QNAS

QnAs with Christopher Monroe QNAS

Chris Samoray, Science Writer

Classic computing can take credit for technology ranging from mobile phones to supercomputers. But in recent years, a budding counterpart to these con- ventional devices has emerged: quantum computers. Whereas classic computing sometimes fails to solve complex calculations, such as factoring hundred-digit numbers, holds the potential to easily tackle such problems. The field of quantum computing has attracted researchers, such as National Academy of Sciences member Christopher Monroe. An experimental atomic at the University of Maryland and fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science in Maryland, Monroe uses lasers to exploit atomic particles to study complex computa- tional problems. Monroe spoke to PNAS about how quantum computing might evolve. Christopher Monroe. PNAS: Can you describe some of the basic mechanics of quantum computing? We manipulate them without looking. In our platform of Monroe: It all comes down to the superposition atoms, lasers allow us to change the atomic state principle, in which a quantum system can exist in without revealing it. multiple states at the same time. Consider the funda- PNAS: mental unit of information: the bit. A bit is binary Since the early 1980s, and mathe- information that can be a 0 or a 1. A quantum bit, or maticians have posited that the quantum world could qubit, can be in a superposition of both 0 and 1 as long be harnessed to assist the development of advanced as it’s isolated and unobserved. A single qubit is pretty computing systems (1). What is the current state of trivial, but when you put many together, there quantum computing? become exponentially many possibilities. Monroe: Let’s consider 300 qubits. That’s an interesting It started about 25 years ago and has taken number because the number of possible configura- place in labs almost entirely at universities and tions, 2300, is more than the number of particles in government labs. In the last few years, it has evolved the universe. You could never simulate what hap- toward industry. pens with 300 qubits on a classical machine; there’s Right now there are two types of physical not enough stuff in the universe to do it. The magic systems being developed by industry. One of them with quantum is the power of exponential growth: is a trapped-ion system and the other is a super- every time you add a qubit, you double the number conducting circuit. Superconducting circuits can of configurations. have current flowing without loss, and the current But to control these qubits and exploit this massive canflowintwodirectionsatthesametime.The storage, we must be able to manipulate the quantum qubits are hooked up with real wires. That’sagood bits without betraying their information. As an analogy, quantum system. consider a coin in a black box that lies in some unknown In our lab, we use trapped atomic ions. Our atom state. Whatever that state is, let’s say I want to flip it. How qubits are connected through their motion. When doyoudoitwithoutlooking?Youjustturntheboxover. we shine a laser beam onto an atom, the laser pushes That’s analogous to what we do with qubits in the lab. theatominadirectionthatdependsonthequbit

This is a QnAs with a recently elected member of the National Academy of Sciences to accompany the member’s Inaugural Article on page 3305 in issue 13 of volume 114.

