A group of physicists has proposed a modular approach for expanding quantum processors, offering a flexible method to connect qubits over long distances for collaborative quantum operations. This interconnected functionality, known as ‘entanglement,’ is what gives quantum computing its superior capabilities over traditional computer systems.
Quantum computers process information using quantum bits, or qubits, which can represent data as a mix of two states simultaneously – unlike the binary bits in classical computing, which are either one or zero. This unique characteristic allows qubits to perform specific calculations much faster than even the most powerful supercomputers available today.
For quantum computers to achieve their full potential, they require vast numbers of qubits. However, challenges arise as systems scale up due to the complex electronics needed to manage even a small number of qubits, making it difficult to expand the circuitry.
In a recent theoretical study led by Vanita Srinivasa, a professor at the University of Rhode Island, the team has envisioned a modular system for scaling quantum processors. This system proposes a flexible method for linking qubits over significant distances, enabling collaboration on quantum operations. The ability to perform entangled operations between connected qubits is foundational to the increased power of quantum computing compared to current technology. The findings from their research, co-authored by Jacob M. Taylor from the University of Maryland and National Institute of Standards and Technology, along with Jason R. Petta from UCLA, have been published in the journal PRX Quantum.
“Every qubit operates at its specific frequency. To leverage the unique advantages of a quantum computer, it’s essential to control each qubit distinctly at its frequency and also to connect pairs of qubits by aligning their frequencies,” explained Srinivasa, who directs URI’s Quantum Information Science program and serves as an assistant physics professor. “As we scale up the number of qubits, managing both these operations at once for each qubit becomes incredibly challenging. Our research illustrates how using oscillating voltages can effectively create additional frequencies for each qubit, linking them without needing to match all of their original frequencies. This enables the connection of qubits while each retains its unique frequency for individual control.”
The prospect of utilizing semiconductors for developing quantum processors appears promising for increasing the number of qubits available. Current semiconductor technology supports the production of chips packed with billions of tiny transistors, which can be adapted to create compact qubits. Furthermore, storing qubits in a property of electrons and semiconductor particles known as spin offers improved protection against the quantum information degradation commonly found in all quantum computing systems.
However, merely adding more spin qubits and their necessary control systems to a single array to scale up a quantum processor is quite complex. The theoretical work by Srinivasa and her team offers a structured approach that discusses various methods to entangle spin qubits over long distances, providing flexibility in frequency matching.
This newfound flexibility presents a pathway toward semiconductor-based modular quantum processing, serving as an alternative method to create numerous qubit systems by linking small arrays of qubit modules—already feasible today—through strong, long-range entangling connections.
Srinivasa likens this scaling method to constructing a larger system with standard-sized LEGO blocks, where each block represents a module connected by longer pieces that can endure while keeping the modules attached until external influences sever their connection. “If we can establish fast and reliable long-distance links between qubits, this modular strategy allows for scaling while also providing additional space for the control circuitry of the spin qubits.” It’s important to note that fully modular, semiconductor-based quantum processors have not yet been demonstrated.
While various types of qubits and interaction methods exist, the researchers focused on quantum dot-based spin qubits that interact via microwave photons within a superconducting cavity. Quantum dots are small, atom-like structures that confine electrons and other particles used as qubits, allowing for individual control through applied voltages. Conversely, superconducting cavities are larger constructs that confine photons, their size determined by microwave wavelength.
Recent experiments have showcased long-distance connections between quantum dot spin qubits via microwave cavity photons. (The pioneering demonstration involving two spin qubits in silicon was conducted by co-author Jason Petta’s experimental team.)
Nonetheless, a challenge remains in tuning all qubit and photon frequencies to achieve resonance for energy exchange—an essential condition for linking even at a two-qubit level, the paper highlights. To overcome this obstacle, the researchers propose a highly adjustable method to connect qubits using microwave photons without the need for all original qubit and cavity frequencies to resonate simultaneously.
In their work, the authors offer detailed guidelines on customized long-distance entangling connections that allow flexibility by providing various frequencies for each qubit to interact with microwave cavity photons of a specific frequency, akin to having multiple keys that fit a single lock, according to Srinivasa.
By applying an oscillating voltage to each spin qubit, it creates a back-and-forth motion within the quantum dots, generating additional frequencies—one higher and one lower than the base qubit frequency. If this oscillation occurs rapidly enough, it results in three tuning methods for each qubit to align with microwave cavity photons, leading to nine different scenarios for linking two qubits.
This flexibility in resonance significantly eases the integration of new qubits into a system, as they don’t all need to be matched to the same frequency. Additionally, the multiple ways for connecting two qubits enable various types of entangling operations to be chosen simply by adjusting the oscillating voltages, without altering the structures of the quantum dots or the cavity photons.
The diversity in entangling connections broadens the range of fundamental quantum operations available for computations. Ultimately, the researchers reveal that their proposed sideband frequency-based entangling technique is less prone to photon leakage from the cavity than previous methods, facilitating more stable, long-distance links between spin qubits.
“The combination of flexible frequency matching, versatile entangling operation types, and reduced sensitivity to photon leakage from the cavity makes our sideband frequency-based approach a highly promising strategy for developing modular quantum processors with semiconductor qubits,” Srinivasa expressed. “I look forward to the next phase, which is to implement these concepts in actual quantum devices in the lab and discover what further steps are needed to enable this approach practically.”
