175 research outputs found
Electrically controlling single spin qubits in a continuous microwave field
Large-scale quantum computers must be built upon quantum bits that are both
highly coherent and locally controllable. We demonstrate the quantum control of
the electron and the nuclear spin of a single 31P atom in silicon, using a
continuous microwave magnetic field together with nanoscale electrostatic
gates. The qubits are tuned into resonance with the microwave field by a local
change in electric field, which induces a Stark shift of the qubit energies.
This method, known as A-gate control, preserves the excellent coherence times
and gate fidelities of isolated spins, and can be extended to arbitrarily many
qubits without requiring multiple microwave sources.Comment: Main paper: 13 pages, 4 figures. Supplementary information: 25 pages,
13 figure
Improved placement precision of implanted donor spin qubits in silicon using molecule ions
Donor spins in silicon-28 (Si) are among the most performant qubits in
the solid state, offering record coherence times and gate fidelities above 99%.
Donor spin qubits can be fabricated using the semiconductor-industry compatible
method of deterministic ion implantation. Here we show that the precision of
this fabrication method can be boosted by implanting molecule ions instead of
single atoms. The bystander ions, co-implanted with the dopant of interest,
carry additional kinetic energy and thus increase the detection confidence of
deterministic donor implantation employing single ion detectors to signal the
induced electron-hole pairs. This allows the placement uncertainty of donor
qubits to be minimised without compromising on detection confidence. We
investigate the suitability of phosphorus difluoride (PF) molecule ions
to produce high quality P donor qubits. Since F nuclei have a spin of , it is imperative to ensure that they do not hyperfine couple to P donor
electrons as they would cause decoherence by adding magnetic noise. Using
secondary ion mass spectrometry, we confirm that F diffuses away from the
active region of qubit devices while the P donors remain close to their
original location during a donor activation anneal. PF-implanted qubit
devices were then fabricated and electron spin resonance (ESR) measurements
were performed on the P donor electron. A pure dephasing time of s and a coherence time of s were
extracted for the P donor electron-values comparable to those found in previous
P-implanted qubit devices. Closer investigation of the P donor ESR spectrum
revealed that no F nuclear spins were found in the vicinity of the P
donor. Molecule ions therefore show great promise for producing high-precision
deterministically-implanted arrays of long-lived donor spin qubits.Comment: 8 pages, 5 figures, 1 tabl
Beating the thermal limit of qubit initialization with a Bayesian Maxwell's demon
Fault-tolerant quantum computing requires initializing the quantum register
in a well-defined fiducial state. In solid-state systems, this is typically
achieved through thermalization to a cold reservoir, such that the
initialization fidelity is fundamentally limited by temperature. Here, we
present a method of preparing a fiducial quantum state with a confidence beyond
the thermal limit. It is based on real time monitoring of the qubit through a
negative-result measurement -- the equivalent of a `Maxwell's demon' that
triggers the experiment only upon the appearance of a qubit in the
lowest-energy state. We experimentally apply it to initialize an electron spin
qubit in silicon, achieving a ground-state initialization fidelity of 98.9(4)%,
corresponding to a 20 reduction in initialization error compared to the
unmonitored system. A fidelity approaching 99.9% could be achieved with
realistic improvements in the bandwidth of the amplifier chain or by slowing
down the rate of electron tunneling from the reservoir. We use a nuclear spin
ancilla, measured in quantum nondemolition mode, to prove the value of the
electron initialization fidelity far beyond the intrinsic fidelity of the
electron readout. However, the method itself does not require an ancilla for
its execution, saving the need for additional resources. The quantitative
analysis of the initialization fidelity reveals that a simple picture of
spin-dependent electron tunneling does not correctly describe the data. Our
digital `Maxwell's demon' can be applied to a wide range of quantum systems,
with minimal demands on control and detection hardware.Comment: 15 pages, 7 figure
Conditional quantum operation of two exchange-coupled single-donor spin qubits in a MOS-compatible silicon device
Silicon nanoelectronic devices can host single-qubit quantum logic operations
with fidelity better than 99.9%. For the spins of an electron bound to a single
donor atom, introduced in the silicon by ion implantation, the quantum
information can be stored for nearly 1 second. However, manufacturing a
scalable quantum processor with this method is considered challenging, because
of the exponential sensitivity of the exchange interaction that mediates the
coupling between the qubits. Here we demonstrate the conditional, coherent
control of an electron spin qubit in an exchange-coupled pair of P
donors implanted in silicon. The coupling strength, MHz,
is measured spectroscopically with unprecedented precision. Since the coupling
is weaker than the electron-nuclear hyperfine coupling MHz which
detunes the two electrons, a native two-qubit Controlled-Rotation gate can be
obtained via a simple electron spin resonance pulse. This scheme is insensitive
to the precise value of , which makes it suitable for the scale-up of
donor-based quantum computers in silicon that exploit the
Metal-Oxide-Semiconductor fabrication protocols commonly used in the classical
electronics industry.Comment: 10 pages, 5 figures, plus Supplementary Information. v2 contains
additional references, and a simpler explanation of two-qubit CROT gates for
donors in silico
Methods for transverse and longitudinal spin-photon coupling in silicon quantum dots with intrinsic spin-orbit effect
In a full-scale quantum computer with a fault-tolerant architecture, having
scalable, long-range interaction between qubits is expected to be a highly
valuable resource. One promising method of achieving this is through the
light-matter interaction between spins in semiconductors and photons in
superconducting cavities. This paper examines the theory of both transverse and
longitudinal spin-photon coupling and their applications in the silicon
metal-oxide-semiconductor (SiMOS) platform. We propose a method of coupling
which uses the intrinsic spin-orbit interaction arising from orbital
degeneracies in SiMOS qubits. Using theoretical analysis and experimental data,
we show that the strong coupling regime is achievable in the transverse scheme.
We also evaluate the feasibility of a longitudinal coupling driven by an AC
modulation on the qubit. These coupling methods eschew the requirement for an
external micromagnet, enhancing prospects for scalability and integration into
a large-scale quantum computer
Spatio-temporal correlations of noise in MOS spin qubits
In quantum computing, characterising the full noise profile of qubits can aid
the efforts towards increasing coherence times and fidelities by creating error
mitigating techniques specific to the type of noise in the system, or by
completely removing the sources of noise. Spin qubits in MOS quantum dots are
exposed to noise originated from the complex glassy behaviour of two-level
fluctuators, leading to non-trivial correlations between qubit properties both
in space and time. With recent engineering progress, large amounts of data are
being collected in typical spin qubit device experiments, and it is beneficiary
to explore data analysis options inspired from fields of research that are
experienced in managing large data sets, examples include astrophysics, finance
and climate science. Here, we propose and demonstrate wavelet-based analysis
techniques to decompose signals into both frequency and time components to gain
a deeper insight into the sources of noise in our systems. We apply the
analysis to a long feedback experiment performed on a state-of-the-art
two-qubit system in a pair of SiMOS quantum dots. The observed correlations
serve to identify common microscopic causes of noise, as well as to elucidate
pathways for multi-qubit operation with a more scalable feedback system.Comment: updated referenc
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