Measurement-based entanglement of single-dopant As spin qubits

基于测量的单掺杂剂 As 自旋量子位的纠缠

• 批准号: 2723776
• 金额: --
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2723776
• 关键词: Measurement; based; entanglement; single; dopant

The elementary unit of quantum information is the quantum bit or qubit. Like the classical bit, the qubit is a two-level system but with the intriguing ability to exist in a superposition of states. This means it can be in the on and off state at the same time which has profound implications if we consider quantum systems of more than one qubit. Instead of each qubit carrying any well-defined information of its own, the information is encoded in their joint properties. In quantum mechanics, the qubits are described as being entangled. The challenge is to find ways to harness quantum phenomena such as superposition and entanglement to construct a quantum computer that is able to perform computational tasks that are unattainable in a classical context.A very natural qubit is the electron spin. The energy difference between spin states of an electron can be precisely controlled by magnetic fields and, using the electron's charge, it is also possible to isolate and manipulate individual spins electrically. One route to achieve entanglement between spin qubits is to use the interaction of their electron wavefunction overlap by placing them in close proximity. While such an approach is feasible for a small number of qubits, a large-scale quantum processor which relies on direct nearest neighbour coupling becomes rapidly impractical. Here we therefore propose an alternative strategy which makes use of an intriguing quantum mechanical effect by which two spatially separated quantum bits become entangled if a measurement cannot tell them apart.As has been shown theoretically, measurement-based entanglement can be used to couple large numbers of physically separated qubits, building up so-called graph states. Computation is then achieved by a sequence of measurements on individual qubits that consumes the entanglement - known as one-way quantum computation - which is entirely different from the standard circuit-based approach. In practise this also requires the presence of a quantum memory where quantum information is stored to allow graph-state growth without the risk of losing existing entanglement. Here we propose to use a solid-state implementation which is ideally suited to this task: single As-dopants in isotopically pure Si-28.To fabricate the devices, we will use the most precise silicon dopant incorporation technique available: scanning tunnelling microscopy (STM) hydrogen resist lithography. The atomically precise incorporation of individual As-dopants is essential in satisfying a key requirement of the measurement-based entanglement protocol: qubit indistinguishability.Having fabricated the devices, we will be able to manipulate the electron spins of the As-dopants and create entanglement between remote qubits using projective measurements. For this we will be using radio-frequency reflectometry techniques which allows us to perform these tasks on a timescale significantly faster than electron spin lifetimes. Once entanglement generation has been achieved, hyperfine coupling will be used to transfer the quantum information from the electron to the As nuclear spin states. This approach takes advantage of record nuclear spin coherence, in the 10-100 second range, of dopants in Si and allows us to grow the entangled graph state. Moreover, since the As nucleus has a non-zero electric quadrupole moment and a four dimensional Hilbert space we will be able to control the nuclear spins electrically and store and control the equivalent of two qubits in each dopant.For a proof-of-principle demonstrator we will entangle four spatially separated devices, each consisting of two As-dopant atom qubits with all-to-all qubit connectivity, equivalent to a 16-qubit processor. The experimental efforts will be supported by theoretical studies to further develop the most efficient strategies for growing a resilient remote network taking into account realistic experimental parameters such as spin dephasing and signal loss.

量子信息的基本单位是量子比特或量子位。像经典比特一样，量子比特是一个两级系统，但具有存在于叠加状态的有趣能力。这意味着它可以同时处于开和关状态，如果我们考虑超过一个量子位的量子系统，这将产生深远的影响。每个量子位不是携带任何定义良好的信息，而是将信息编码在它们的联合属性中。在量子力学中，量子比特被描述为纠缠态。挑战在于找到利用叠加和纠缠等量子现象的方法来构建量子计算机，使其能够执行在经典环境中无法实现的计算任务。一个非常自然的量子位是电子自旋。电子自旋状态之间的能量差可以通过磁场精确地控制，并且，利用电子的电荷，也可以用电隔离和操纵单个自旋。实现自旋量子比特之间纠缠的一种途径是通过将它们放置在近距离来利用它们的电子波函数重叠的相互作用。虽然这种方法对于少量量子比特是可行的，但依赖于直接近邻耦合的大规模量子处理器很快就变得不切实际。因此，我们在这里提出了一种替代策略，该策略利用了一种有趣的量子力学效应，即如果测量不能将两个空间分离的量子比特分开，它们就会纠缠在一起。理论上已经证明，基于测量的纠缠可以用来耦合大量物理上分离的量子位，建立所谓的图态。然后，计算是通过对单个量子比特进行一系列测量来实现的，这些测量消耗了纠缠——被称为单向量子计算——这与标准的基于电路的方法完全不同。在实践中，这也需要量子存储器的存在，量子存储器存储量子信息，以允许图形状态的增长，而不会有失去现有纠缠的风险。在这里，我们建议使用固态实现，这非常适合于这项任务：在同位素纯Si-28中添加单一as掺杂剂。为了制造这些器件，我们将使用最精确的硅掺杂技术：扫描隧道显微镜（STM）抗氢光刻技术。单个as掺杂剂的原子精确结合对于满足基于测量的纠缠协议的关键要求：量子位不可区分性至关重要。在制造出这些设备后，我们将能够操纵砷掺杂剂的电子自旋，并使用投影测量在远程量子比特之间产生纠缠。为此，我们将使用射频反射技术，这使我们能够在比电子自旋寿命快得多的时间尺度上执行这些任务。一旦实现了纠缠的产生，超精细耦合将用于将量子信息从电子转移到As核自旋态。这种方法利用了硅中掺杂剂在10-100秒范围内创纪录的核自旋相干性，并允许我们生长纠缠图态。此外，由于As核具有非零电四极矩和四维希尔伯特空间，我们将能够电控制核自旋，并在每个掺杂剂中存储和控制相当于两个量子位的量子比特。对于原理验证演示器，我们将纠缠四个空间分离的设备，每个设备由两个as掺杂原子量子位组成，具有全对全量子位连接，相当于16量子位处理器。实验工作将得到理论研究的支持，以进一步制定最有效的策略，以建立一个考虑到实际实验参数（如自旋减相和信号损失）的弹性远程网络。