[Paper Review] A single-atom quantum memory in silicon

Solomon Freer, Stephanie Simmons+11 more|arXiv (Cornell University)|Aug 25, 2016

Quantum and electron transport phenomena3 references3 citations

TL;DR

This paper demonstrates a single-atom quantum memory in isotopically enriched 28Si, using the electron and nuclear spins of a single 31P donor as qubits. Quantum information is transferred from the fast electron spin to the long-lived nuclear spin via electron-nuclear resonance pulses, achieving a process fidelity of 81% and memory storage times up to 80 ms using dynamical decoupling, limited by a pulse-induced electron spin resonance frequency shift.

ABSTRACT

Long coherence times and fast gate operations are desirable but often conflicting requirements for physical qubits. This conflict can be resolved by resorting to fast qubits for operations, and by storing their state in a `quantum memory' while idle. The $^{31}$P donor in silicon comes naturally equipped with a fast qubit (the electron spin) and a long-lived qubit (the $^{31}$P nuclear spin), coexisting in a bound state at cryogenic temperatures. Here, we demonstrate storage and retrieval of quantum information from a single donor electron spin to its host phosphorus nucleus in isotopically-enriched $^{28}$Si. The fidelity of the memory process is characterised via both state and process tomography. We report an overall process fidelity of $F_p =$81${\pm}$7%, a memory fidelity ($F_m$) of over 90%, and memory storage times up to 80 ms. These values are limited by a transient shift of the electron spin resonance frequency following high-power radiofrequency pulses.

Motivation & Objective

• To implement a quantum memory at the single-atom level using a donor in silicon, leveraging the electron and nuclear spins of 31P as qubits.
• To achieve high-fidelity storage and retrieval of quantum information from the electron spin to the nuclear spin.
• To extend memory coherence times beyond the intrinsic nuclear spin coherence by applying dynamical decoupling (DD) pulses.
• To identify and characterize the dominant decoherence mechanism limiting fidelity and storage time.
• To enable scalable, high-fidelity quantum information processing in silicon-based quantum computing architectures.

Proposed method

• A single 31P donor is implanted in an isotopically enriched 28Si epilayer to minimize magnetic decoherence from 29Si isotopes.
• A single-electron transistor (SET) is used for single-shot readout of the electron spin state.
• Microwave and radiofrequency (RF) pulses are applied via a broadband antenna to coherently manipulate the electron and nuclear spins.
• Electron-nuclear resonance (ENDOR) pulses transfer quantum coherence from the electron spin to the nuclear spin for storage.
• Dynamical decoupling (DD) pulses, specifically CPMG sequences, are applied to the nuclear spin during storage to extend coherence time.
• State and process tomography are performed using Ramsey interferometry and ancillary microwave pulses to measure fidelity in arbitrary bases.

Experimental results

Research questions

• RQ1 Can quantum information be coherently transferred from a single electron spin to its host 31P nuclear spin in silicon with high fidelity?
• RQ2 What is the maximum achievable memory storage time for a single-atom quantum memory in silicon?
• RQ3 What are the dominant decoherence mechanisms limiting process and memory fidelity in this system?
• RQ4 How does dynamical decoupling affect the coherence of the nuclear spin memory?
• RQ5 Can the memory fidelity be improved by mitigating pulse-induced frequency shifts?

Key findings

• The overall process fidelity of the quantum memory protocol reaches 81% ± 7%, significantly exceeding the classical limit of 2/3.
• The memory fidelity is measured at 92%, indicating high-fidelity state transfer and storage.
• Memory storage times of up to 80 ms are achieved using 256 CPMG pulses, matching the nuclear spin coherence time.
• The coherence time of the memory protocol scales with the number of CPMG pulses as N^0.36, approaching the nuclear spin CPMG time.
• A transient shift in the electron spin resonance frequency after high-power RF pulses is identified as the primary source of fidelity and coherence degradation.
• The nuclear spin coherence time is significantly longer than in earlier experiments on the same device, suggesting an undetected pulse-induced shift on the nuclear spin may also contribute.