Connectivity between qubits in a quantum computer may be as important as clock speed and gate fidelity when it comes time to build large-scale quantum computers. We run several quantum algorithms on two 5-qubit programmable quantum computers: our fully-connected ion trap system, and the IBM Quantum Experience superconducting system.  The performance is seen to mirror the connectivity of the systems, with the ion trap system out-performing the superconducting system on all results, but particularly when the algorithm demands more connections.  This first comparison of algorithms on different platforms shows the power of having a programmable and reconfigurable system, which will be critical to successfully adapt to new quantum algorithms as they are discovered.

• “Experimental Comparison of Two Quantum Computing Architectures,” N. M. Linke, D. Maslov, M. Roetteler, S. Debnath, C. Figgatt, K. A. Landsman, K. Wright, C. Monroe, Proc. Natl. Acad. Sci. 114, 13 (2017). [News Release]
• “Comparison of Cloud-Based Ion Trap and Superconducting Quantum Computer Architectures,” S. Blinov, B. Wu, and C. Monroe, AVS Quantum Sci. 3, 033801 (2021)

In a delicate balance between strong interactions, weak disorder, and a periodic driving force, a collection of trapped ions qubits has been made to pulsate with a period that is relatively insensitive to the drive. This is a time crystal, where the stable pulses emerge and break time symmetry – just like a freezing liquid breaks spatial symmetry and forms a spatial crystal.

 Trapped ion qubits can pulsate on their own with excellent passive stability, but this observation may guide the stabilization of complex solid-state systems, where true quantum behavior is usually masked by defects and impurities.

• Nature news article on time crystals
• News Release

In a pair of forward-looking articles, Christopher Monroe, Jungsang Kim, and Kenneth Brown layout the only known method for scaling quantum computers based on demonstrated science and technology. The proposed architecture is based on trapped atomic ion qubits, highlighting the need for “co-design” of applications to the machine and modularity.

The cover portrays a photograph of a surface trap that was fabricated by Sandia National Labs and used to trap ions at the Duke Quantum Center and IonQ, among other laboratories.

• “Scaling the Ion Trap Quantum Processor,” C. Monroe and J. Kim, Science 339, 1164 (2013).
• “Co-designing a Scalable Quantum Computer with Trapped Atomic Ions,” K. R. Brown, J. Kim, and C. Monroe, Nature Quantum Information 2, 16034 (2016).
• See also, “Quantum Connections and the Modular Quantum Computer,” C. Monroe, R. J. Schoelkopf, and M. D. Lukin, Scientific American, p. 50 (May, 2016)
• Video animation of trapped ion quantum computer concept

It’s just a five-qubit quantum computer, and anything it does is easily simulated on a laptop. However, these trapped ion qubits are fully connected, with entangling gates between all possible pairs. The qubits are dynamically “wired” from the outside with patterns of laser beams, so we can run any algorithm through software without modifying the base hardware.  While the individual gate operations are only about 98% pure, it should be possible to exceed the >99.9% purity others have demonstrated with two isolated ions. Most importantly, we have blueprints for scaling this system up to useful dimensions.

• “Demonstration of a small programmable quantum computer with atomic qubits,”   S. Debnath, N. M. Linke, C. Figgatt, K. A. Landsman, K. Wright, and C. Monroe, Nature 536, 63 (2016).
• News Release
• YouTube video animation of how it works
• Wall Street Journal Business/Technology section coverage (4 August, 2016)

Modularity is everywhere, from social networks and transportation hubs to biological function. Modular systems are always necessary for mitigating complexity, especially in computer systems where the latest processors have up to 256 modular cores.  We propose a realistic modular quantum computing design that is scalable to huge numbers of qubits, while resistant to errors. Entanglement within a module is afforded through local phonon interactions, which can be extended to other qubit modules through photonic interfaces. Experimentally, we report the first step in such an architecture by entangling remote ions in different ion traps while also showing local entanglement between ions in a single ion trap module as a demonstration of both photon and phonon buses in a single network.  The entanglement rate between modules is nearly 10/sec, orders of magnitude faster than previous results, and much faster than the observed decoherence rate, thus representing the first demonstration of a scalable quantum network in any photonic platform.  Moreover, we show how to phase-lock gates over space and time between multiple modules, a crucial prerequisite for scalability.  We finally show that even if the photons from different modules have different optical frequencies, entanglement fidelity of the linked quantum memories can be recovered, without sacrificing entanglement rate, by feed-forwarding timing information on the coincidence interference.

• “Large Scale Modular Quantum Computer Architecture with Atomic Memory and Photonic Interconnects,” Phys. Rev. A 89, 022317 (2014).      [AIP Synopsis]
• “Modular Entanglement of Atomic Qubits using Photons and Phonons,”  Nature Physics 11, 37-42 (2015).
• “Quantum gates with phase stability over space and time,” Phys. Rev. A 90, 042316 (2014).
• “Entanglement of distinguishable quantum memories,” Phys. Rev. A 90, 040302(R) (2014).
• News Release and Animation of concept