Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot

Abstract

Nanofabricated quantum bits permit large-scale integration but usually suffer from short coherence times due to interactions with their solid-state environment 1 . The outstanding challenge is to engineer the environment so that it minimally affects the qubit, but still allows qubit control and scalability. Here we demonstrate a long-lived single-electron spin qubit in a Si/SiGe quantum dot with all-electrical two-axis control. The spin is driven by resonant microwave electric fields in a transverse magnetic field gradient from a local micromagnet 2,3 , and the spin state is read out in single-shot mode 4 . Electron spin resonance occurs at two closely spaced frequencies, which we attribute to two valley states. Thanks to the weak hyperfine coupling in silicon, Ramsey and Hahn echo decay timescales of s µ 1 and s µ 40 , respectively, are observed. This is almost two orders of magnitude longer than the intrinsic timescales in III-V quantum dots 5,6 , while gate operation times are comparable to those achieved in GaAs 3,7,8 . This places the single-qubit rotations in the fault-tolerant regime 9 and strongly raises the prospects of quantum information processing based on quantum dots.The proposal by Loss and DiVincenzo 10 to define quantum bits by the state of a single electron spin in a gate-defined semiconductor quantum dot has guided research for the past 15 years 7 . Most progress was made in well-controlled III-V quantum dots, where spin manipulation with two 6,11 , three 12 and four 13 dots has been realized, but gate fidelities and spin coherence times are limited by the unavoidable interaction with the fluctuating nuclear spins in the host substrate 5,6 . While the randomness of the nuclear spin bath could be mitigated to some extent by feedback techniques 14 , eliminating the nuclear spins by using group IV host materials offers the potential for extremely long electron spin coherence times that exceed one second in P impurities in bulk 28 Si 15,16 .Much effort has been made to develop stable spin qubits in quantum dots defined in carbon nanotubes 17,18 , Ge/Si core/shell nanowires 19 , Si MOSFETs 20,21 and Si/SiGe 2D electron gases 16,22,23 . However, coherent control in these group IV quantum dots is so far limited to a Si/SiGe singlet-triplet qubit with only single-axis control 23 and a carbon nanotube single-electron spin qubit, with a Hahn echo decay time of 65 ns 17 .Our device is based on an undoped Si/SiGe heterostructure with two layers of electrostatic gates (Fig. 1a). Compared to conventional, doped heterostructures, this technology strongly improves charge stability 23 . First, accumulation gates ( mV 150 a + V ) are used to induce a twodimensional electron gas (2DEG) in a 12 nm wide Si quantum well 37 nm below the surface. Second, a set of depletion gates, labelled 1-12 in Fig. 1a, is used to form a single or double quantum dot in the 2DEG, flanked by a quantum point contact and another dot intended as charge sensors. Two μm 1 -wide, 200 nm-thick, and μm 5 . 1 -long Co magnets are placed...

Computer-automated tuning of semiconductor double quantum dots into the single-electron regime

“…Recently, several experiments have shown coherent manipulation of such spins for the purpose of spin-based quantum computation. [4][5][6][7][8][9][10] Enabled by advances in device technology, the number of quantum dots that can be accessed is quickly increasing from very few to many. 11,12 Up to date, all these quantum dots have been tuned by "hand."…”

Controlled-NOT gate sequences for mixed spin qubit architectures in a noisy environment

“…Universal qubit control of the spin qubit is achieved adopting magnetic fields pulses. From a practical point of view the approaches implemented are based on: electron spin resonance (ESR) techniques using local AC magnetic fields [7], the application of a global magnetic field that allow local manipulation or through all-electrical manipulation via AC electric fields in a magnetic field gradient [5]. Although the manipulation requires sophisticated techniques involving magnetic fields or gradient of them, such spin qubit has a great advantage represented by long coherence times of the order of milliseconds [4].…”

“…The choice of the most suitable qubit and the consequent implementation of universal one and two-qubit gates are the main challenges for the realization of efficient quantum computers. A spin based quantum computer can be created exploiting different architectures that have been realized with great reliability in III-V compounds such as GaAs [1,2,3] as well as in Si [4,5,6], besides theoretically investigated. A significant improvement in coherence times comes from the realization of QDs in Si, which can be isotopically purified.…”

Dephasing due to Nuclear Spins in Large-Amplitude Electric Dipole Spin Resonance

“…These two aspects are unavoidable for EDSR based on III-V semiconductors, in contrast to ESR or spin manipulation based on group-IV materials [24,25]. Nuclear fluctuations, on the other hand, do not represent a fun-Supplemental material for "Dephasing due to nuclear spins in large-amplitude electric dipole spin resonance"…”