Duke University and IonQ researchers today reported developing a powerful new approach to executing multi-qubit entanglement gates that “can dramatically simplify quantum circuit structures, speed up their execution, and extend the power of quantum computer systems facing decoherence.”

IonQ contends the technique, at least at present, can only be executed on its quantum platform. “[N]o other available quantum computing architectures – not even other ion-based quantum computers – are able to utilize this new family of N-qubit gates,” according to Christopher Monroe, co-founder and chief scientist at IonQ.

Effective implementation of multi-entangled-qubits gates has long proven tricky. Most systems can do two-qubit gates and even those aren’t easy. Entanglement, of course, is one of the two key attributes that give quantum computing its powerful advantage over classical computing. Broadly entanglement connects qubits in such a way as that when the entangled state of one qubit is altered the others all know it instantly and also change their state (true parallelism). This is the spooky-action-at-a-distance phenomenon no one understands but which experiment consistently confirms. (Superposition is the other distinguishing quantum computing attribute.)

Cleverly developed algorithms, Shor’s algorithm is one example, can take advantage of the entanglement and superstition characteristics of quantum computers to perform select computations more quickly and tackle other computations that are impractical on classical computers.

The IonQ/Duke advance is significant and their paper (N-body interactions between trapped ion qubits via spin-dependent squeezing) was posted today on arXiv.org. The paper’s introduction summarizes the challenge nicely:

“The central ingredient in a quantum computer is the controllable quantum entanglement of its degrees of freedom, allowing the system to evolve over an exponentially large state space that can encode certain problems that are otherwise intractable. The qubit and gate model of a quantum computer employs a universal set of operations, such as single-qubit rotations and two-qubit controlled-NOT gates.

“While such few-qubit interactions are sufficient for general computation, and can be used to construct many-body entangled states, many-qubit interactions can dramatically simplify quantum circuit structures, speed up their execution, and extend the power of quantum computer systems facing decoherence. For example, direct N-qubit operations such as the N-qubit Toffoli gate are expected to find native use in quantum adders and multipliers, Grover searches, error-correction encoding, variational quantum algorithms for calculating electronic properties of molecules and materials, and simulations of nuclear structure and lattice gauge theories.”

As explained by the researchers, the central idea behind trapped ion quantum gates is the coupling between spins and motion (phonons) through spin-dependent forces. “Owing to the Coulomb interaction between the trapped ions, their motion around equilibrium can be expressed by collective normal modes of harmonic oscillation. We focus on the coupling through a single phonon mode through a near-resonance driving force, although generalization to multiple modes is straightforward. (Shown below is a figure from the paper with an overview – it’s a bit lengthy but worth scanning; click to enlarge.)

The new quantum gate, report the researchers, is a novel way to operate on many connected qubits at once and leverages the multi-qubit communication bus available only on IonQ and Duke Quantum Center (DQC) computers. “The new gate family includes the N-qubit Toffoli gate, which flips a select qubit if and only if all the other qubits are in a particular state. Unlike standard two-qubit quantum computing gates, the N-qubit Toffoli gate acts on many qubits at once, leading to more efficient operations. The gate appears naturally in many common quantum algorithms,” said IonQ.

There are many underlying qubit technologies at various stages of development. Trapped ion systems have strength coherence but are sometimes criticized as being difficult to scale.

IonQ says its quantum computers uniquely feature full connectivity and a wide communication bus that allows all qubits to talk to each other simultaneously. Noteworthy, IonQ was the first one of the first pure-play quantum companies to go public which it did in last fall using a SPAC approach. Rigetti, which is betting on a different underlying qubit technology – semiconductor-based superconducting qubits – has announced plans to go public via SPAC and this week D-Wave announced similar plans.

Pressure on the new public and soon-to-be-public quantum computing companies will no doubt escalate to show results. IBM has said 2023 is the year it will bring broader quantum advantage to market.

In today’s announcement, IonQ said, “This discovery follows a series of announcements around IonQ’s research efforts and preparations for scale. In December, IonQ announced that it plans to use barium ions as qubits in its systems, bringing about a wave of advantages it believes will enable advanced quantum computing architectures. Last year, the team also debuted the industry first reconfigurable multicore quantum architecture and evaporated glass trap technology, both of which are expected to contribute to scaling the number of qubits in IonQ’s quantum computers.”

The company says this new work may lead to significant efficiency gains in solving fundamental quantum algorithms, such as Grover’s search algorithm, variational quantum eigensolvers (VQEs), and arithmetic operations like addition and multiplication. “These use cases are ubiquitous across quantum computing applications, and are core to IonQ’s work in quantum chemistry, quantum finance, and quantum machine learning. They are also key components of commonly accepted industry benchmarks for quantum computers,” reported IonQ

As always, it best to read the latest paper directly for a fuller understanding.

Link to paper, https://arxiv.org/abs/2202.04230

Link to announcement, https://www.hpcwire.com/off-the-wire/duke-university-and-ionq-develop-new-quantum-computing-gate/

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