18 research outputs found
Fast and accurate circularization of a Rydberg atom
Preparation of a so-called circular state in a Rydberg atom where the
projection of the electron angular momentum takes its maximum value is
challenging due to the required amount of angular momentum transfer. Currently
available protocols for circular state preparation are either accurate but slow
or fast but error-prone. Here, we show how to use quantum optimal control
theory to derive pulse shapes that realize fast and accurate circularization of
a Rydberg atom. In particular, we present a theoretical proposal for optimized
radio-frequency pulses that achieve high fidelity in the shortest possible
time, given current experimental limitations on peak amplitudes and spectral
bandwidth. We also discuss the fundamental quantum speed limit for
circularization of a Rydberg atom, when lifting these constraints.Comment: 10 pages, 6 figure
Real-time quantum feedback prepares and stabilizes photon number states
Feedback loops are at the heart of most classical control procedures. A
controller compares the signal measured by a sensor with the target value. It
adjusts then an actuator in order to stabilize the signal towards its target.
Generalizing this scheme to stabilize a micro-system's quantum state relies on
quantum feedback, which must overcome a fundamental difficulty: the
measurements by the sensor have a random back-action on the system. An optimal
compromise employs weak measurements providing partial information with minimal
perturbation. The controller should include the effect of this perturbation in
the computation of the actuator's unitary operation bringing the incrementally
perturbed state closer to the target. While some aspects of this scenario have
been experimentally demonstrated for the control of quantum or classical
micro-system variables, continuous feedback loop operations permanently
stabilizing quantum systems around a target state have not yet been realized.
We have implemented such a real-time stabilizing quantum feedback scheme. It
prepares on demand photon number states (Fock states) of a microwave field in a
superconducting cavity and subsequently reverses the effects of
decoherence-induced field quantum jumps. The sensor is a beam of atoms crossing
the cavity which repeatedly performs weak quantum non-demolition measurements
of the photon number. The controller is implemented in a real-time computer
commanding the injection, between measurements, of adjusted small classical
fields in the cavity. The microwave field is a quantum oscillator usable as a
quantum memory or as a quantum bus swapping information between atoms. By
demonstrating that active control can generate non-classical states of this
oscillator and combat their decoherence, this experiment is a significant step
towards the implementation of complex quantum information operations.Comment: 12 pages, 4 figure
Fast Navigation in a Large Hilbert Space Using Quantum Optimal Control
The precise engineering of quantum states, a basic prerequisite for technologies such as quantum-enhanced sensing or quantum computing, becomes more challenging with increasing dimension of the system Hilbert space. Standard preparation techniques then require a large number of operations or slow adiabatic evolution and give access to only a limited set of states. Here, we use quantum optimal control theory to overcome this problem and derive shaped radio-frequency pulses to experimentally navigate the Stark manifold of a Rydberg atom. We demonstrate that optimal control, beyond improving the fidelity of an existing protocol, also enables us to accurately generate a nonclassical superposition state that cannot be prepared with reasonable fidelity using standard techniques. Optimal control thus substantially enlarges the range of accessible states. Our joint experimental and theoretical work establishes quantum optimal control as a key tool for quantum engineering in complex Hilbert spaces
Towards quantum simulation with circular Rydberg atoms
The main objective of quantum simulation is an in-depth understanding of
many-body physics. It is important for fundamental issues (quantum phase
transitions, transport, . . . ) and for the development of innovative
materials. Analytic approaches to many-body systems are limited and the huge
size of their Hilbert space makes numerical simulations on classical computers
intractable. A quantum simulator avoids these limitations by transcribing the
system of interest into another, with the same dynamics but with interaction
parameters under control and with experimental access to all relevant
observables. Quantum simulation of spin systems is being explored with trapped
ions, neutral atoms and superconducting devices. We propose here a new paradigm
for quantum simulation of spin-1/2 arrays providing unprecedented flexibility
and allowing one to explore domains beyond the reach of other platforms. It is
based on laser-trapped circular Rydberg atoms. Their long intrinsic lifetimes
combined with the inhibition of their microwave spontaneous emission and their
low sensitivity to collisions and photoionization make trapping lifetimes in
the minute range realistic with state-of-the-art techniques. Ultra-cold
defect-free circular atom chains can be prepared by a variant of the
evaporative cooling method. This method also leads to the individual detection
of arbitrary spin observables. The proposed simulator realizes an XXZ spin-1/2
Hamiltonian with nearest-neighbor couplings ranging from a few to tens of kHz.
All the model parameters can be tuned at will, making a large range of
simulations accessible. The system evolution can be followed over times in the
range of seconds, long enough to be relevant for ground-state adiabatic
preparation and for the study of thermalization, disorder or Floquet time
crystals. This platform presents unrivaled features for quantum simulation
Ultrahigh finesse Fabry-Perot superconducting resonator
We have built a microwave Fabry-Perot resonator made of diamond-machined
copper mirrors coated with superconducting niobium. Its damping time (Tc = 130
ms at 51 GHz and 0.8 K) corresponds to a finesse of 4.6 x 109, the
highest ever reached for a Fabry-Perot in any frequency range. This result
opens novel perspectives for quantum information, decoherence and non-locality
studies
Atom-photon interactions in a system of coupled cavities
We give a theoretical treatment of single atom detection in an compound,
optical micro cavity. The cavity consists of a single mode semiconductor
waveguide with a gap to allow atoms to interact with the optical field in the
cavity. Optical losses, both in the semiconductor and induced by the gap are
considered and we give an estimate of the cavity finesse. We also compute the
cooperativity parameter and show how it depends on the gap width and cavity
length. Maximization of the cooperativity does not always correspond to
maximization of the coupling