www.pnas.org/cgi/doi/10.1073/pnas.1704441114 PNAS Early Edition | 1of3 Downloaded by guest on September 26, 2021 state. This force doesn’t betray which qubit state the turns out that the most popular form of encryption is an atom is in, and the motion allows us to communicate algorithm that relies on the inability to factor numbers. with another atom qubit. We understand the basic About 20 years ago, Peter Shor, a mathematician at MIT atomic physics of how this happens. The question [Massachusetts Institute of Technology], showed that a now is how do you go from five to five million qubits, quantum computer would be able to factor numbers and this is what much of the community is looking at. exponentially faster than any known classical algorithm Between 30 and 100 qubits, quantum computing (4). Now, it’s going to be a while before this is accom- can achieve some problems that classical [comput- plished. But organizations like the National Security ing] can’t. So that’s where the field is now. Agency are interested. What I think is more interesting is the more PNAS: In your Inaugural Article, you compared two general application where all of the possible outputs five-qubit quantum computer platforms (2). One come down to only a few possible answers. A good platform was an IBM superconducting device, example of this is the classic “traveling salesman and the other was based on your laboratory’s problem.” Here, a salesman is given a map of cities trapped-ion system. What did you learn about the to visit and his job is to find the path with the least technology platforms? total distance traveled. That’s a hard problem be- cause there are exponentially many possible paths. Monroe: IBM is one of the leaders in the field of It’s applicable to logistics, military deployment, eco- superconducting quantum computers, and coinci- nomics, pattern recognition, and machine learning. dentally, both their system and our trapped-ion Quantum computers have the potential to approxi- system is composed of five qubits. Let’s think about mate the best solutions to very complex systems. I the flexibility of the hardware. Classical computing think that’s going to be where we find the use for mainly uses an operation called a NAND gate, which quantum computing, and it’s an important problem. is an expression of a logical AND operation. If you can use this tool on any pair of bits, you can do PNAS: What are some of the challenges associated anything, so we call it a universal gate. Quantum with quantum computing? computing has similar types of universal gates. The question is: Can you perform an operation between Monroe: The challenges are huge. Basically, we any pair, or only a few pairs? want to know if we can execute classical controls at a The IBM system has five qubits arranged in a high enough level to make it interesting when we shape that looks like a five on rolling die, with the have 100 qubits. You need lots of qubits, lots of connections forming a cross shape. You can do an connectivity, and great control. The systems have to operation between any pair connected by a line. Our be engineered exquisitely well. With trapped-ion system, still with five qubits, looks like a pentagon. systems, we have a vision to scale to huge sizes, not The outside of the pentagon is connected, and by putting all these ions in one place, but by using there’s an interior star-shaped connection. This photons in an optical fiber to couple the qubit pattern is fully connected. Unlike the supercon- of an atom to one of another atom. We’ve actually ducting circuit, the qubits don’thavetobenext performed gate operations between atoms about to each other. 1 meter apart (5). We’re going to perfect the indi- We found that algorithms with a lot of symmetry vidual module, and then we can think about stamp- among the qubits work fine on either system. But ing them out in scale and hooking them together algorithms in which every possible qubit is con- with optical fibers. nected can be done much easier on the trapped-ion At least with the ions, I don’t see that we need system. It has all of the links. On the IBM system, any breakthroughs. All ofthepieceshavebeen the overhead associated with moving information demonstrated in small numbers. With other plat- around results in added noise. It’s an architectural forms, especially in solid-state systems like the su- result independent of the hardware, but it speaks perconductors, they need to figure out a way to to how the hardware is connected. It’s the connec- hook-up many groups of circuits. You can’tjuststuff tivity that matters. Of course there are other issues, a bunch of superconductors in one chip. They’re all a like clock speed and gate fidelity, but when the sys- little different, and the more you make, the more tem is made very large, connectivity will become that difference matters. extremely important. PNAS: For your contributions to quantum information PNAS: Some potential applications of quantum research, you were elected a member of the National computing include increasing the precision of atomic Academy of Sciences in 2016. What does the induction clocks, unraveling unbreakable code, and guaran- mean to you? teeing privacy in storing and communicating infor- mation (3). How might quantum computing be useful Monroe: It’s a statement that my colleagues appre- in such applications? ciate the research directions I am taking. For that reason, it’s quite an honor and it means a great deal Monroe: I have to say what excites me most about the to me. This is a pat on the back from the community, field is that we don’t really know the application. But it and I appreciate it greatly.

2of3 | www.pnas.org/cgi/doi/10.1073/pnas.1704441114 Samoray Downloaded by guest on September 26, 2021 1 Feynman R (1982) Simulating physics with computers. Int J Theor Phys. Available at https://people.eecs.berkeley.edu/∼christos/ classics/Feynman.pdf. Accessed February 7, 2017. 2 Linke NM, et al. (2017) Experimental comparison of two quantum computing architectures. Proc Natl Acad Sci USA, 10.1073/ pnas.1618020114. 3 Monroe C, Schoelkopf R, Lukin M (2016) Quantum connections. Scientific American. Available at iontrap.umd.edu/wp-content/ uploads/2016/06/SciAm_May2016.pdf. Accessed on February 7, 2017. 4 Shor P (1996) Polynomial-Time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer. Available at https://arxiv.org/abs/quant-ph/9508027. Accessed on February 8, 2017. 5 Olmschenk S, et al. (2009) Quantum teleportation between distant matter qubits. Science 323:486–489.

Samoray PNAS Early Edition | 3of3 Downloaded by guest on September 26, 2021