A team of physicists, led by University of Rhode Island professor Vanita Srinivasa, has made a breakthrough in advancing modular quantum information processing. Quantum computers rely on encoding and processing information in the form of quantum bits, or qubits, which can exist in multiple states simultaneously, allowing for exponentially faster calculations than classical computers. However, scaling up to millions of qubits is a major hurdle due to the complex electronics required to control them.
Srinivasa’s team has proposed a modular system that enables linking qubits over long distances using oscillating voltages to generate extra frequencies, allowing for individual control and entangling operations between linked qubits. This approach, published in PRX Quantum, uses semiconductor technology and could lead to the development of compact, robust quantum processors. The research involved collaboration with Jacob M. Taylor of the University of Maryland and the National Institute of Standards and Technology, and Jason R. Petta of the University of California, Los Angeles.
Modular Quantum Information Processing: A Promising Approach for Scaling Qubits
The operation of a quantum computer relies on encoding and processing information in the form of quantum bits, or qubits. Unlike binary bits used in classical computers, qubits can exist in a combination of zero and one simultaneously, allowing them to perform certain calculations exponentially faster than today’s largest supercomputers. However, scaling up a quantum processor by simply adding more and more spin qubits and their associated control circuitry to a single array of qubits is very challenging in practice.
The Challenge of Scaling Qubits
To reach their full potential, quantum computers need millions of qubits. However, as the number of qubits increases, so does the complexity of the control circuitry required to manipulate them. This leads to a scalability problem, where the added complexity makes it difficult to maintain the fragile quantum states necessary for computation.
A Modular Approach to Scaling Qubits
Researchers have proposed a modular approach to scaling qubits, which involves building small arrays of qubits, or modules, and connecting them with robust, long-range entangling links. This approach is like building a larger system using fixed-size LEGO blocks, where each block is an individual module, and connecting them using longer pieces that are strong enough to maintain the connection between the blocks for a sufficient time.
Tuning Qubits and Photons for Entanglement
One of the key challenges in establishing entangling links between qubits is tuning all of the qubit and photon frequencies so they precisely match and can exchange energy. Researchers have demonstrated long-distance links between quantum dot spin qubits using microwave cavity photons, but tuning all of the frequencies has been a problem even at just the two-qubit level.
A Highly Tunable Approach to Linking Qubits
To address this problem, researchers have proposed a highly tunable approach for linking qubits using microwave photons that does not rely on simultaneous resonance among all original qubit and cavity frequencies. This approach involves generating extra frequencies by applying an oscillating voltage to each spin qubit, which moves the spins back and forth in the quantum dots.
Flexibility in Resonance Conditions
The addition of sideband frequencies results in three ways to tune each qubit into resonance with microwave cavity photons, and consequently nine different conditions under which two qubits can be linked. This flexibility in resonance conditions would make it much easier to add qubits to a system because they do not need to all be tuned to the same frequency.
Versatility in Entangling Operations
The versatility in types of entangling links enables an expanded set of elementary quantum operations with which to perform calculations. Furthermore, the researchers show that their proposed entangling method is less sensitive to leakage of photons out of the cavity than previous approaches, allowing for more robust long-distance links between spin qubits.
A Promising Approach for Realizing Modular Quantum Processors
The combination of flexibility in matching frequencies, versatility in tailoring the types of quantum entangling operations between qubits, and reduced sensitivity to cavity photon leakage renders this approach promising for realizing a modular quantum processor using semiconductor qubits. The next step is to apply these ideas to real quantum devices in the laboratory and find out what needs to be done to make the approach work in practice.
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