2017 Talk Abstracts
Quantum supremacy
Scott Aaronson, Texas, Austin
(Session 10: Saturday from 8:30am - 9:15am)
In the near future, there will likely be special-purpose quantum computers with 40-50 high-quality qubits. In this talk, I'll discuss general theoretical foundations for how to use such devices to demonstrate "quantum supremacy": that is, a clear quantum speedup for *some* task, motivated by the goal of overturning the Extended Church-Turing Thesis (which says that all physical systems can be efficiently simulated by classical computers) as confidently as possible.
Techniques for scaling trapped-ion QIP
Christopher Ballance, Oxford
(Session 11: Saturday from 10:15am - 11:00am)
Demonstration experiments on small numbers of qubits are approaching the fidelity needed for large scale computation. However scaling these systems to the size needed to build a useful quantum computer presents significant challenges. We are mounting a two-pronged attack on these challenges for trapped-ion systems: using microwave control fields instead of lasers to simplify the control requirements, and pursing a networked modular scheme based on many simple nodes with complexity close to the current state of the art. Here we present the realization of high-fidelity single- and two-qubit gates (99.9999% and 99.7% respectively) driven with microwaves generated by electrodes embedded in the ion-trap chip, and discuss the potential for scaling this design. Furthermore, we present initial results on a modular architecture, involving mapping information from a memory qubit to an interface qubit, and from an interface qubit to a photon.
Experimentally generated random numbers certified by the impossibility of superluminal signaling
Peter Bierhorst, NIST Boulder
(Session 7: Friday from 11:00am - 11:30am)
Random numbers are an important resource for applications such as numerical simulation and secure communication. However, it is difficult to certify whether a physical random number generator is truly unpredictable. Here, we exploit the phenomenon of quantum entanglement in a loophole-free photonic Bell test experiment to obtain data containing randomness that cannot be predicted within any non-superdeterministic physical theory that does not also allow the sending of signals faster than the speed of light. To certify and quantify the randomness, we develop a new protocol that performs well in an experimental regime characterized by low violation of Bell inequalities. Applying an extractor function to our data, we obtain 256 new random bits, uniform to within 10-3.
Necessary adiabatic run times in quantum optimization
Lucas Brady, UC Santa Barbara
(Session 9c: Friday from 4:45pm - 5:15pm)
Quantum annealing is guaranteed to find the ground state of optimization problems in the adiabatic limit. Recent work [Phys. Rev. X 6, 031010 (2016)] has found that for some barrier tunneling problems, quantum annealing can be run much faster than is adiabatically required. Specifically, an n-qubit optimization problem was presented for which a non-adiabatic, or diabatic, annealing algorithm requires only constant runtime, while an adiabatic annealing algorithm requires a runtime polynomial in n. Here we show that this non-adiabatic speed-up is a direct result of a specific symmetry in the studied problems. In the more general case, no such non-adiabatic speed-up occurs. We furthermore show why the special case achieves this speed-up compared to the general case. We conclude with the observation that the adiabatic annealing algorithm has a necessary and sufficient runtime that is quadratically better than the standard quantum adiabatic condition suggests.
Quantum Gibbs sampling
Fernando Brandao, IQIM, Caltech
(Session 6: Friday from 8:30am - 9:15am)
Quantum thermal states, aka Gibbs states, are fundamental objects in physics. I will discuss recent results on the role of Gibbs states on quantum information theory. First I will show how we can use them to give quantum speed-ups for semidefinite programming. Then I will discuss how to prepare Gibbs states efficiently on a quantum computer.
Classical simulation of quantum circuits by dynamical localization: analytic results for Pauli-observable propagation in time-dependent disorder
Adrian Chapman, CQuIC, New Mexico
(Session 12: Saturday from 2:15pm - 2:45pm)
Matchgate circuits have been extensively studied due to (i) their classical simulability properties and (ii) their close connection to the physics of noninteracting fermions in one dimension. We extend (i) by introducing a classically efficient algorithm for the Lieb-Robinson commutator norm of a local observable under Heisenberg evolution by a nearest-neighbor matchgate circuit. This is surprising in light of the fact that the Heisenberg evolution itself cannot even be stored efficiently by a classical computer in general. We apply our result by (ii) to the study of fermions propagating through a one-dimensional lattice in the presence of spatio-temporally fluctuating disorder and demonstrate a method to classify this propagation into either the localized, diffusive, or ballistic dynamical phase. We find that our results coincide with known classifications of Anderson localization in the statically disordered case, but the localized phase can also be seen to survive in the presence of weakly fluctuating disorder. We expect our results to inspire the application of localization as a classical simulation technique for more general classes of quantum circuits.
