Nanophotonic waveguides enhance atom-light coupling with a “weak” local field

Strong atom-light coupling is the key to quantum information processing with atoms and photons. It is a common belief that a strong optical field is required to generate a strong atom-light coupling. However, a recent theoretical study conducted by researchers in CQuIC and the Sandia National Labs demonstrated that, by placing the atoms at an azimuthal position where the guided probe mode of a waveguide has the lowest intensity, the atom-light coupling is the strongest for quantum measurement applications.

Fig 1. Schematic diagram of the QND measurement and spin squeezing protocol with nanophotonic waveguides based on Faraday effect.

In this study, the authors consider an atom-nanophotonic waveguide interface, where atoms are trapped in the evanescent field of a waveguide which has an effective diameter (a few hundred nano-meters) less than the wavelength of the light and supports two orthogonal guided modes. They use cooperativity to quantitatively characterize the atom-light coupling and study the enhancement of cooperativity in the atom-light interface near a nanophotonic waveguide for application to quantum nondemolition (QND) measurement of atomic spins. Here the
cooperativity per atom is determined by the ratio between the measurement strength and the decoherence rate. The two orthogonal guided modes are adiabatically connected to two orthogonal linearly polarized modes as the cross-section of the waveguide become large compared to optical wavelength. In the QND measurement protocol, as shown in the figure above, a horizontally polarized light is sent into the waveguide and becomes the H-mode in the interaction region where the atoms are trapped. By preparing the trapped alkali atoms at a certain state known as a spin coherent state, the light detected at the measurement equipment will no longer be horizontally polarized due to the Faraday interaction with the atoms. How much the polarization state of the light
is changed is a signature of the state of the collective spin of the atoms. Following the principles of quantum mechanics, when a measurement is done there is “backaction” on the state. By measuring the direction of the collective spin projected to the z-axis we dramatically reduced our uncertainty in this value from the superposition that existed in the initial spin coherent state. On the other hand, the uncertainty of the collective spin state projected to the orthogonal direction, the y-direction in this case, on the Bloch sphere has been elongated. This state is called a spin squeezed state, which has applications in precision measurements and other quantum information processing protocols. How much the spin is squeezing we can generation is determined by how strong the atoms are coupled to the probe light.

Fig 2. Spin squeezing illustrated on the Bloch sphere.

What is surprising about this study is that the authors discovered that the optimal configuration to generate the strongest spin squeezing effect is defined by placing the atoms at the weakest probe mode position, which is along the vertical direction–not the strongest mode position as the top 1 common guess of people–which is the horizontal direction in the xy-plane. This arises because the QND measurement strength relies on the interference between the probe and scattered light guided into an orthogonal polarization–V-mode, while the decoherence rate depends on the local intensity of the probe defined by the H-mode.  At the position of the atom the vacuum V-Mode is strongest. Thus, by placing the atoms on the y-axis, the ratio of good to bad scattering can be strongly enhanced for highly anisotropic modes. The researchers of this work apply this idea to study spin squeezing resulting from QND measurement of spin projection noise via the Faraday effect in two nanophotonic geometries, a cylindrical nanofiber and a square waveguide. They find, with about 2500 atoms using realistic experimental parameters, about 6.3 dB and 13 dB of squeezing can be achieved on the nanofiber and square waveguide, respectively. In contrast, to generate the same amount of spin squeezing using an atomic cloud trapped by Gaussian laser beams in free space, it might require billions of atoms.

This study gives a new perspective on the design of the atom-nanophotonic waveguide interface.  Based on the approach taken here, one can prepare and readout collective spin squeezing while avoiding strongly disturbing the mechanical motion and internal states of atoms, which are often big challenges if atoms are sitting at strong field spots of the probe. By using some additional techniques, such as adding an optical cavity on the waveguide, one can generate highly nonclassical nonGaussian states, which have a variety of applications in quantum information processing. This work also marks a milestone towards atom-nanophotonic interface based quantum simulations and quantum computing applications.

Related paper: arXiv:1712.02916.

Demonstration of the Jaynes-Cummings ladder with Rydberg-dressed atoms

THIS PAPER WAS CHOSEN AS AN “EDITORS’ SUGGESTION” IN PHYSICAL REVIEW A, Vol. 95, Iss. 4 — April 2017.

Demonstration of the Jaynes-Cummings ladder with Rydberg-dressed atoms

Jongmin Lee, Michael J. Martin, Yuan-Yu Jau, Tyler Keating, Ivan H. Deutsch, and Grant Biedermann

The Jaynes-Cummings model, a widely employed theoretical framework in cavity quantum electrodynamics, is experimentally tested on a platform involving Rydberg-blockaded atomic ensembles. The work opens the way to a richer exploration of protocols for quantum control or, more broadly, quantum computing.

The full article is available online at Phys. Rev. A 95, 041801(R) (2017)

 

FIG 1.  Experimental setup.  The Rydberg laser and the Raman lasers are  aligned along the x axis.  Two optical tweezers are formed by two lasers  with an angular separation θ..  In this setup, eight electrodes control the electric fields near the trapped atoms.  The bias magnetic field is applied along the x axis.

 

 

Integrated quantum information processing in the dispersive regime with fewer atoms

Nanofiber Scheme
The integration of nanophotonics with ultracold atoms opens the door to new protocols in quantum information processing. Strong entangling interactions between atoms and photons are the key ingredient. Whereas a resonant interaction can lead to the strongest entanglement per atom, this requires special geometries that limit decoherence. Off-resonant dispersive interactions, where a phase shift is associated with the atom-photon interaction, provides an alternative route to strong entanglement. This can be achieved due to the “cooperativity” of a large ensemble of atoms that can be homogeneously trapped in the evanescent field of an optical nanofiber using well-known techniques (see figure above). The optical scattering cross section closely matches the guided beam mode area across the entire length of the nanofiber. Our recent paper pedagogically develops the theory to describe how the light dispersively responds to an ensemble of atoms in the optical nanofiber waveguide platform and how this can yield large cooperativity. As an application of the theory, we study the creation of spin squeezed states for application in improved precision of atomic clocks. With only a few thousand Cesium atoms, a nontrivial squeezed state can be created using an anisotropic property of the nanofiber modes, which is not available or hard to implement in free space. This is a first step towards more general protocols involving the production of nonGaussian atomic states and their interaction with nonclassical light.

 

Comments and discussions can go to its Github repository.

Reference:

Xiaodong Qi, Ben Q. Baragiola, Poul S. Jessen, and Ivan H. Deutsch, Dispersive response of atoms trapped near the surface of an optical nanofiber with applications to quantum nondemolition measurement and spin squeezing, Phys. Rev. A 93, 023817 (2016). [PDF]

“Quanta Magazine” Interviews Ivan Deutsch

Ivan Deutsch Q&A in "Quanta Magazine"

Ivan Deutsch Q&A in “Quanta Magazine”

 

The Ivan Deutsch group at CQuIC received a welcome surprise with the publication of an interview with Dr. Deutsch in “Quanta Magazine“. The Deutsch group’s research has focused on the control of a 16 dimensional quantum system, or “qudit” in the quantum information parlance. This system offers the opportunity to study robust quantum control techniques (where errors introduced in the control protocols do not introduce too many errors into the behavior of the system), as well as the opportunity to examine novel quantum effects.