Vaccine and drug development, artificial intelligence, transportation and logistics, climate science – these are all areas that need to be transformed by the development of a full-scale quantum computer. And there has been an explosive growth in investment in quantum computers over the past decade.
Yet the current quantum processors are relatively small in scale with less than 100 qubits – the basic building blocks of a quantum computer. Bits are the smallest unit of information in data processing, and the term qubits is derived from “quantum bits”.
While early quantum processors have been crucial in demonstrating the potential of quantum computing, the realization of globally significant applications is likely to require processors of up to one million qubits.
Our new research tackles a core problem at the heart of upscaling quantum computers: how do we go from controlling just a few qubits to controlling millions? In research recently published in Science Advances, we reveal a new technology that can offer a solution.
Quantum computers use qubits to store and process quantum information. Unlike the pieces of information in classical computers, qubits make use of the quantum properties of nature, known as “superposition” and “entanglement”, to perform some calculations much faster than their classical counterparts.
Unlike a classic bit, which is represented by either 0 or 1, a qubit can exist in thaw modes (i.e. 0 and 1) at the same time. This is what we refer to as a superposition state.
Demonstrations from Google and others have shown that even current, early quantum computers can outperform the most powerful supercomputers on the planet for a highly specialized (though not very useful) task – to reach a milestone we call quantum domination.
Google’s quantum computer, built of superconducting electrical circuits, had only 53 qubits and was cooled to a temperature below -273 ℃ in a high-tech refrigerator.
This extreme temperature is necessary to remove heat, which can introduce defects to the fragile qubits. While such demonstrations are important, the challenge now is to build quantum processors with many more qubits.
Great efforts are underway at UNSW Sydney to make quantum computers from the same material used in everyday computer chips: silicon. A conventional silicon chip is the size of multi-billion-bit thumbnails and packets, so the prospect of using this technology to build a quantum computer is compelling.
The control problem
In silicon quantum processors, information is stored in individual electrons trapped under small electrodes on the surface of the chip. Specifically, the qubit is encoded into the electron spin.
It can be depicted as a small compass inside the electron. The needle of the compass can point north or south, representing the 0 and 1 states.
Putting a qubit in superposition mode (both 0 and 1), an operation that occurs in all quantum calculations, a control signal must be routed to the desired qubit.
For qubits in silicon, this control signal is in the form of a microwave field, much like those used to conduct telephone calls over a 5G network. The microwaves interact with the electron and cause its spin (compass needle) to rotate.
Currently, each qubit requires its own microwave control field. It is supplied to the quantum chip through a cable running from room temperature down to the bottom of the refrigerator at close to -273 ℃. Each cable carries heat, which must be removed before it reaches the quantum processor.
With about 50 qubits, which is state-of-the-art today, this is difficult but manageable. The current refrigerator technology can handle the cable heat load. However, it represents a huge obstacle if we are to use systems with a million qubits or more.
An elegant solution to the challenge of delivering control signals to millions of spin qubits was proposed in the late 1990s. The idea of ”global control” was simple: emit a single microwave control field across the entire quantum processor.
Voltage pulses can be applied locally to qubit electrodes to cause the individual qubits to interact with the global field (and produce superposition states).
It is much easier to generate such voltage pulses on chip than it is to generate multiple microwave fields. The solution requires only a single control cable and removes intrusive microwave control circuits on the chip.
For more than two decades, global control in quantum computers remained an idea. Scientists could not devise a suitable technology that could be integrated with a quantum chip and generate microwave fields at suitably low forces.
In our work, we show that a component known as a dielectric resonator could finally allow this. The dielectric resonator is a small, transparent crystal that captures microwaves for a short period of time.
The capture of microwaves, a phenomenon known as resonance, allows them to interact with spin-qubits longer and greatly reduces the power of microwaves needed to generate the control field. This was essential to be able to operate the technology inside the refrigerator.
In our experiment, we used the dielectric resonator to generate a control field over an area that could hold up to four million qubits. The quantum chip used in this demonstration was a unit with two qubits. We were able to show that the microwaves produced by the crystal could reverse the spin state of each one.
There is still work to be done before this technology is ready for the task of controlling a million qubits. For our study, we managed to reverse the state of qubits, but not yet produce arbitrary superposition states.
Experiments are underway to demonstrate this critical ability. We will also need to further study the effect of the dielectric resonator on other aspects of the quantum processor.
That said, we believe that these technical challenges will ultimately be overcome – removing one of the biggest obstacles to realizing a large-scale spin-based quantum computer.
This story originally appeared on The Conversation website and is being republished with permission. To view the original, please clickk here.