Quantum Computing with an Electron Spin Ensemble Proposed to Make Hundreds of Qubits

The physical setup of the quantum computer consists of a superconducting transmission line cavity coupled to an ensemble of electron spins and a transmon Cooper pair box. The cavity dimensions allow 100 billion electron spins to be coupled to the cavity mode, which could be used to make hundreds of physical qubits. Image copyright: J.H. Wesenberg, et al.

Physics Review Letter A: UK and Denmark researchers propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper pair box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations.

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Scientists have recently proposed a quantum computing scheme that uses an ensemble of about 100 billion electron spins. They show that hundreds of physical qubits can be made from these collective electron spin excitations.

The system can also perform qubit encoding and provide one- and two-bit gates for quantum computing. In the setup, the electron spins are coupled to a superconducting transmission line cavity. In turn, this cavity is coupled to a transmon Cooper pair box that carries out the gate operations.

“A single electron spin only interacts very weakly with its environment: this makes it a good quantum memory, except that it is very hard to initialize or read out,” Wesenberg explained to PhysOrg.com. “In the ensemble register we make use of the fact that the collective interaction between an ensemble of billions of spins and a microwave cavity is greatly enhanced by the so-called superradiant effect. This makes it possible to transfer a microwave photon (carrying a qubit), from the cavity to the spin ensemble in a few tens of nanoseconds compared to a significant fraction of a second for a single spin. Once the photon has been transferred to the ensemble, it lives as an delocalized excitation.

“The state of the system is a quantum superposition of each spin being excited, that is, flipped relative to the very strong magnetic field that has been applied to the system. There is an infinite number of ways in which a single excitation can be superpositioned in this way, and these can be described in terms of spin waves. By applying a magnetic gradient pulse, we can transfer an excitation that lives as one kind of spin wave to another kind of spin wave.”

Depending on the materials used, the system could achieve spin coherence times of up to tens of milliseconds, which could be used to build a solid-state device.

“The immediate plan is to demonstrate experimentally that this works,” Wesenberg said. “First in the semi-classical setting (which is essentially electron spin resonance spectroscopy), and later in the quantum regime. Experiments to this end are underway at Yale and Oxford.”

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