For The Longest Time: Quantum Computing Engineers Set New Standard In Silicon Chip Performance

The ability of "spin qubits," which are the fundamental units of information in quantum computers, to store information for up to two milliseconds has now been shown by a team of researchers at UNSW Sydney. The milestone, which is 100 times longer than prior benchmarks in the same quantum processor, extends the amount of time known as "coherence time," which is the amount of time qubits may be used in increasingly complex operations.

The achievement was made possible by the work of Ph.D. student Ms. Amanda Seedhouse, whose work in theoretical quantum computing was instrumental in making it possible. "Longer coherence time means you have more time over which your quantum information is stored—which is exactly what you need when doing quantum operations," she says.

The coherence time essentially indicates how long you have before losing all of the information stored in your qubits while using any algorithm or sequence.

The greater the ability to hold spins in motion, the more likely it is that information will be preserved throughout computations in quantum computing. The calculation fails and the values each spin qubit was representing are lost when the spin qubits cease spinning. Quantum engineers at UNSW previously empirically verified the idea of extending coherence in 2016.

The fact that working quantum computers of the future will need to keep track of the values of millions of qubits in order to resolve some of humanity's most difficult problems, such as the search for efficient vaccines, modeling weather systems, and forecasting the effects of climate change, makes the task even more difficult.

The same UNSW Sydney team late last year found a technological solution to a conundrum that has baffled researchers for years: how to control millions of qubits without producing extra heat and interference. The study team discovered a technique to utilize just one antenna to control all the qubits in the device by inserting a crystal called a dielectric resonator, rather than adding thousands of small antennas to control millions of electrons using magnetic waves. Science Advances published these findings.

This resolved the space, heat, and noise issues that would inevitably arise as more and more qubits are made operational in order to perform the mind-bending computations made possible when qubits can simultaneously represent both 1 and 0, using a phenomenon known as quantum superposition, in addition to 1 or 0 like conventional binary computers.

Global vs individual control

This proof-of-concept accomplishment did leave some problems, though. In a series of studies that have been published in the journals Physical Review B, Physical Review A, and Applied Physics Reviews—the most recent one just this week—lead researchers Ms. Ingvild Hansen and Ms. Seedhouse have addressed these challenges.

It was a significant advance to be able to manage millions of qubits with just one antenna. Although controlling millions of qubits at once is an impressive achievement, operating quantum computers will also require manipulating each qubit separately. The values of all the spin qubits will be the same if they rotate at essentially the same frequency. How can we manage each one separately such that they may represent various values in a calculation?

Theoretically, we first demonstrated that spinning the qubits continually can increase the coherence time, according to Ms. Hansen.

"If you picture a circus performer spinning plates, the act may go on as long as the dishes are spinning. In the same manner, qubits may store information for longer if they are driven continually. We demonstrated that such "dressed" qubits have coherence durations greater than 230 microseconds."

The next challenge was to strengthen the protocol and demonstrate that the globally controlled electrons can also be controlled individually so that they can hold different values needed for complex calculations after the team demonstrated that coherence times could be extended with so-called "dressed" qubits.

The scientists did this by developing the "SMART" qubit protocol, which stands for Sinusoidally Modulated, Always Rotating, and Tailored.

Qubits were adjusted to rock back and forth like a metronome rather than spinning in circles. Then, each qubit may be made to move at a different pace from its neighbors while still maintaining the same rhythm if an electric field is provided to it individually, knocking it out of resonance.

According to Ms. Seedhouse, "imagine it as two toddlers on a swing who is very much moving forward and backward in rhythm." If we push one of them, we may make them reach the ends of their arcs at the opposite ends, allowing one to be a 0 while the other is now a 1.

As a result, a qubit may be individually controlled (electronically) while being subject to global control (magnetically), and the coherence period is, as previously said, significantly longer and adequate for quantum computations.

Dr. Henry Yang, a senior member of the team, states, "We have demonstrated a simple and elegant technique to manage all qubits at once that also comes with greater performance."

A possible route for full-scale quantum computers is the SMART protocol.

Professor Andrew Dzurak, CEO and creator of Diraq, a UNSW spin-out business creating quantum computer processors that can be produced using conventional silicon chip manufacturing, is in charge of the research team.

Next steps

After demonstrating our proof-of-concept in our experimental study with one qubit, Ms. Hansen states that the next step is to demonstrate this functioning with two-qubit computations.

"Then, to demonstrate that the theory is validated in actuality, we aim to show that we can also achieve this with a small number of qubits."