Coupled layer construction of three dimensional topological codes with 'Fractons'
Xie Chen, IQIM, Caltech
(Session 12: Saturday from 1:30pm - 2:15pm)
Three dimensional quantum codes can exhibit exotic topological properties compared to their two dimensional counterparts. In two dimensions, topological excitations are point like and can move freely in space, inducing nontrivial braiding statistics as they wind around each other. In three dimensions, topological excitations can also be point like. But it has been discovered that sometimes their motion are highly restricted, such that they can only move in a sub-dimensional manifold or their motion are correlated with each other. Such point excitations are dubbed 'fractons' and in this talk we try to address the question of where they come from. We show that one class of 'fractons' can emerge by coupling layers of two dimensional topological codes and inducing a condensation of 'particles loops'. By making connections between the 'fracton' topological order and the more conventional two dimensional ones, we are able to generalize 'fractons' models beyond the stabilizer framework.
Quantum light matter interfaces using erbium doped yttrium orthosilicate
Ioana Craiciu, IQIM, Caltech
(Session 13: Saturday from 3:15pm - 3:45pm)
Rare earth quantum light-matter interfaces (QLMIs), consisting of optical resonators coupled to ensembles of rare earth ions, are uniquely suited for various quantum information applications, including quantum memories and quantum optical-to-microwave transducers. Among rare earths, erbium is particularly appealing due to its highly coherent resonance within a telecom band, allowing integration with existing optical communication technology and infrastructure. Micro-resonator QLMIs have various advantages over bulk rare earth crystals. They permit on-chip integration with other elements, such as microwave resonators for optical-to-microwave conversion. In the context of quantum memories, they provide enhanced coupling to the ions, and when the resonator is impedance matched to the ions, they can raise the theoretical memory efficiency to 100%. For spectral hole-burning based quantum memories, the coupling of rare earth ions to the resonator can provide improved memory initialization via Purcell enhancement of optical lifetimes. We present nano scale quantum light matter interfaces in erbium doped yttrium orthosilicate (Er:YSO). Our two types of devices take the form of nanobeam photonic crystal resonators milled directly into Er:YSO and of amorphous silicon ring resonators on Er:YSO. This latter hybrid design represents our newest efforts in a scalable on chip QLMI architecture. We have fabricated ring resonators with measured quality factors of over 10^5, and evanescent coupling to an ensemble of erbium ions characterized by a cooperativity of 0.54. The nanobeam resonator design has a measured quality factor of around 25,000, and a cooperativity of 2.4. We present simulation and experimental results of the optical properties of these cavities, and their coupling to erbium ions, including a demonstration of Purcell enhancement of the erbium telecom transition. We then analyze their potential as quantum memories.
Conditional mutual information of bipartite unitaries and scrambling
Dawei Ding, Stanford
(Session 6: Friday from 9:15am - 9:45am)
One way to diagnose chaos in bipartite unitary channels is via the tripartite information of the corresponding Choi state, which for certain choices of the subsystems reduces to the negative conditional mutual information (CMI). We study this quantity from a quantum information-theoretic perspective to clarify its role in diagnosing scrambling. When the CMI is zero, we find that the channel has a special normal form consisting of local channels between individual inputs and outputs. However, we find that arbitrarily low CMI does not imply arbitrary proximity to a channel of this form, although it does imply a type of approximate recoverability of one of the inputs. When the CMI is maximal, we find that the residual channel from an individual input to an individual output is completely depolarizing when the other input is maximally mixed. However, we again find that this result is not robust. We also extend some of these results to the multipartite case and to the case of Haar-random pure input states. Finally, we look at the relationship between tripartite information and its Renyi-2 version which is directly related to out-of-time-order correlation functions. In particular, we demonstrate an arbitrarily large gap between the two quantities.
