorithmic qubits, useful qubits when the algorithm runs) to about 100; a SPAM fidelity of 99.96% can reach 2000 #AQ.

Professor Jungsang Kim, co-founder, and CTO of IonQ said: "We have shown that captured ions produce more algorithmic qubits than any other quantum computer architecture, and the results released today show that our new barium qubits have paved the way for increased fidelity, increasing the ability to reduce state measurement errors, which is another aspect of IonQ's clear leadership in this field. ”

Schematic of qubit readouts in the IonQ system – To read the state of the ion qubits, the system shines a specially tuned laser beam on the qubits and then measures the emitted light.

99.9904% of Quantinuum

Using qubits formed by non-radioactive barium-137, the quantinuum team achieved 99.9904% state preparation and measurement (SPAM) fidelity, the highest of any quantum technology to date. The researchers believe that SPAM fidelity needs to reach 99.97% to 99.99% to reach a level where the logic error rate exceeds the primary physical error rate. 

Researchers can use lasers in the visible spectrum, a more mature and readily available technique, to initialize and manipulate qubits. They used two schemes, microwave-assisted optical pumping (MAOP) and 1762nm narrowband optical pumping (NBOP), which measured SPAM error rates of (14.8±1.7) × 10−5 and (9.6±1.4) ×10−5, respectively the latter has a lower SPAM error rate and corresponds to a fidelity of 99.9904%.

Quantinuum says being able to initialize non-radioactive barium-137 ions is only the first step. The goal is to integrate these ions into future quantum hardware technologies. "We believe that using non-radioactive barium-137 ions as qubits is an attractive pathway to increasingly powerful, scalable quantum hardware," said Tony Uttley, president and chief operating officer of Quantinumum[3].

Why barium?

Neutral ytterbium atoms have long been the source of ions for ion trap quantum computers. ytterbium ions are converted into qubits by lasers. but there are challenges with using ytterbium. because expensive UV lasers are required to manipulate ytterbium ions, and the results can be difficult to measure.

In contrast, barium ions are easier to measure and can be manipulated by cheaper, more stable green lasers. But before Quantinuum's non-radioactive barium-137 study, researchers could only achieve lower SPAM fidelity for barium-133 atoms because barium-133 atoms are radioactive and require special handling.

Dr. Anthony Ransford, a physicist and head of technology at Quantinumum, said: "Everyone doesn't believe you can achieve a fast, powerful SPAM with non-radioactive barium-137. We were able to design a scheme that allowed us to initialize the qubits and measure them better than any other qubit. We were the first to do so. ”

Link:

[1]https://ionq.com/news/march-03-2022-barium-demonstrates-leading-readout

[2]https://assets.website-files.Com/617730fbcf7b7c387194556a/62211b26eec38769e5954732_SPAM%20Results.pdf

[3]https://www.quantinuum.com/pressrelease/quantinuum-announces-a-world-record-in-fidelity-for-quantum-computing-qubits