Werner Heisenberg’s Matrix Mechanics, Erwin Schrodinger's Wave Mechanics, proved to be equivalent by Paul Dirac, dubbed Quantum Mechanics by Max Born, turned into an Ant-Man film by Jeff Loveness. All these men have laid out the rules for how matter behaves at the smallest scale, which the modern applied physicist is working to leverage for a quantum computer.
The race for a quantum computer is not one happening away from the public eye, with recent explosions in the stocks of companies drowning in a sea of advantages they have “technically” achieved. Headlines about quantum bits that are capable of beating classical computers in data organization, ignoring that this trick only works with data that is completely randomly generated. This article won’t focus on this race however, we will instead shine light on the quiet quest for a quantum network. If quantum computers parallel the chips in our technology that do trillions of calculations a second, then quantum networks mirror the internet that connects them all. But ultimately, to talk about quantum networks we first need to talk about the quantum computers they connect.
Quantum computers are built off the basic idea of identifying a physical system with two distinguishable quantum states, getting together a lot of these systems, and being able to manipulate each one and their interactions with each other very precisely. Think of Google and IBM’s Superconducting qubits, which are built out of superconducting electrical circuits that simulate a standard parabolic potential for which a set of evenly spaced quantum states have been analytically found, but with a perturbation that shifts the values of these states such that the gaps between them are distinguishable. Other approaches do similar things with light and its polarization states, or the discrete energy levels of atoms and molecules.
These systems are all extremely delicate, and they are designed to be as stable and silent as possible. That’s why we introduce multiple layers of shielding and cool these systems down to the order of sub-kelvin. We want to minimize outside interference so that the only interactions we are probing are those between qubits, which we dub their “entanglement”. Now, consider two qubits, one given to Alice and the other to Bob. Normally, you would explore the interesting things Alice and Bob could do with these two qubits. But the question of a quantum network is not what they can do with them, but how they get them in the first place. With so many carefully engineered layers of shields, noise reduction techniques, and extreme cooling, it wouldn't be practical to just “give” each of them a superconducting circuit or an atom, or a photon, after coupling them together. This is the issue of a quantum network, with such a big focus on keeping qubits still, quiet, and stable, how are we supposed to start moving them?
The defect qubit begins with the clean lattice pattern of a host material. This pattern is disrupted via the introduction of a local deformity, which creates a localized electronic state within that deformity with well-defined energy levels. The defect is coupled to optical fiber, which channels incoming light sent in by the experimenter and captures the emissions of the defect. The information of the quantum state is thus not limited to the defect itself; it is mobile through the coupled fiber [1]. This property addresses the key challenge of the quantum network problem, providing an alternative to transporting fragile circuitry or atoms with the stationary defect and mobile photon interface. The entanglement of two defects or generalized qubits can be mediated through a mutual entanglement with the low-loss fiber between them. Being able to use modern fiber like this would be revolutionary, when you consider experiments have shown light can circle Manhattan for weeks in room temperature fiber without a loss of coherence [2].
The defect archetype covers a range of research being done on different types of defects, like the work being done on Silicon T-Centers at Columbia University by the optics group of Zhang Lab. The advantage of these T-centers is that the energy gap between their primary energy levels is excited by light in the O-band, which is the same frequency that the internet operates on. This system could piggyback off the fiber already laid across our continents and under our oceans, which minimizes the new infrastructure needed to build a quantum network [3]. Another type of defect is the nitrogen-vacancy (NV), used by groups like Dr. de Leon’s to build quantum devices [4]. While it doesn’t share the same advantage of operating in the O-band, and would require new fiber to be laid, these defects can operate at significantly higher temperatures. Where other defects would need to be in the 1-4 Kelvin range, NV defects can operate at around 7 K. In a race where outside noise is the biggest hurdle to overcome, an inherent resistance to noise is an enormous advantage.
We should consider the defect as a living oxymoron. A structure that breaks the beautiful symmetry of its host, yet itself is beautiful as it creates a functional bridge for quantum information. An underdog in quantum computing with many complications, so much so that it lacks any notoriety amongst the public, yet one of the most viable proposals for a quantum network. In a sense, defects and the overblown headlines of quantum computing aren’t all that different. A good physicist can add an asterisk to the “advantages” claimed by modern quantum computers or defects, but a great one can recognize that they are both still a marvel of physics and engineering as they are, and push the limits of their potential every news cycle.
Works Cited
[1] Weber, J. R., W. F. Koehl, J. B. Varley, et al. “Quantum Computing with Defects.” Proceedings of the National Academy of Sciences 107, no. 19 (2010): 8513–18. https://doi.org/10.1073/pnas.1003052107.
[2] Craddock, Alexander N., Anne Lazenby, Gabriel Bello Portmann, Rourke Sekelsky, Mael Flament, and Mehdi Namazi. “Automated Distribution of Polarization-Entangled Photons Using Deployed New York City Fibers.” PRX Quantum 5, no. 3 (2024): 030330. https://doi.org/10.1103/PRXQuantum.5.030330.
[3] Bergeron, L., C. Chartrand, A. T. K. Kurkjian, et al. “Silicon-Integrated Telecommunications Photon-Spin Interface.” PRX Quantum 1, no. 2 (2020): 020301. https://doi.org/10.1103/PRXQuantum.1.020301.