Most experts agree that quantum computing is still in an experimental era. The current state of quantum technology has been compared to the same stage that classical computing was in during the late 1930s.
Quantum computing uses various computation technologies, such as superconducting, trapped ion, photonics, silicon-based, and others. It will likely be a decade or more before a useful fault-tolerant quantum machine is possible. However, a team of researchers at MIT Lincoln Laboratory has developed a vital step to advance the evolution of trapped-ion quantum computers and quantum sensors.
Most everyone knows that classical computers perform calculations using bits (binary digits) to represent either a one or zero. In quantum computers, a qubit (quantum bit) is the fundamental unit of information. Like classical bits, it can represent a one or zero. Still, a qubit can also be a superposition of both values when in a quantum state.
Superconducting qubits, used by IBM and several others, are the most commonly used technology. Even so, trapped-ion qubits are the most mature qubit technology. It dates back to the 1990s and its first use in atomic clocks. Honeywell and IonQ are the most prominent commercial users of trapped ion qubits.
Trapped-Ion quantum computers
Honeywell and IonQ both create trapped-ion qubits using an isotope of rare-earth metal called ytterbium. In its chip using integrated photonics, MIT used an alkaline metal called strontium. The process to create ions is essentially the same. Precision lasers remove an outer electron from an atom to form a positively charged ion. Then, lasers are used like tweezers to move ions into position. Once in position, oscillating voltage fields hold the ions in place. One main advantage of ions lies in the fact that it is natural instead of fabricated. All trapped-ion qubits are identical. A trapped-ion qubit created on earth would be the perfect twin of one created on another planet.
Dr. Robert Niffenegger, a member of the Trapped Ion and Photonics Group at MIT Lincoln Laboratory, led the experiments and is first author on the Nature paper. He explained why strontium was used for the MIT chip instead of ytterbium, the ion of choice for Honeywell and IonQ. “The photonics developed for the ion trap are the first to be compatible with violet and blue wavelengths,” he said. “Traditional photonics materials have very high loss in the blue, violet and UV. Strontium ions were used instead of ytterbium because strontium ions do not need UV light for optical control.”
All the manipulation of ions takes place inside a vacuum chamber containing a trapped-ion quantum processor chip. The chamber protects the ions from the environment and prevents collisions with air molecules. In addition to creating ions and moving them into position, lasers perform necessary quantum operations on each qubit. Because lasers and optical components are large, it is by necessity located outside the vacuum chamber. Mirrors and other optical equipment steer and focus external laser beams through the vacuum chamber windows and onto the ions.
The largest number of trapped-ion qubits being used in a quantum computer today is 32. For quantum computers to be truly useful, millions of qubits are needed. Of course, that means many thousands of lasers will also be required to control and measure the millions of ion qubits. The problem becomes even larger when two types of ions are used, such as ytterbium and barium in Honeywell’s machine. The current method of controlling lasers makes it challenging to build trapped-ion quantum computers beyond a few hundred qubits.
Rather than resorting to optics and bouncing lasers off mirrors to aim beams into the vacuum chamber, MIT researchers have developed another method. They have figured out how to use optical fibers and photonics to carry laser pulses directly into the chamber and focus them on individual ions on the chip.
A trapped-ion strontium quantum computer needs lasers of six different frequencies. Each frequency corresponds to a different color that ranges from near-ultraviolet to near-infrared. Each color performs a different operation on an ion qubit. The MIT press release describes the new development this way, “Lincoln Laboratory researchers have developed a compact way to deliver laser light to trapped ions. In the Nature paper, the researchers describe a fiber-optic block that plugs into the ion-trap chip, coupling light to optical waveguides fabricated in the chip itself. Through these waveguides, multiple wavelengths [colors] of light can be routed through the chip and released to hit the ions above it.”
In other words, rather than using external mirrors to shine lasers into the vacuum chamber, MIT researchers used multiple optical fibers and photonic waveguides instead. A block equipped with four optic fibers delivering a range of colors was mounted on the quantum chip’s underside. According to Niffenegger, “Getting the fiber block array aligned to the waveguides on the chip and applying the epoxy felt like performing surgery. It was a very delicate process. We had about half a micron of tolerance, and it needed to survive cool down to 4 Kelvin.”
I asked Dr. Niffenegger his thoughts about the long-term implications of his team’s development. His reply was interesting.
“I think many people in the quantum computing field think that the board is set and all of the leading technologies at play are well defined. I think our demonstration, together with other work integrating control of trapped ion qubits, could tip the game on its head and surprise some people that maybe the rules aren’t what they thought. But really I just hope that it spurs more out of the box ideas that could enable quantum computing technologies to break through towards practical applications.”
- Integrating optical waveguides into ion traps represents a step forward toward the goal of building a useful quantum computer with thousands to millions of qubits.
- MIT’s technique also provides a development path for portable trapped-ion quantum sensors and clocks.
- Integrated photonics is inherently resistant to vibrations. With external lasers, vibrations cause pulses to miss the ion. Integrated optics should eliminate most effects of vibrations.
- The stability offered by integrated photonics will help qubits maintain quantum states longer so that deeper and more complex computations can be performed.
- Initially I had some concerns about loss of optical power due to compromises that may have been made in the grating coupler to accommodate different wavelengths. Keep in mind there are four fibers and six colors. The shortest of the six laser wavelengths is 405 nm and the longest is 1092 nm. Dr. Niffenegger pointed out there are separate gratings for the shortest and longest wavelengths. He also said there are some power losses, but they are in the path from where light enters the optical waveguide to where it exits the coupler grating. Despite this minor optical power loss, tighter focus provided by the existing diffraction gratings provides enough power for operations on the ions.
- Dr. Niffenegger and the MIT research team will focus future research on reducing two qubit gate errors caused by heating of the motional state of ion qubits. The rate at which ions heat up is much higher in traps with integrated photonic chips than traditional surface traps without photonics
Note: Moor Insights & Strategy writers and editors may have contributed to this article.