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Quantum circuits are akin to the circuits or programs in classical computing. Given the limited size of existing quantum hardware, it is particularly important that quantum circuits be short and efficient. A close analogy can be drawn with the early days of programmable classical computers when hardware was small and expensive and an efficient program had to be crafted carefully to reduce its length and memory footprint. For instance, variable reuse was commonly practiced, but is no longer necessary nowadays due to cheap and abundant memory.
Dmitri Maslov Chief Software Architect, IBM Quantum
IBM’s Dmitri Maslov laid out early foundational work to make quantum circuits efficient. His contributions include the design of efficient circuits for a variety of quantum subroutines (including single-qubit fault-tolerant unitaries, multiply-controlled gates, Approximate QFT, Select-V) and classes of quantum transformations (reversible circuits, Clifford circuits), frameworks for quantum circuit optimization (templates, phase polynomials), quantum circuit placement, technology optimization, and resource tradeoffs. The circuits he designed have been used across multiple state-of-the-art experimental demonstrations of quantum algorithms.
For example, IBM Quantum Experience platform users may recognize the multiply-controlled Toffoli gate implementation, a core transformation in Grover’s search algorithm — thanks to Maslov.
Owing to this pioneering research, IEEE has named Maslov a 2021 Fellow for his “contributions to quantum circuit synthesis and optimization, and compiling for quantum computers.” In the following Q&A, he explains quantum circuits, their compilers, and what it means to be named an IEEE Fellow.
How does “quantum circuit synthesis and optimization, and compiling” improve what a quantum computer is capable of?
Dmitri Maslov: Compilation research boils down to studying a range of problems related to executing quantum algorithms on quantum hardware efficiently. This includes quantum algorithm design, quantum circuit synthesis, quantum circuit optimization (at all levels of abstraction), quantum circuit placement, layout, scheduling, resource tradeoffs, and quantum architectures, often united under the roof of quantum compiling. Efficient compilation is what gives short, practical, and ready-to-use quantum programs that solve the problems of interest on quantum hardware.
The work for which I was named an IEEE Fellow lies at the foundation of efficient quantum computations. Every quantum computation needs to be expressed as a schedule of instructions (a circuit) that a physical quantum computing hardware can directly execute. Those schedules need to be as short as possible for today’s quantum computations to improve upon their classical counterparts’ capabilities. Given a problem that can be solved on a quantum computer, my goal is to produce a schedule that uses as few quantum resources as possible solving it — say, one relying on M quantum gates (which can also be circuit width, depth, or other parameters of value). Once a solution with M gates is found, I literally try to find one with M-1 gates, and repeat until no further improvement can be found. I am ultimately satisfied when an exact optimal circuit is found and proved to be optimal—a feat that does not happen very often, since finding optimal circuits and proving their optimality is very difficult, but highly desirable due to guaranteeing the best performance.
A quantum computer would, naturally, need to be equipped with an efficient compiler. An inefficient compiler may render a quantum computer completely useless by the simple virtue of synthesizing longer-than-classical quantum schedules.
“The topic that interests me most right now is space-time tradeoffs in quantum computing and how they compare to those in classical computations.” — Dr. Dmitri Maslov, IBM
What is a quantum compiler?
DM: Much like a classical computer, a quantum computer will consist of hardware and software. One of the most important aspects of the software is the compiler — it is responsible for determining the amount of quantum resources needed to solve a certain problem, and providing an executable sequence of physical operations.
The fewer quantum resources required, the faster an algorithm can be executed and the cleaner (higher fidelity) the answer is. For quantum computations of limited size (e.g., those with limited number of qubits or limited number of gates that can be applied), a good compiler is something that may enable a computation to be performed when a bad compiler may ask for resources in excess of those available.
It may turn out that a particularly efficient circuit calls for the use of a certain kind of instructions orchestrated over a particular type of architecture. This, if shown to be far better than alternatives, calls for designing quantum hardware optimized for executing such a circuit (akin to an ASIC). This illustrates that quantum compilers carry the potential to alter the quantum computing roadmaps or bring about the milestones much sooner than anticipated. This has happened in classical computing a number of times, so it should not surprise.
What significance does being named an IEEE Fellow have for you, and for the field of quantum computing?
DM: IEEE had a crucial influence on classical computers. It is safe to say that without the work done by the members of IEEE, classical computers as we know them today would not exist. Likewise, if quantum computing is to become a wide-spread mature technology comparable to classical computers, IEEE will need to play a major role in fostering the quantum computing field, facilitating the exchange of ideas, assisting with preparing qualified personnel, working on the standards, etc.
“Congratulations to Dmitri, my IBM Quantum colleague. His appointment as Fellow further highlights the importance of circuit design and compilation as part of the progress in quantum computing. As part of IEEE, I know first-hand how collaborative and engaged its members are to push for technological progress.” — Dr. Matthias Steffen, IBM Fellow, IEEE Senior Member
Being among the first few Fellows recognized by the IEEE in the quantum computing field is a great honor, as it puts me at the source of what I believe is bound to become a significant part of both IEEE as an organization, and quantum computing as an industry. I hope to see a formation of a Quantum Computing Society within IEEE not too many years down the road and believe it may become one of the larger societies at IEEE. Indeed, it would include a “quantum” version of many of the existing classical IEEE computing societies.
I furthermore believe the significance of the IEEE Fellow award in the field of quantum computing is in recognizing a future in which both IEEE and the Quantum Computing Community will work closely with each other.
“The number of IEEE Fellows elevated in a year is no more than one-tenth of one percent of the total IEEE voting membership.” — IEEE
Maslov is IBM Quantum’s Chief Software Architect, responsible for the development of IBM’s quantum compilers, which includes a wide range of problems needed to be solved to efficiently execute algorithms of interest on today’s available quantum hardware.
It’s possible to run circuits on quantum computers, today. Try step-by-step tutorials, or code your own programs on the IBM Quantum Experience, using Qiskit.
Quantum starts here