Speed limits for quantum control of local spin systems
Jeffrey Epstein, UC Berkeley
(Session 2: Thursday from 11:45am - 12:15pm)
We show that the fundamental limits on quantum many-body dynamics from the Lieb-Robinson bound yield speed limits on two quantum control tasks, state transfer and entanglement sharing. We derive analytic speed limits on these tasks in nearest-neighbor coupled spin chains and lattices, providing optimal speeds for comparison with numerical optimal control results in the many-body setting.
Protecting quantum information from noise -- a passive approach
Ryan Epstein, Northrop Grumman
(Session 9a: Friday from 5:15pm - 5:45pm)
The steady improvement in coherence times and gate fidelities over the past several years has largely been due to reductions in noise and energy loss mechanisms. Achieving highly integrated quantum hardware, however, may necessitate tolerance of noisier signals and dirtier materials. Over the past couple of years, we have been looking at practical ways to design noise-resilience into quantum devices. In this talk, I’ll present theoretical work on methods for performing gates that are robust to control noise and that reduce qubit overhead and coupling complexity, building off of Bacon and Flammia’s Adiabatic Gate Teleportation technique. I’ll also talk about more fully noise-protected qubits and gates using blocks of qubits coupled together in Bacon-Shor-like codes.
Four wave mixing in a cold atomic ensemble for the generation of correlated photons pairs
Andrew Ferdinand, CQuIC, New Mexico
(Session 13: Saturday from 3:45pm - 4:15pm)
Photon pairs generated by spontaneous four-wave mixing (FWM) in atomic ensembles provide a natural path toward quantum light-matter interfaces due to their intrinsic compatibility with atomic quantum memories. These photons are narrow band and have frequencies at or close to atomic resonances, and their temporal and spectral properties can be efficiently tailored to make them compatible with specific quantum memory protocols [1]. In addition, conservation of orbital angular momentum (OAM) in the FWM process enables the generated photons to form entangled qudits, which have applications in high-dimensional quantum information and communication. We study experimentally the generation of light from FWM in a cold ensemble of cesium atoms. We investigate theoretically the correlation and distribution of OAM modes occupied by photon pairs produced in spontaneous FWM as a function of experimentally accessible parameters of the process. These studies provide the basis for future investigations of photonic OAM correlation generated with FWM in atomic ensembles. [1] Du et al., Phys. Rev. Lett. 100, 183603. (2008)
Secrets of PRL
Robert Garisto, Physical Review Letters, APS
(Session 14: Saturday from 5:30pm - 6:15pm)
I'll give a view inside of Physical Review Letters (PRL). I'll describe how the review process works, how we pick our highlighted papers, and how things have changed since -- notably how we are asking more of authors, referees, and ourselves. I will also present some illuminating statistics and talk about journal metrics. If there is time, I will give you some advice on how to succeed with your PRL submissions.
Single-shot quantum resource theories
Gilad Gour, Calgary
(Session 9b: Friday from 5:45pm - 6:15pm)
One of the main goals of any resource theory such as entanglement, quantum thermodynamics, quantum coherence, and asymmetry, is to find necessary and sufficient conditions (NSC) that determine whether one resource can be converted to another by the set of free operations. In this talk I will present such NSC for a large class of quantum resource theories which we call affine resource theories (ARTs). ARTs include the resource theories of athermality, asymmetry, and coherence, but not entanglement. Remarkably, the NSC can be expressed as a family of inequalities between resource monotones (quantifiers) that are given in terms of the conditional min entropy. The set of free operations is taken to be (1) the maximal set (i.e. consists of all resource non-generating (RNG) quantum channels) or (2) the self-dual set of free operations (i.e. consists of all RNG maps for which the dual map is also RNG). As an example, I will discuss the applications of the results to quantum thermodynamics with Gibbs preserving operations, and several other ARTs. Finally, I will discuss the applications of these results to resource theories that are not affine.
