In the field of quantum computing, two milliseconds, or two thousandths of a second, is a very long period of time. The blink of an eye, which lasts one tenth of a second, feels like an eternity on these timescales.
Now, a team of scientists from UNSW Sydney has made significant progress by demonstrating that "spin qubits," which are the electron-based fundamental units of information in quantum computers, can store information for up to two milliseconds. The accomplishment is 100 times longer than prior benchmarks in the same quantum processor for what is known as "coherence time," the amount of time qubits may be moved around in increasingly complex calculations.
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 calculations in quantum computing. The calculation fails and the values each spin qubit was representing are lost when the spin qubits stop 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, modelling 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 technical 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 use just one antenna to control all the qubits in the chip by inserting a crystal called a dielectric resonator, rather than adding thousands of tiny antennas to control millions of electrons with 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.
Individual vs. global control
This proof-of-concept accomplishment did leave some problems, though. In a series of studies that were published in the journals Physical Review B, Physical Review A, and Applied Physics Reviews—the most recent one just this week—lead researcher Ms. Ingvild Hansen collaborated with Ms. Seedhouse to solve these challenges.
It was a significant advance to be able to manage millions of qubits using just one antenna. Although controlling millions of qubits at once is an impressive feat, 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 manipulate them separately so that they each represent a different value in a calculation? First, we demonstrated theoretically that rotating the qubits continually can increase the coherence duration, according to Ms. Hansen.
"Consider a circus performer spinning plates; the act can go on while the plates are still spinning. In the same way, qubits can store information for longer if they are driven continually. We demonstrated the coherence durations of such "dressed" qubits, which were greater than 230 microseconds [230 millionths of a second].
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.
This was done 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. Any qubit can then be made to move at a different tempo from its neighbours while still maintaining the same rhythm if an electric field is applied to it independently, taking it out of resonance.
According to Ms. Seedhouse, "imagine it like two kids on a swing who are very much going forward and backward in sync." If we push one of them, we can 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 can be individually controlled (electronically) while being subject to global control (magnetically), and the coherence period is, as previously mentioned, significantly longer and adequate for quantum calculations.
One of the team's senior members, Dr. Henry Yang, claims that they have demonstrated a straightforward and elegant method for simultaneously controlling all qubits that also offers improved performance.
A possible route for full-scale quantum computers is the SMART protocol.
Professor Andrew Dzurak, CEO and founder of Diraq, a UNSW spin-out business developing quantum computer processors that can be produced using conventional silicon chip manufacturing, is in charge of the research team.
Next actions
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 calculations.
"Then, to demonstrate that the theory is validated in actuality, we aim to show that we can also achieve this for a small number of qubits
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