Operation of a silicon quantum processor unit cell above one kelvin

Abstract

Quantum computers are expected to outperform conventional computers for a range of important problems, from molecular simulation to search algorithms, once they can be scaled up to large numbers of quantum bits (qubits), typically millions [1][2][3]. For most solid-state qubit technologies, e.g. those using superconducting circuits or semiconductor spins, scaling poses a significant challenge as every additional qubit increases the heat generated, while the cooling power of dilution refrigerators is severely limited at their operating temperature below 100 mK [4][5][6]. Here we demonstrate operation of a scalable silicon quantum processor unit cell, comprising two qubits confined to quantum dots (QDs) at ∼1.5 Kelvin. We achieve this by isolating the QDs from the electron reservoir, initialising and reading the qubits solely via tunnelling of electrons between the two QDs [7-9]. We coherently control the qubits using electrically-driven spin resonance (EDSR) [10,11] in isotopically enriched silicon 28 Si [12], attaining single-qubit gate fidelities of 98.6% and coherence time T * 2 = 2 µs during 'hot' operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures [13][14][15][16]. Furthermore, we show that the unit cell can be operated at magnetic fields as low as 0.1 T, corresponding to a qubit control frequency of 3.5 GHz, where the qubit energy is well below the thermal energy. The unit cell constitutes the core building block of a full-scale silicon quantum computer, and satisfies layout constraints required by error correction architectures [8,17]. Our work indicates that a spin-based quantum computer could be operated at elevated temperatures in a simple pumped 4 He system, offering orders of magnitude higher cooling power than dilution refrigerators, potentially enabling classical control electronics to be integrated with the qubit array [18,19].Electrostatically gated QDs in Si/SiGe or Si/SiO 2 heterostructures are prime candidates for spin-based quantum computing due to their long coherence times, high control fidelities, and industrial manufacturability [13,14,[20][21][22][23]. In large scale quantum processors the qubits will be arranged in either 1D chains [17] or 2D arrays [3] to enable quantum error correction schemes. For architectures relying on exchange coupling for twoqubit operation [15,16,24,25], the QDs are expected to be densely packed. Until now, two-qubit QD systems have been tunnel-coupled to a nearby charge reservoir that has typically been used for initialisation and readout using spin-to-charge conversion [26]. Here we demonstrate an isolated double QD system that requires no tunnel-coupled reservoir [7-9] to perform full two-qubit initialisation, control and readout -thus realising the elementary unit cell of a scalable quantum processor (see Figure 1h).

Tunable Coupling and Isolation of Single Electrons in Silicon Metal-Oxide-Semiconductor Quantum Dots

“…However, current two-qubit logic with single spins in SiMOS is based on controlling the exchange using the detuning only [17] or is executed at fixed exchange interaction [18].In SiMOS, a first step toward the required control to materialize architectures for large-scale quantum computation [1,[19][20][21][22][23][24] has been the demonstration of tunable coupling in a double quantum dot system operated in the many-electron regime, where gaining control is more accessible owing to the larger electron wave function [25]. More recently, exchange-controlled two-qubit operations have been shown with three-electron quantum dots [26]. However, tunnel couplings between single electrons that can be switched off and turned on for qubit operation, still remain to be shown in SiMOS.In this work we show a high degree of control over the tunnel coupling of single electrons residing in two gatedefined quantum dots in a SiMOS device.…”

Is all-electrical silicon quantum computing feasible in the long term?

“…The challenging problem of the increase of the heat load because of scaling versus the fixed amount of cooling power of dilution refrigerators is investigated in Ref. [76]. It consists of the first report of operation of two qubits confined by quantum dots at 1.5 K. In 28 Si the single-qubit gate fidelity is 98.6% and the coherence time T * 2 = 2µs.…”

A Scalable Cryo-CMOS Controller for the Wideband Frequency-Multiplexed Control of Spin Qubits and Transmons

“…In this work, the focus is on the design of a controller operating at 3 K because of the higher available cooling power. This does not restrict a future co-integration with qubits at the same temperature as the electronics since "hot" qubits operating at temperatures above 1 K have recently been demonstrated and are likely to evolve further in the next few years [20], [21].…”