Research Bits: June 8

Five-second coherence for silicon carbide qubits

Researchers from the University of Chicago, National Institutes for Quantum Science and Technology, and Linköping University built a qubit from silicon carbide and was able to retain its coherence, or the length of time the quantum state persists, for over five seconds.

“It’s uncommon to have quantum information preserved on these human timescales,” said David Awschalom, professor in Molecular Engineering and Physics at University of Chicago and senior scientist at Argonne National Laboratory. “Five seconds is long enough to send a light-speed signal to the moon and back. That’s powerful if you’re thinking about transmitting information from a qubit to someone via light. That light will still correctly reflect the qubit state even after it has circled the earth almost 40 times—paving the way to make a distributed quantum internet.”

“This essentially brings silicon carbide to the forefront as a quantum communication platform,” said Elena Glen, graduate student at University of Chicago. “This is exciting because it’s easy to scale up, since we already know how to make useful devices with this material.”

First, the team worked to make silicon carbide qubits easier to read compared to standard laser readout methods. They first used carefully designed laser pulses to add a single electron to their qubit depending on its initial quantum state, either 0 or 1. Then the qubit is read out using a laser, as is typical.

“Only now, the emitted light reflects the absence or presence of the electron, and with almost 10,000 times more signal,” said Glen. “By converting our fragile quantum state into stable electronic charges, we can measure our state much, much more easily. With this signal boost, we can get a reliable answer every time we check what state the qubit is in. This type of measurement is called ‘single-shot readout,’ and with it, we can unlock a lot of useful quantum technologies.”

The chips used in the experiment are made from silicon carbide, an inexpensive and commonly used material. (Photo by David Awschalom / University of Chicago)

To extend the length of time quantum states lasted, the team grew highly purified samples of silicon carbide that reduced the background noise that tends to interfere with their qubit functioning. Then, by applying a series of microwave pulses to the qubit, they extended the amount of time that their qubits preserved their coherence.

“These pulses decouple the qubit from noise sources and errors by rapidly flipping the quantum state,” said Chris Anderson, at the time a researcher at University of Chicago and now a postdoctoral scholar at Stanford University. “Each pulse is like hitting the undo button on our qubit, erasing any error that may have happened between pulses.”

The longer coherence makes more complex operations possible, Anderson noted. “For example, this new record time means we can perform over 100 million quantum operations before our state gets scrambled.”

“The ability to perform single-shot readout unlocks a new opportunity: using the light emitted from silicon carbide qubits to help develop a future quantum internet,” said Glen. “Essential operations such as quantum entanglement, where the quantum state of one qubit can be known by reading out the state of another, are now in the cards for silicon carbide-based systems.”

“We’ve essentially made a translator to convert from quantum states to the realm of electrons, which are the language of classical electronics, like what’s in your smartphone,” said Anderson. “We want to create a new generation of devices that are sensitive to single electrons, but that also host quantum states. Silicon carbide can do both, and that’s why we think it really shines.”

Constructing quantum computers atom-by-atom

Researchers at the University of Melbourne, University of New South Wales Sydney, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Leibniz Institute of Surface Engineering, and RMIT University propose a way to construct quantum computers using a method that allows for embedding of single atoms in silicon wafers.

The technique can create large scale patterns of counted atoms that are controlled so their quantum states can be manipulated, coupled and read-out.

“We believe we ultimately could make large-scale machines based on single atom quantum bits by using our method and taking advantage of the manufacturing techniques that the semiconductor industry has perfected,” said David Jamieson, a professor at University of Melbourne.

The method uses an atomic force microscope. A tiny hole is drilled in the AFM’s cantilever so that when it was showered with phosphorus atoms one would occasionally drop through the hole and embed in the silicon substrate.

The kinetic energy of the atom as it embeds into the silicon crystal and dissipates its energy by friction can be exploited to make a tiny electronic ‘click,’ which enabled the team to determine just one atom had become embedded in the substrate.

“One atom colliding with a piece of silicon makes a very faint click, but we have invented very sensitive electronics used to detect the click, it’s much amplified and gives a loud signal, a loud and reliable signal,” said Jamieson.  “That allows us to be very confident of our method. We can say, ‘Oh, there was a click. An atom just arrived. Now we can move the cantilever to the next spot and wait for the next atom.’”

Andrea Morello, Scientia professor from the University of New South Wales, said the new qubit ‘chip’ created by the technique can be used in lab experiments to test designs for large scale devices. “This will allow us to engineer the quantum logic operations between large arrays of individual atoms, retaining highly accurate operations across the whole processor. Instead of implanting many atoms in random locations and selecting the ones that work best, they will now be placed in an orderly array, similar to the transistors in conventional semiconductors computer chips.”

Hybrid quantum internet

Researchers from Delft University of Technology (TU Delft) and the University of Campinas (UNICAMP) demonstrated that information encoded in a quantum bit consisting of a single photon can be teleported to the mechanical motion of an optomechanical device comprising billions of atoms.

“As occurs in the classical internet, a quantum internet will require a network of signal repeaters to distribute information to any part of the world. In our study, we obtained the faithful transfer of an unknown quantum state to a remote quantum system. This result enables us to visualize a long-distance quantum communication scenario, necessary for the construction of a future quantum internet,” said Thiago Alegre, a professor at the University of Campinas’ Gleb Wataghin Institute of Physics.

In the experiment, an arbitrary quantum state was encoded in a photonic qubit by optical polarization. The photon was transported over tens of meters of optic fiber and then teleported to two silicon resonators, each with billions of atoms. To do this, the researchers had to generate an entangled state between the mechanical modes of the two micro-oscillators, permitting manipulation of these states at a distance. Lastly, they demonstrated the reliability of the process by faithfully retrieving the original quantum state from the resonators’ memory.

While quantum teleportation has previously been demonstrated, using optimechanical devices to receive the signal is novel, the researchers noted.

“These devices can be designed to operate at any wavelength, including infrared. That’s the wavelength with the least transmission loss, so the distance between repeaters can be greater,” said Simon Gröblacher, a professor at TU Delft.

Alegre said that a next step in making this result available for real-world application will be to design optomechanical systems that are resilient to parasitic optical absorptions. “This can be done thanks to the outstanding flexibility of these nanofabricated devices.”

The work is a step toward a hybrid quantum internet, which would operate heterogeneously with several physical systems intercommunicating and performing different functions, according to Gröblacher. “We could have optomechanical quantum repeater nodes connected to a quantum computer or memory comprising superconducting qubits or spin quantum systems. All these components will have to be compatible with each other and operate at the same wavelength so as to transfer quantum information faithfully.”

“The study is important because it demonstrates the possibility of creating a versatile platform capable of interconnecting several of these systems,” Alegre added.