24 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
Injection locking of an electro-optomechanical device
The techniques of cavity optomechanics have enabled significant achievements
in precision sensing, including the detection of gravitational waves and the
cooling of mechanical systems to their quantum ground state. Recently, the
inherent non-linearity in the optomechanical interaction has been harnessed to
explore synchronization effects, including the spontaneous locking of an
oscillator to a reference injection signal delivered via the optical field.
Here, we present the first demonstration of a radiation-pressure driven
optomechanical system locking to an inertial drive, with actuation provided by
an integrated electrical interface. We use the injection signal to suppress
drift in the optomechanical oscillation frequency, strongly reducing phase
noise by over 55 dBc/Hz at 2 Hz offset. We further employ the injection tone to
tune the oscillation frequency by more than 2 million times its narrowed
linewidth. In addition, we uncover previously unreported synchronization
dynamics, enabled by the independence of the inertial drive from the optical
drive field. Finally, we show that our approach may enable control of the
optomechanical gain competition between different mechanical modes of a single
resonator. The electrical interface allows enhanced scalability for future
applications involving arrays of injection-locked precision sensors.Comment: Main text: 10 pages, 7 figures. Supplementary Information: 5 pages, 4
figure
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
Free spectral range electrical tuning of a high quality on-chip microcavity
Reconfigurable photonic circuits have applications ranging from
next-generation computer architectures to quantum networks, coherent radar and
optical metamaterials. However, complete reconfigurability is only currently
practical on millimetre-scale device footprints. Here, we overcome this barrier
by developing an on-chip high quality microcavity with resonances that can be
electrically tuned across a full free spectral range (FSR). FSR tuning allows
resonance with any source or emitter, or between any number of networked
microcavities. We achieve it by integrating nanoelectronic actuation with
strong optomechanical interactions that create a highly strain-dependent
effective refractive index. This allows low voltages and sub-nanowatt power
consumption. We demonstrate a basic reconfigurable photonic network, bringing
the microcavity into resonance with an arbitrary mode of a microtoroidal
optical cavity across a telecommunications fibre link. Our results have
applications beyond photonic circuits, including widely tuneable integrated
lasers, reconfigurable optical filters for telecommunications and astronomy,
and on-chip sensor networks.Comment: Main text: 7 pages, 3 figures. Supplementary information: 7 pages, 9
figure
Propagation and imaging of mechanical waves in a highly-stressed single-mode phononic waveguide
We demonstrate a single-mode phononic waveguide that enables robust
propagation of mechanical waves. The waveguide is a highly-stressed silicon
nitride membrane that supports the propagation of out-of-plane modes. In direct
analogy to rectangular microwave waveguides, there exists a band of frequencies
over which only the fundamental mode is allowed to propagate, while multiple
modes are supported at higher frequencies. We directly image the mode profiles
using optical heterodyne vibration measurement, showing good agreement with
theory. In the single-mode frequency band, we show low-loss propagation
(~dB/cm) for a ~MHz mechanical wave. This design is well suited
for phononic circuits interconnecting elements such as non-linear resonators or
optomechanical devices for signal processing, sensing or quantum technologies.Comment: 6 pages, 5 figure
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
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
Inertial injection locking in an electro-optomechanical system
Electro-optomechanical systems are a platform for exploring rich physics such as frequency conversion and synchronization. Here we demonstrate the first case of inertial injection locking in a radiation pressure driven electro-optomechanical system