51 research outputs found
Robust two-qubit gates for donors in silicon controlled by hyperfine interactions
We present two strategies for performing two-qubit operations on the electron
spins of an exchange-coupled pair of phosphorus donors in silicon, using the
ability to set the donor nuclear spins in arbitrary states. The effective
magnetic detuning of the two electron qubits is provided by the hyperfine
interaction when the P nuclei are prepared in opposite spin states. This
can be exploited to switch on and off SWAP operations with modest tuning of the
electron exchange interaction. Furthermore, the hyperfine detuning enables
high-fidelity conditional rotation gates based on selective resonant
excitation. The latter requires no dynamic tuning of the exchange interaction
at all, and offers a very attractive scheme to implement two-qubit logic gates
under realistic experimental conditions.Comment: 8 pages, 3 figure
Climbing the Jaynes-Cummings ladder by photon counting
We present a new method to observe direct experimental evidence of
Jaynes--Cummings nonlinearities in a strongly dissipative cavity quantum
electrodynamics system, where large losses compete with the strong light-matter
interaction. This is a highly topical problem, particularly for quantum dots in
microcavities where transitions from higher rungs of the Jaynes--Cummings
ladder remain to be evidenced explicitly. We compare coherent and incoherent
excitations of the system and find that resonant excitation of the detuned
emitter make it possible to unambiguously evidence few photon quantum
nonlinearities in currently available experimental systems.Comment: 4 pages, 4 figures (color online). Updated bb
Transport of Spin Qubits with Donor Chains under Realistic Experimental Conditions
The ability to transport quantum information across some distance can
facilitate the design and operation of a quantum processor. One-dimensional
spin chains provide a compact platform to realize scalable spin transport for a
solid-state quantum computer. Here, we model odd-sized donor chains in silicon
under a range of experimental non-idealities, including variability of donor
position within the chain. We show that the tolerance against donor placement
inaccuracies is greatly improved by operating the spin chain in a mode where
the electrons are confined at the Si-SiO interface. We then estimate the
required timescales and exchange couplings, and the level of noise that can be
tolerated to achieve high fidelity transport. We also propose a protocol to
calibrate and initialize the chain, thereby providing a complete guideline for
realizing a functional donor chain and utilizing it for spin transport.Comment: 19 pages, 12 figure
All-electron hyperfine coupling of Si-, Ge- and Sn-vacancy defects in diamond
Colour centres in diamond are attractive candidates for numerous quantum
applications due to their good optical properties and long spin coherence
times. They also provide access to the even longer coherence of hyperfine
coupled nuclear spins in their environment. While the NV centre is well
studied, both in experiment and theory, the hyperfine couplings in the more
novel centres (SiV, GeV, and SnV) are still largely unknown. Here we report on
the first all-electron \textit{ab-initio} calculations of the hyperfine
constants for SiV, GeV, and SnV defects in diamond, both for the respective
defect atoms (Si, Ge, Sn, Sn), as well as for the
surrounding C atoms. Furthermore, we calculate the nuclear quadrupole
moments of the GeV defect. We vary the Hartree-Fock mixing parameter for
Perdew-Burke-Ernzerhof (PBE) exchange correlation functional and show that the
hyperfine couplings of the defect atoms have a linear dependence on the mixing
percentage. We calculate the inverse dielectric constant to predict an
\textit{ab-initio} mixing percentage. The final hyperfine coupling predictions
are close to the experimental values available in the literature. Our results
will help to guide future novel experiments on these defects.Comment: 8 pages, 3 figures. Supplementary data (Tables S1-S12) in sourc
Controlling spin-orbit interactions in silicon quantum dots using magnetic field direction
Silicon quantum dots are considered an excellent platform for spin qubits,
partly due to their weak spin-orbit interaction. However, the sharp interfaces
in the heterostructures induce a small but significant spin-orbit interaction
which degrade the performance of the qubits or, when understood and controlled,
could be used as a powerful resource. To understand how to control this
interaction we build a detailed profile of the spin-orbit interaction of a
silicon metal-oxide-semiconductor double quantum dot system. We probe the
derivative of the Stark shift, -factor and -factor difference for two
single-electron quantum dot qubits as a function of external magnetic field and
find that they are dominated by spin-orbit interactions originating from the
vector potential, consistent with recent theoretical predictions. Conversely,
by populating the double dot with two electrons we probe the mixing of singlet
and spin-polarized triplet states during electron tunneling, which we conclude
is dominated by momentum-term spin-orbit interactions that varies from 1.85 MHz
up to 27.5 MHz depending on the magnetic field orientation. Finally, we exploit
the tunability of the derivative of the Stark shift of one of the dots to
reduce its sensitivity to electric noise and observe an 80 % increase in
. We conclude that the tuning of the spin-orbit interaction will be
crucial for scalable quantum computing in silicon and that the optimal setting
will depend on the exact mode of qubit operations used
Phonon-assisted transitions from quantum dot excitons to cavity photons
For a single semiconductor quantum dot embedded in a microcavity, we
theoretically and experimentally investigate phonon-assisted transitions
between excitons and the cavity mode. Within the framework of the independent
boson model we find that such transitions can be very efficient, even for
relatively large exciton-cavity detunings of several millielectron volts.
Furthermore, we predict a strong detuning asymmetry for the exciton lifetime
that vanishes for elevated lattice temperature. Our findings are corroborated
by experiment, which turns out to be in good quantitative and qualitative
agreement with theory
High-fidelity adiabatic inversion of a electron spin qubit in natural silicon
The main limitation to the high-fidelity quantum control of spins in
semiconductors is the presence of strongly fluctuating fields arising from the
nuclear spin bath of the host material. We demonstrate here a substantial
improvement in single-qubit gate fidelities for an electron spin qubit bound to
a P atom in natural silicon, by applying adiabatic inversion instead of
narrow-band pulses. We achieve an inversion fidelity of 97%, and we observe
signatures in the spin resonance spectra and the spin coherence time that are
consistent with the presence of an additional exchange-coupled donor. This work
highlights the effectiveness of adiabatic inversion techniques for spin control
in fluctuating environments.Comment: 4 pages, 2 figure
Bell's inequality violation with spins in silicon
Bell's theorem sets a boundary between the classical and quantum realms, by
providing a strict proof of the existence of entangled quantum states with no
classical counterpart. An experimental violation of Bell's inequality demands
simultaneously high fidelities in the preparation, manipulation and measurement
of multipartite quantum entangled states. For this reason the Bell signal has
been tagged as a single-number benchmark for the performance of quantum
computing devices. Here we demonstrate deterministic, on-demand generation of
two-qubit entangled states of the electron and the nuclear spin of a single
phosphorus atom embedded in a silicon nanoelectronic device. By sequentially
reading the electron and the nucleus, we show that these entangled states
violate the Bell/CHSH inequality with a Bell signal of 2.50(10). An even higher
value of 2.70(9) is obtained by mapping the parity of the two-qubit state onto
the nuclear spin, which allows for high-fidelity quantum non-demolition
measurement (QND) of the parity. Furthermore, we complement the Bell inequality
entanglement witness with full two-qubit state tomography exploiting QND
measurement, which reveals that our prepared states match the target maximally
entangled Bell states with 96\% fidelity. These experiments demonstrate
complete control of the two-qubit Hilbert space of a phosphorus atom, and show
that this system is able to maintain its simultaneously high initialization,
manipulation and measurement fidelities past the single-qubit regime.Comment: 10 pages, 3 figures, 1 table, 4 extended data figure
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