QInfer: Statistical inference software for quantum applications
Christopher Granade, Sydney
(Session 9b: Friday from 3:45pm - 4:15pm)
Characterizing quantum systems through experimental data is critical to applications as diverse as metrology and quantum computing. Analyzing this experimental data in a robust and reproducible manner is made challenging, however, by the lack of readily-available software for performing principled statistical analysis. In this talk, we introduce an open-source library, QInfer, to address this need and to improve the robustness and reproducibility of characterization experiments. We will show examples of how our library makes it easy to analyze data from tomography, randomized benchmarking, and Hamiltonian learning experiments either in post-processing, or online as data is acquired. We will discuss how QInfer also provides functionality for predicting the performance of proposed experimental protocols from simulated runs. By delivering easy-to-use characterization tools based on principled statistical analysis, QInfer helps address many outstanding challenges facing quantum technology. All source code and examples for this talk may be found online at qinfer.org.
Fundamental percolation thresholds for ballistic linear optical quantum computing
Saikat Guha, Raytheon BBN Technologies
(Session 8: Friday from 1:30pm - 2:15pm)
Any quantum algorithm can be implemented by an adaptive sequence of single node measurements on an entangled cluster of qubits in a square lattice topology. Photons are a promising candidate for encoding qubits but assembling a photonic entangled cluster with linear optical elements relies on probabilistic operations.
Improved spin squeezing of an atomic ensemble through internal state control
Daniel Hemmer, Arizona
(Session 1: Thursday from 9:30am - 10:00am)
Squeezing of collective atomic spins is typically generated by quantum backaction from a QND measurement of the relevant spin component. In this scenario the degree of squeezing is determined by the measurement resolution relative to the quantum projection noise (QPN) of a spin coherent state (SCS). When starting from a SCS our current experiment generates ~3dB of metrologically relevant spin squeezing, closely matching theoretical predictions. Going forward, our main objective is to use control of the internal atomic spin to improve squeezing. For example, we can coherently map the internal spins from the SCS to a “cat” state, which increases the QPN by a factor of 2f=8 relative to the SCS [1]. This leads to increased backaction and entanglement produced by our QND measurement. The squeezing generated in the cat state basis can in principle be mapped back to the SCS basis where it will correspond to squeezing of the physical spin. A preliminary experimental result suggests that up to 8dB of metrologically useful squeezing can be generated in this way. However, more complex internal state preparation brings additional vulnerability to control errors. The main source of error for internal state control in our experiment appears to be fluctuating background magnetic fields at frequencies up to tens of kHz. We are currently developing a toolbox of composite pulses in order to diagnose and compensate for their presence. [1] L.M. Norris et al., Phys. Rev. Lett. 109, 173603 (2012)
Decoherence-free quantum computing in Kondo-coupled optical tweezers
Leonid Isaev, JILA, NIST, CU Boulder
(Session 8: Friday from 2:15pm - 2:45pm)
We propose a basis for decoherence-free quantum computing that uses neutral atoms and encodes qubits in the collective atomic spin and motional degrees of freedom. The physical qubit consists of three spin-\(\frac{1}{2}\) atoms in a double-well, two localized in the lowest vibrational mode and one atom in an excited delocalized state, subject to a staggered Zeeman field whose direction is opposite in the two traps. An interplay between this field gradient and exchange interactions gives rise to a local singlet-triplet degeneracy, and defines a logical qubit subspace. For strong interactions this subspace enjoys full protection against longitudinal magnetic-field noise, and is protected by an energy gap against transverse spin-flipping perturbations. Arbitrary single-qubit rotations are performed by virtue of resonant transfer of two-atom singlet-triplet states between the wells. Moreover, a two-qubit entangling control-z gate can be implemented. We design a qubit initialization protocol that employs Landau-Zener adiabatic tunneling to efficiently create a spin-singlet state in one well, and argue that our proposal can be realized using optical tweezers to create the double-well, hyperfine states of bosonic \(^{87} {\rm Rb}\) atoms to implement spin degrees of freedom, and laser-induced AC Stark shifts to impose the Zeeman field gradient.
A circuit-based quantum search algorithm driven by transverse fields
Zhang Jiang, NASA Ames
(Session 9c: Friday from 3:45pm - 4:15pm)
We designed a quantum search algorithm, giving the same quadratic speedup achieved by Grover's original algorithm; we replace Grover's diffusion operator (hard to implement) with a product diffusion operator generated by transverse fields (easy to implement). In our algorithm, the problem Hamiltonian (oracle) and the transverse fields are applied to the system alternatively. We construct such a sequence that the corresponding unitary generates a closed transition between two states; one has a big overlap with the initial state (even superposition of all states), and the other has a big overlap with the target state. Let N = 2n be the size of the search space. The transition rate is of order O(N-1/2), yielding a O(N1/2) algorithm. Our algorithm belongs to a class of algorithms recently proposed by Farhi et al., namely the Quantum Approximate Optimization Algorithm (QAOA).
