Symmetric Phases of Universal Quantum Computation

Jacob Miller and Akimasa Miyake have recently published a paper in Physical Review Letters giving strong evidence that certain forms of symmetric topological quantum matter can be utilized ubiquitously to power quantum computation. Their work is carried out within measurement-based quantum computation, where computation is extracted from a fixed quantum “resource state” using local measurements. In this setting, the power of computation attributes the physical properties of the resource state, but the properties which guarantee a state can carry out universal quantum computation are still unknown.
In their work, the authors study a model of symmetric topological matter and identify special states in each phase which enable universal quantum computation precisely when they possess nontrivial quantum order. This gives an infinite family of new universal resource states whose structure perfectly mirrors a recent classification of symmetric quantum order coming from condensed matter physics. These special resource states are distinguished by their “fractional symmetry”, a property already noticed in previous universal resource states, but which hadn’t been investigated systematically. Overall, the work provides a concrete research program for identifying phases of universal quantum computation within the setting of symmetric quantum matter.
The full article can be found online at Phys. Rev. Lett. 120, 170503 (2018).
Classification of special resource states with fractional symmetry.

Becerra’s group publishes article on multiple coherent states in Journal of Optical Society of America B

Dr. Elohim Becerra Chavez’s research group recently published Implementation of a single-shot receiver for quaternary phase-shift keyed coherent states in Journal of Optical Society of America B

Measurement strategies for multiple coherent states based on single-shot measurements with photon counting can be useful for high bandwidth communications with high spectral efficiency. The quantum-optics group led by Elohim Becerra at UNM investigated implementations of optimized multi-state discrimination strategies based on single-shot measurements extending previous work to include realistic situations with noise and imperfections, which impact the achievable performance of the measurement. The implementation with noise and imperfections allows us to identify the experimental requirements to outperform the sensitivity limit of an ideal heterodyne measurement and can guide future demonstrations of these measurements with high efficiency single-photon detectors surpassing the heterodyne limit.

The full article can be found online.

Enhance atom-light coupling with a “weaker” 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. Following this school of thought, great challenges including heating and decoherent photon scattering problems have been encountered in experiments to enhance atom-light coupling by placing atoms in the strongest trapping field, which hindered the implementing of quantum communication and quantum computing applications using atoms. Surprisingly, 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.

Fig 3. (a) Field components of Rel[Ex], Re[Ey] and Im[Ez] distributions for a nanofiber and a square waveguide. (b) Atoms (black stars) are better to put at the weak field positions.

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 paperPhys. Rev. A 97, 033829 – Published 16 March 2018

arXiv version: arXiv:1712.02916.

Related presentation: SQuInT 2018 workshop at Santa Fe, NM, USA.

Data and source files repository: on GitHub (comments are welcome).

Becerra Chavez article illustration

Becerra’s group publishes Low Power Light article in Nature Partner Journals

Dr. Elohim Becerra Chavez’s research group recently published Quantum measurements: surpassing conventional sensitivity limits at low powers in Nature Partner Journals

Becerra Chavez article illustration

Optimized discrimination of multiple states.

Light has intrinsic quantum noise, which limits how well we can measure it, especially at low powers, and bounds how much information we can communicate. A team led by F. Elohim Becerra at the University of New Mexico demonstrated optimized measurements for light pulses with different phases at low powers, such as those used in coherent optical communication. These optimized measurements can surpass the ultimate sensitivity limits of ideal conventional detectors, even in the presence of loss and noise encountered in realistic situations. The measurements are based on combining the input pulse with a reference field, and counting single photons in a fraction of the pulse. By analyzing the detection outcome, the reference field can be optimized to enhance the measurement’s sensitivity. Optimized measurements at low powers may lead to more-efficient optical communication in realistic environments.

The full article can be found online at npj Quantum Information 3, Article number: 43 (2017) .