How can one verify the performance of a near-term quantum device?

Jacob Miller, Keith Sanders, and Akimasa Miyake have recently published a paper in Physical Review A presenting a distinctive means of demonstrating the unique computational power inherent in quantum mechanics. Their work follows other proposals in the growing topic of “quantum computational supremacy”, which aims to construct a realistic device implementing a sampling-based computational task which is otherwise impossible with any modern digital computer. Such sampling tasks must achieve a careful balance, where they are both easier to implement in a laboratory than full quantum computation, but must also be hard enough to require genuinely quantum effects to solve.
The proposal put forward by Miller, Sanders, and Miyake has several desirable features. First, it can carry out its computational task in a constant amount of time, helping to mitigate the harmful effects of experimental noise. Secondly, it is capable of seamlessly verifying the correct operation of the difficult sampling task with exactly the same resources required to perform the sampling itself. This latter property is important, since the difficulty of the sampling generally makes it extremely hard to check whether or not a realistic device is actually achieving quantum supremacy. While previous works had satisfied one or the other of these properties, the current proposal uses the framework of measurement-based quantum computation and insights from the study of quantum phases of matter to simultaneously achieve both.
The full article can be found online at Phys. Rev. A 96, 062320 (2017).

(a) Quantum circuit to sample probability distributions related to certain Boolean functions. The task is expected to be intractable to modern computers. (b) Our measurement-based implementation, which realizes a sampling task and its verification procedure under the same resource requirement.

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.

 

 

Quantum Computation from Symmetric Quantum Matter

Jacob Miller and Akimasa Miyake have recently published a paper in Nature Partner Journal Quantum Information showing that quantum computers with distinct operational advantages can be built using a form of symmetric topological matter. Their work deals with measurement-based quantum computation (MQC), a way of powering computation with only single-spin measurements and a large entangled quantum “resource” state. MQC is a natural way to study the connection between different forms of quantum topological order, the types of quantum computation they can power, and what measurements are needed for this computation.

The authors show that two complementary forms of symmetric topological order are particularly useful for quantum computation. While previous resource states have only utilized the first form, a new Union Jack state is constructed to study the second, stronger form. While both are sufficient to achieve any desired computation, the quantum order found in the Union Jack state can achieve this using simpler measurements, pointing towards a surprising new resource for simplifying quantum computation.

The full article is available online at http://www.nature.com/articles/npjqi201636.

 

Do I smell percolation?
(Left) The Union Jack state, a new state with a strong form of symmetric quantum order which makes it useful for quantum computation. (Right) One step in the procedure for computing with the Union Jack state. Measurements on half of the sites drive the system through a quantum phase transition, which leads to a computationally useful graph state being randomly condensed on the unmeasured spins.

 

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]