Sub-shot noise measurement strategies for precision atomic sensors
Mark Kasevich, Stanford
(Session 1: Thursday from 8:45am - 9:30am)
Bounding the costs of quantum simulation of many-body physics in real space
Ian Kivlichan, Harvard
(Session 4: Thursday from 4:00pm - 4:30pm)
Simulating many-particle dynamics, such as first-quantized quantum chemistry, with logarithmic dependence on the accuracy has proven to be a challenge. This is because the traditional approach, based on the quantum Fourier transform, introduces Hamiltonians with large max-norms. We solve this problem by using a new approach based on high-order finite difference formulae. This change makes the approach practical, and we further demonstrate that it can simulate n interacting particles using Õ(n^4) calculations of the pairwise interactions for a fixed spatial grid spacing, versus the Õ(n^5) time required by previous methods, assuming the number of particles is proportional to the number of orbitals. We also show that previous work has overlooked the fact that discretization errors can remove these exponential speedups, and address this by providing bounds on the discretization error and sufficient conditions to guarantee efficiency.
Squeezed state ansatz for quantum Sherrington-Kirkpatrick model and its applications to quantum annealing
Sergey Knysh, NASA Ames
(Session 9c: Friday from 5:15pm - 5:45pm)
A question of fundamental importance in the physics of quantum annealing is its scalability. Recent work predicts a crossover from polynomial to exponential complexity for quantum annealing of spin glasses and relates the problem size at which this occurs to the "density" of spin glass bottlenecks [1]. An exact solution has been obtained for a toy problem, but rigorous analysis has remained elusive for realistic spin glass models where naive mean field fails. The present work takes a step in that direction by investigating thermodynamics of quantum Sherrington-Kirkpatrick model without resorting to replicas. The approach uses hard-core boson representation of a spin-1/2 model, with "modes" corresponding to delocalized eigenvectors of the interaction matrix. Hard-core nature of bosons is taken into account by appropriate renormalization factors. In this formulation, the ground state of paramagnetic phase is approximated by applying mode-dependent amount of squeezing/anti-squeezing to a vacuum, and the low-energy excitations correspond to Bogolyubov quasiparticles. Spin-glass phase is characterized by macroscopic occupation of a finite fraction of modes. Theoretical predictions are compared with known numerical results. [1] S. Knysh, "Zero-temperature quantum annealing bottlenecks in the spin-glass phase", Nature Communications 7, 12370 (2016).
Quantum optimal control of superconducting circuits
Christiane Koch, Kassel
(Session 2: Thursday from 10:30am - 11:15am)
Quantum optimal control has grown into a versatile tool for quantum technology. Its key application is to identify performance bounds, for tasks such as state preparation or quantum gate implementation, within a given architecture. One such bound is the quantum speed limit, which determines the shortest possible duration to carry out the task at hand. Typical examples include the creation of entanglement or quantum error correction. To date, these tasks have been optimized for known, fixed parameters of the system. I will show that a fully numerical quantum optimal control approach can go even further and, using the most advanced control techniques, map out the entire parameter landscape for two superconducting transmon qubits. This allows to determine the global quantum speed limit for a universal set of gates with gate errors limited solely by the qubit lifetimes. While the interaction of qubits with their environment is typically regarded as detrimental, this does not need to be the case. I will show that the back-flow of amplitude and phase encountered in non-Markovian dynamics can be exploited to carry out quantum control tasks for a superconducting circuit that could not be realized if the system was isolated. The control is facilitated by a few strongly coupled, sufficiently isolated environmental modes. These can be found in a variety of solid-state devices other than superconducting circuits, for example in color centers in nanodiamonds or nanomechanical oscillators.
