Quantum science holds promise for many technological applications, such as building hacker-proof communication networks or quantum computers that could help discover new drugs. These applications require a quantum version of a computer bit, called a “qubit”, which stores quantum information.
But researchers are still struggling with how to easily read the information in these qubits, and struggle with the short memory time, or “coherence,” of qubits, which is typically limited to microseconds or milliseconds.
A team of researchers from the University of Chicago achieved two major breakthroughs to overcome these common challenges in quantum systems: they were able to read their qubit on demand and then keep the quantum state intact for more than five seconds – a new record for this class of devices. Additionally, the researchers’ qubits are made from an easy-to-use material called silicon carbide, which is widely found in light bulbs, electric vehicles and high-voltage electronics.
“It’s rare for quantum information to be preserved on these human timescales,” said David Awschalom, Liew Family Professor of Molecular Engineering and Physics, Principal Investigator at Argonne National Laboratory and Principal Investigator of the project. “Five seconds is enough to send a speed-of-light signal to the moon and back. This is powerful if you plan to transmit information from a qubit to someone via light. This light will still correctly reflect the qubit’s state even after circling the Earth nearly 40 times, paving the way for the creation of a distributed quantum internet.
By creating a qubit system that can be fabricated in common electronics, the researchers hope to open a new avenue for quantum innovation using technology that is both scalable and cost-effective.
“This essentially brings silicon carbide to the fore as a quantum communication platform,” said graduate student Elena Glen, the paper’s co-first author. “It’s exciting because it’s easy to scale, since we already know how to make useful devices out of this material.”
The results were published on February 2 in the journal Scientists progress.
‘10,000 times more signals’
The first breakthrough for researchers was to make silicon carbide qubits easier to read.
Every computer needs a way to read the information encoded in its bits. For solid-state qubits like those measured by the team, the typical readout method is to address the qubits with lasers and measure the light emitted back. This procedure is difficult, however, because it requires very efficient detection of single particles of light called photons.
Instead, the researchers used carefully designed laser pulses to add a single electron to their qubit based on its initial quantum state, either 0 or 1. Then the qubit is read out the same way as before, with a laser. “Only now does the emitted light reflect the absence or presence of the electron, and with almost 10,000 times more signal,” said Elena Glen, co-first author of the paper. “By converting our fragile quantum state into stable electronic charges, we can measure our state much, much more easily.
“With this signal amplification, we can get a reliable answer whenever we check what state the qubit is in,” Glen explained. “This type of measurement is called ‘single reading,’ and with it we can unlock many useful quantum technologies.”
Armed with the one-shot readout method, scientists could focus on lasting their quantum states for as long as possible – a notorious challenge for quantum technologies, as qubits easily lose their information due to noise in their environment. .
The researchers grew highly purified samples of silicon carbide that reduced background noise that tends to interfere with their qubit functioning. Then, by applying a series of microwave pulses to the qubit, they extended the length of time their qubits retained their quantum information, a concept called “coherence.”
“These pulses decouple the qubit from noise sources and errors by rapidly inverting the quantum state,” said Chris Anderson, PhD’20, co-first author of the paper. “Each pulse is like pressing the cancel button on our qubit, erasing any errors that may have occurred between pulses.”
