Proximal micromagnets increases the speed of quantum manipulation in silicon without adding measurable noise

Researchers report the fabrication and operation of a qubit in a double-quantum dot in a silicon/silicon–germanium (Si/SiGe) heterostructure in which the qubit states are singlet and triplet states of two electrons. The significant advance over previous work is that a proximal micromagnet is used to create a large local magnetic field difference between the two sides of the quantum dot, which increases the manipulability significantly without introducing measurable noise.

The integrated micromagnet provides a promising path toward fast manipulation in materials with small concentrations of nuclear spins, including both natural silicon (Si) and isotopically enriched 28S.

“The next steps in our research are to increase both the magnitude of the field difference between the quantum dots, and the number of qubits by increasing the number of quantum dots,” Coppersmith tells “Both steps are being implemented in new devices that have been designed and are currently being fabricated. We’re also working on other qubit implementations in silicon quantum dots all of which use electrical initialization, manipulation and readout, and therefore have the potential advantages of integrability and scalability.” Moreover, Eriksson points out that being able to control local magnetic fields in a nanoelectronic device could be very useful for spintronics.

Arxiv – Two-axis control of a singlet-triplet qubit with an integrated micromagnet

Researchers demonstrate coherent quantum control around two axes of the Bloch sphere of a singlet-triplet qubit in a silicon quantum dot. The relatively large magnetic field difference between the dots required to achieve two-axis control is implemented using a proximal micromagnet. By measuring the inhomogeneous spin coherence time T∗2 at many different values of the exchange coupling J and two different ∆B fields, we provide evidence that the dominant limits on T∗2 arise from charge noise and from coupling to nuclear spins.

Nature – Quantum control and process tomography of a semiconductor quantum dot hybrid qubit

The similarities between gated quantum dots and the transistors in modern microelectronics—in fabrication methods, physical structure and voltage scales for manipulation—have led to great interest in the development of quantum bits (qubits) in semiconductor quantum dots. Although quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins or the addition of a third quantum dot. Here we demonstrate a qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. We demonstrate full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions, which is more than an order of magnitude faster than any other double-dot qubit. The speed arises from the qubit’s charge-like characteristics, and its spin-like features result in resistance to decoherence over a wide range of gate voltages. We achieve full process tomography in our electrically controlled semiconductor quantum dot qubit, extracting high fidelities of 85 per cent for X rotations (transitions between qubit states) and 94 per cent for Z rotations (phase accumulation between qubit states).

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