25 research outputs found
Quantum oscillations without quantum coherence
We study numerically the damping of quantum oscillations and the dynamics of the density matrix in model many-spin systems decohered by a spin bath. We show that oscillations of some density matrix elements can persist with considerable amplitude long after other elements, along with the entropy, have come close to saturation, i.e., when the system has been decohered almost completely. The oscillations exhibit very slow decay, and may be observable in experiments.
Controlling the quantum dynamics of a mesoscopic spin bath in diamond
Understanding and mitigating decoherence is a key challenge for quantum
science and technology. The main source of decoherence for solid-state spin
systems is the uncontrolled spin bath environment. Here, we demonstrate quantum
control of a mesoscopic spin bath in diamond at room temperature that is
composed of electron spins of substitutional nitrogen impurities. The resulting
spin bath dynamics are probed using a single nitrogen-vacancy (NV) centre
electron spin as a magnetic field sensor. We exploit the spin bath control to
dynamically suppress dephasing of the NV spin by the spin bath. Furthermore, by
combining spin bath control with dynamical decoupling, we directly measure the
coherence and temporal correlations of different groups of bath spins. These
results uncover a new arena for fundamental studies on decoherence and enable
novel avenues for spin-based magnetometry and quantum information processing
Towards a large-scale quantum simulator on diamond surface at room temperature
Strongly-correlated quantum many-body systems exhibits a variety of exotic
phases with long-range quantum correlations, such as spin liquids and
supersolids. Despite the rapid increase in computational power of modern
computers, the numerical simulation of these complex systems becomes
intractable even for a few dozens of particles. Feynman's idea of quantum
simulators offers an innovative way to bypass this computational barrier.
However, the proposed realizations of such devices either require very low
temperatures (ultracold gases in optical lattices, trapped ions,
superconducting devices) and considerable technological effort, or are
extremely hard to scale in practice (NMR, linear optics). In this work, we
propose a new architecture for a scalable quantum simulator that can operate at
room temperature. It consists of strongly-interacting nuclear spins attached to
the diamond surface by its direct chemical treatment, or by means of a
functionalized graphene sheet. The initialization, control and read-out of this
quantum simulator can be accomplished with nitrogen-vacancy centers implanted
in diamond. The system can be engineered to simulate a wide variety of
interesting strongly-correlated models with long-range dipole-dipole
interactions. Due to the superior coherence time of nuclear spins and
nitrogen-vacancy centers in diamond, our proposal offers new opportunities
towards large-scale quantum simulation at room temperatures
Controllable effects of quantum fluctuations on spin free-induction decay at room temperature
Fluctuations of local fields cause decoherence of quantum objects. Usually at high temperatures, thermal noises are much stronger than quantum fluctuations unless the thermal effects are suppressed by certain techniques such as spin echo. Here we report the discovery of strong quantum-fluctuation effects of nuclear spin baths on free-induction decay of single electron spins in solids at room temperature. We find that the competition between the quantum and thermal fluctuations is controllable by an external magnetic field. These findings are based on Ramsey interference measurement of single nitrogen-vacancy center spins in diamond and numerical simulation of the decoherence, which are in excellent agreement
Solid state quantum memory using the 31P nuclear spin
The transfer of information between different physical forms is a central
theme in communication and computation, for example between processing entities
and memory. Nowhere is this more crucial than in quantum computation, where
great effort must be taken to protect the integrity of a fragile quantum bit.
Nuclear spins are known to benefit from long coherence times compared to
electron spins, but are slow to manipulate and suffer from weak thermal
polarisation. A powerful model for quantum computation is thus one in which
electron spins are used for processing and readout while nuclear spins are used
for storage. Here we demonstrate the coherent transfer of a superposition state
in an electron spin 'processing' qubit to a nuclear spin 'memory' qubit, using
a combination of microwave and radiofrequency pulses applied to 31P donors in
an isotopically pure 28Si crystal. The electron spin state can be stored in the
nuclear spin on a timescale that is long compared with the electron decoherence
time and then coherently transferred back to the electron spin, thus
demonstrating the 31P nuclear spin as a solid-state quantum memory. The overall
store/readout fidelity is about 90%, attributed to systematic imperfections in
radiofrequency pulses which can be improved through the use of composite
pulses. We apply dynamic decoupling to protect the nuclear spin quantum memory
element from sources of decoherence. The coherence lifetime of the quantum
memory element is found to exceed one second at 5.5K.Comment: v2: Tomography added and storage of general initial state
Effect of pulse error accumulation on dynamical decoupling of the electron spins of phosphorus donors in silicon
Dynamical decoupling (DD) is an efficient tool for preserving quantum coherence in solid-state spin systems. However, the imperfections of real pulses can ruin the performance of long DD sequences. We investigate the accumulation and compensation of different pulse errors in DD using the electron spins of phosphorus donors in silicon as a test system. We study periodic DD sequences based on spin rotations about two perpendicular axes, and their concatenated and symmetrized versions. We show that pulse errors may quickly destroy some spin states, but maintain other states with high fidelity over long times. Pulse sequences based on spin rotations about x and y axes outperform those based on x and z axes due to the accumulation of pulse errors. Concatenation provides an efficient way to suppress the impact of pulse errors, and can maintain high fidelity for all spin components: pulse errors do not accumulate (to first order) as the concatenation level increases, despite the exponential increase in the number of pulses. A symmetrized DD sequence cancels the first-order pulse errors. Our theoretical model gives a clear qualitative picture of the error accumulation and produces results in quantitative agreement with the experiments