Novel method helps researchers scale up quantum devices easier, Now the MIT team has developed a method to “recruit” neighboring quantum bits made of nanoscale defects in diamond, so that instead of causing disruptions they help carry out quantum operations.

Quantum devices perform operations using quantum bits, called “qubits,” that can represent the two states corresponding to classic binary bits — a 0 or 1 — or a “quantum superposition” of both states simultaneously.

The unique superposition state can enable quantum computers to solve problems that are practically impossible for classical computers, potentially spurring breakthroughs in biosensing, neuroimaging, machine learning, and other applications.

In experiments, the team generated and detected quantum coherence among three electronic spins scaling up the size of the quantum system from a single qubit (the NV center) to three qubits (adding two nearby spin defects). Novel method helps researchers scale up quantum devices easier.

NV centers occur where carbon atoms in two adjacent places in a diamond’s lattice structure are missing one atom is replaced by a nitrogen atom, and the other space is an empty vacancy.

The NV center essentially functions as an atom, with a nucleus and surrounding electrons that are extremely sensitive to tiny variations in surrounding electrical, magnetic, and optical fields.

Sweeping microwaves across the center, for instance, makes it change, and thus control, the spin states of the nucleus and electrons.

In their work, the researchers identified, located, and controlled two electron-nuclear spin defects near an NV center.

They first sent microwave pulses at specific frequencies to control the NV center. Simultaneously, they pulse another microwave that probes the surrounding environment for other spins.

The spectrum dipped in several spots when the probing pulse interacted with nearby electron-nuclear spins, indicating their presence. The researchers then swept a magnetic field across the area at different orientations.

For each orientation, the defect would “spin” at different energies, causing different dips in the spectrum. Basically, this allowed them to measure each defect’s spin in relation to each magnetic orientation. Novel method helps researchers scale up quantum devices easier.

They then plugged the energy measurements into a model equation with unknown parameters. This equation is used to describe the quantum interactions of an electron-nuclear spin defect under a magnetic field. Then, they could solve the equation to successfully characterize each defect.

The researchers verified the three-spin coherence by measuring a major spike in the resonance spectrum.

The measurement of the spike recorded was essentially the sum of the frequencies of the three qubits. If the three qubits for instance had little or no entanglement, there would have been four separate spikes of smaller height.

We come into a black box [environment with each NV center]. But when we probe the NV environment, we start seeing dips and wonder which types of spins give us those dips.

Once we [figure out] the spin of the unknown defects, and their interactions with the NV center, we can start controlling their coherence, Researcher says. Then, we have full universal control of our quantum system. Novel method helps researchers scale up quantum devices easier.

Next, the researchers hope to better understand other environmental noise surrounding qubits.

That will help them develop more robust error-correcting codes for quantum circuits.

Furthermore, because on average the process of NV center creation in diamond creates numerous other spin defects, the researchers say they could potentially scale up the system to control even more qubits.

Researcher said. But if we can start finding NV centers with more resonance spikes, you can imagine starting to control larger and larger quantum systems.