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# QnAs with Christopher Monroe

Classic computing can take credit for technology ranging from mobile phones to supercomputers. But in recent years, a budding counterpart to these conventional devices has emerged: quantum computers. Whereas classic computing sometimes fails to solve complex calculations, such as factoring hundred-digit numbers, quantum computing 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 physicist 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 computational problems. Monroe spoke to PNAS about how quantum computing might evolve.

PNAS:Can you describe some of the basic mechanics of quantum computing?

Monroe:It all comes down to the superposition principle, in which a quantum system can exist in multiple states at the same time. Consider the fundamental unit of information: the bit. A bit is binary information that can be a 0 or a 1. A quantum bit, or qubit, can be in a superposition of both 0 and 1 as long as it’s isolated and unobserved. A single qubit is pretty trivial, but when you put many qubits together, there become exponentially many possibilities.

Let’s consider 300 qubits. That’s an interesting number because the number of possible configurations, 2

^{300}, is more than the number of particles in the universe. You could never simulate what happens with 300 qubits on a classical machine; there’s not enough stuff in the universe to do it. The magic with quantum is the power of exponential growth: every time you add a qubit, you double the number of configurations.But to control these qubits and exploit this massive storage, we must be able to manipulate the quantum bits without betraying their information. As an analogy, consider a coin in a black box that lies in some unknown state. Whatever that state is, let’s say I want to flip it. How do you do it without looking? You just turn the box over. That’s analogous to what we do with qubits in the lab. We manipulate them without looking. In our platform of atoms, lasers allow us to change the atomic qubit state without revealing it.

PNAS:Since the early 1980s, physicists and mathematicians have posited that the quantum world could be harnessed to assist the development of advanced computing systems (1). What is the current state of quantum computing?

Monroe:It started about 25 years ago and has taken place in physics labs almost entirely at universities and government labs. In the last few years, it has evolved toward industry.

Right now there are two types of physical systems being developed by industry. One of them is a trapped-ion system and the other is a superconducting circuit. Superconducting circuits can have current flowing without loss, and the current can flow in two directions at the same time. The qubits are hooked up with real wires. That’s a good quantum system.

In our lab, we use trapped atomic ions. Our atom qubits are connected through their motion. When we shine a laser beam onto an atom, the laser pushes the atom in a direction that depends on the qubit state. This force doesn’t betray which qubit state the atom is in, and the motion allows us to communicate with another atom qubit. We understand the basic atomic physics of how this happens. The question now is how do you go from five to five million qubits, and this is what much of the community is looking at. Between 30 and 100 qubits, quantum computing can achieve some problems that classical [computing] can’t. So that’s where the field is now.

PNAS:In your Inaugural Article, you compared two five-qubit quantum computer platforms (2). One platform was an IBM superconducting device, and the other was based on your laboratory’s trapped-ion system. What did you learn about the technology platforms?

Monroe:IBM is one of the leaders in the field of superconducting quantum computers, and coincidentally, both their system and our trapped-ion system is composed of five qubits. Let’s think about the flexibility of the hardware. Classical computing mainly uses an operation called a NAND gate, which is an expression of a logical AND operation. If you can use this tool on any pair of bits, you can do anything, so we call it a universal gate. Quantum computing has similar types of universal gates. The question is: Can you perform an operation between any pair, or only a few pairs?

The IBM system has five qubits arranged in a shape that looks like a five on rolling die, with the connections forming a cross shape. You can do an operation between any pair connected by a line. Our system, still with five qubits, looks like a pentagon. The outside of the pentagon is connected, and there’s an interior star-shaped connection. This pattern is fully connected. Unlike the superconducting circuit, the qubits don’t have to be next to each other.

We found that algorithms with a lot of symmetry among the qubits work fine on either system. But algorithms in which every possible qubit is connected can be done much easier on the trapped-ion system. It has all of the links. On the IBM system, the overhead associated with moving information around results in added noise. It’s an architectural result independent of the hardware, but it speaks to how the hardware is connected. It’s the connectivity that matters. Of course there are other issues, like clock speed and gate fidelity, but when the system is made very large, connectivity will become extremely important.

PNAS:Some potential applications of quantum computing include increasing the precision of atomic clocks, unraveling unbreakable code, and guaranteeing privacy in storing and communicating information (3). How might quantum computing be useful in such applications?

Monroe:I have to say what excites me most about the field is that we don’t really know the application. But it turns out that the most popular form of encryption is an algorithm that relies on the inability to factor numbers. About 20 years ago, Peter Shor, a mathematician at MIT [Massachusetts Institute of Technology], showed that a quantum computer would be able to factor numbers exponentially faster than any known classical algorithm (4). Now, it’s going to be a while before this is accomplished. But organizations like the National Security Agency are interested.

What I think is more interesting is the more general application where all of the possible outputs come down to only a few possible answers. A good example of this is the classic “traveling salesman problem.” Here, a salesman is given a map of cities to visit and his job is to find the path with the least total distance traveled. That’s a hard problem because there are exponentially many possible paths. It’s applicable to logistics, military deployment, economics, pattern recognition, and machine learning. Quantum computers have the potential to approximate the best solutions to very complex systems. I think that’s going to be where we find the use for quantum computing, and it’s an important problem.

PNAS:What are some of the challenges associated with quantum computing?

Monroe:The challenges are huge. Basically, we want to know if we can execute classical controls at a high enough level to make it interesting when we have 100 qubits. You need lots of qubits, lots of connectivity, and great control. The systems have to be engineered exquisitely well. With trapped-ion systems, we have a vision to scale to huge sizes, not by putting all these ions in one place, but by using photons in an optical fiber to couple the qubit of an atom to one of another atom. We’ve actually performed gate operations between atoms about 1 meter apart (5). We’re going to perfect the individual module, and then we can think about stamping them out in scale and hooking them together with optical fibers.

At least with the ions, I don’t see that we need any breakthroughs. All of the pieces have been demonstrated in small numbers. With other platforms, especially in solid-state systems like the superconductors, they need to figure out a way to hook-up many groups of circuits. You can’t just stuff a bunch of superconductors in one chip. They’re all a little different, and the more you make, the more that difference matters.

PNAS:For your contributions to quantum information research, you were elected a member of the National Academy of Sciences in 2016. What does the induction mean to you?

Monroe:It’s a statement that my colleagues appreciate the research directions I am taking. For that reason, it’s quite an honor and it means a great deal to me. This is a pat on the back from the community, and I appreciate it greatly.

## Footnotes

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.

## References

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- Linke NM, et al.

- ↵.
- Monroe C,
- Schoelkopf R,
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- ↵.
- Shor P

- ↵.
- Olmschenk S, et al.

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