In Building 13 on the MIT campus, there is a $500,000 device that looks like an elongated chandelier, a series of gold discs connected by thin silver tubes. The device, called a dilution refrigerator, was a key player in doctoral student Alex Greene’s research because it housed all of their experiments. “My life is shaped around its rhythm,” they said.
The first time Green helped put new samples into the freezer, they worked with a postdoc at midnight on Friday, playing Danish scream music. Since then, Fridge has led them on exciting and frustrating adventures in their doctoral research on reducing errors in quantum computing systems.
Green and their identical twin, Jamie, grew up in northern New Jersey. The pair have been extremely competitive from an early age, and outside of school, they keep themselves busy with running, pole vaulting and rock climbing. Their father, a neurologist, and their mother, a former electrical engineer, worked at Bell Labs, a research lab known for pioneering key technologies in computers and telephony.
In 2010, Alex and Jamie both came to MIT as undergraduates. Alex got interested in biomedical engineering in high school, “but then I found out I hated working in a ‘law’ lab,” they said, where scientists work with chemical and biological materials. Another influence was Carl Sagan’s “Contact,” a science fiction novel about astronomers searching for extraterrestrial intelligence. “It got me hooked on physics,” Green said.
As an undergrad at MIT, Greene majored in physics, electrical engineering, and computer science. They have found a home in the field of quantum computing, where researchers are working to build extremely powerful computers by exploiting physical concepts from quantum mechanics.
Greene stayed at MIT for a master’s degree in quantum computing and worked at Lincoln Laboratory. There, they looked at ways to improve a technique called trapped-ion quantum computing, which uses atoms suspended in air and controlled by lasers.
After completing their master’s degree, they turned to another technique called superconducting quantum computing. Instead of levitating atoms, the technology uses tiny circuits that are excellent at carrying current. To control these circuits, researchers only need to send electrical signals.
For this project, Green hopes to collaborate with MIT professor William Oliver, who directs the Center for Quantum Engineering at the Electronics Research Laboratory. Green chose to stay at the Institute again—this time to pursue a Ph.D.
Adding randomness to quantum computers
One day, quantum computers may solve problems that ordinary classical computers cannot, leading to huge advances in many applications. However, from a technical point of view, manipulating hardware to exhibit quantum behavior is challenging. Currently, quantum computers, including superconducting computers, struggle with high error rates, which limit the length and complexity of the “programs” they can run. Most experimental research on quantum computing has focused on addressing these errors.
Greene is working to make superconducting quantum computers more accurate by reducing the effects of these errors. To test their idea, they needed to conduct experiments on superconducting circuits. But for these circuits to work, they need to be cooled to extremely low temperatures, about -273.13 degrees Celsius — within 0.02 degrees of the coldest temperature possible in the universe.
This is where the chandelier-style dilution refrigerator comes into play. The refrigerator can easily reach the desired low temperature. But sometimes it misbehaves, sending Green to perform side quests to fix its problems.
Green’s toughest side quest involves tracking down a leak in the refrigerator’s pipes. The pipes carry an expensive and rare mixture of gases used to cool refrigerators, which Green cannot afford to lose. Fortunately, even with a leak, the refrigerator is designed to remain functional and not lose any mixture for about two weeks at a time. However, to keep the refrigerator functioning properly, Greene had to constantly restart and clean it over a five-day period. After about seven months of stress, Green and their lab mates finally found and fixed the leak, allowing Green to resume their research at full speed.
To develop strategies to effectively improve the precision of superconducting quantum computers, Greene needs to first take stock of the different types of errors in these systems. In quantum computing, there are two types of errors: incoherent errors and coherent errors. Incoherent errors are random errors that occur even when a quantum computer is idle, while coherent errors are caused by imperfect system control. In quantum computers, coherent errors are often the culprits of system inaccuracy; researchers have shown mathematically that coherent errors recombine much faster than incoherent errors.
To avoid the annoying compound inaccuracies of coherent errors, Green employs a clever tactic: disguise these errors as incoherent ones. “If you [strategically] Introduce a bit of randomness into a superconducting circuit,” they say, and you can make coherent errors recombine as slowly as incoherent errors. Greene points out that other researchers in the field are using random strategies in different ways. Still, Through their research, Greene is helping to pave the way for more precise superconducting quantum computers.
Improving water sanitation in Pakistan
Outside of research, Green is constantly engaged in a variety of activities, adding new hobbies while deliberately removing old ones to make room in his busy schedule. Over the years, their hobbies have included glass blowing, singing in a local queer choir and competitive rock climbing. Currently, they work on home improvement projects with partners at the rainbow-colored co-op on weekends.
For the past year and a half, Green has also been involved in the Water Sanitation Project through classes at MIT’s D-Lab, a program that helps underprivileged communities around the world. Taking classes at the D-Lab, they say, is “something I’ve always wanted to do since undergrad, but I’ve never had the time to do it.” They were finally able to incorporate D-Lab into their schedule by using these courses to meet their PhD requirements.
For one project, they are developing a system to efficiently and inexpensively filter out harmful excess fluoride from Pakistan’s water supply. “Fluoride is bad, it’s not intuitive because our toothpaste has fluoride in it,” they said. “But in reality, too much fluoride changes the hardness of teeth and bones.” One idea they and their collaborators are exploring is to build a water filtration system using clay, a proven but inexpensive method of fluoride removal.
The fluoride filtration project was originally proposed by a visiting assistant professor from Pakistan who took the D-Lab course. After the course, the professor returned to Pakistan, but the project continued. Green is now working virtually with professors to help figure out the best type of clay to filter fluoride. Through their experience at D-Lab, Greene believes that she will continue to volunteer for water sanitation projects in the long term.
Greene plans to complete his Ph.D. this December. After 12 years at MIT, Greene’s goal is to leave the institute and work at a quantum computing company. In industry, “it’s a good time to really get into the field,” they said. “The company started to scale [quantum computing] technology. “