Random bosonic states for robust quantum metrology
Jan Kolodynski, ICFO
(Session 9b: Friday from 4:45pm - 5:15pm)
We study how useful random states are for quantum metrology, i.e., whether they surpass the classical limits imposed on precision in the canonical phase sensing scenario. First, we prove that random pure states drawn from the Hilbert space of distinguishable particles typically do not lead to superclassical scaling of precision even when allowing for local unitary optimization. Conversely, we show that random pure states from the symmetric subspace typically achieve the optimal Heisenberg scaling without the need for local unitary optimization. Surprisingly, the Heisenberg scaling is observed for random isospectral states of arbitrarily low purity and preserved under loss of a fixed number of particles. Moreover, we prove that for pure states, a standard photon-counting interferometric measurement suffices to typically achieve resolutions following the Heisenberg scaling for all values of the phase at the same time. Finally, we demonstrate that metrologically useful states can be prepared with short random optical circuits generated from three types of beam splitters and a single nonlinear (Kerr-like) transformation.
Three-dimensional color code thresholds via statistical-mechanical mapping
Aleksander Kubica, IQIM, Caltech
(Session 9a: Friday from 4:15pm - 4:45pm)
The color code is an example of a topological quantum error-correcting code which recently has gained a lot of attention due to achieving universality without magic-state distillation in three dimensions. Also, the color code illustrates a new and exciting idea of single-shot error correction which might drastically reduce time overhead of quantum computation. In this work we find fundamental bounds on the error-correcting capabilities of the three-dimensional color code, namely the threshold for optimal error correction of bit-flip/phase-flip noise with perfect measurements on the body-centered cubic lattice. In particular, the threshold associated with string-like (one- dimensional) and sheet-like (two-dimensional) logical operators is p_1 ≃ 1.9% and p_2 ≃ 27.5%, respectively. The aforementioned results were obtained by exploiting a connection between error correction and statistical mechanics. We performed parallel tempering Monte Carlo simulations of two previously unexplored three-dimensional statistical-mechanical models: the 4-body and the 6- body random coupling Ising models. We find their phase diagrams in terms of disorder strength and temperature. Our results put constraints on the practical use of the color code from the viewpoint of efficient decoders and bounding overhead.
Preparation and coherent manipulation of pure quantum states of a single molecular ion
Dietrich Leibfried, NIST, Boulder
(Session 11: Saturday from 11:00am - 11:30am)
An amazing level of control is routinely reached in modern experiments with atoms, but similar control over molecules has been an elusive goal. We recently proposed a method based on quantum logic spectroscopy [1] to address this problem for a wide class of molecular ions [2]. We have now realized the basic elements of this proposal. In our demonstration, we trap a calcium ion together with a calcium hydride ion (CaH+) that is a convenient stand-in for more general molecular ions. We cool the two-ion crystal to its motional ground state and then drive the motional sidebands of Raman transitions in the molecular ion. A transition of the molecule is indicated by a single quantum of excitation in the secular motion of the pair. We can efficiently detect this single quantum with the calcium ion, which projects the molecule into the final state of the attempted sideband transition, leaving the molecule in a known, pure quantum state. The molecule can be coherently manipulated after the projection, and its final state read out by another quantum logic state detection. We demonstrate this by driving Rabi oscillations between rotational states. All transitions we address in the molecule are driven by a single, far off-resonant continuous-wave laser. This makes our approach applicable to control and precision measurement of a large class of molecular ions. Other QI projects in the NIST Ion Storage group will be briefly summarized. [1] P.O. Schmidt, et al. Science 309, 749 (2005) [2] D. Leibfried, New J. Phys. 14, 023029 (2012) *supported by ARO, IARPA, ONR, and the NIST Quantum Information program
Quantum simulation of complex dynamics in a quantum kicked top
Nathan Lysne, Arizona
(Session 4: Thursday from 3:30pm - 4:00pm)
Recent advances in quantum control have enabled analog quantum simulation (AQS) as a means to study phase changes, order, and other complex many body phenomena. However, as experimental AQS grows in sophistication, new questions arise about our ability to verify the validity of a given simulation. In the absence of error correction, investigating the effects of imperfections on dynamics that is potentially chaotic and hypersensitive to errors is thus essential to understanding how much and in which ways we can trust AQS. The quantum kicked top (QKT) is an ideal model for such studies. We discuss results from recent experiments that use the d = 16 dimensional hyperfine manifold in the 6S1/2 electronic ground state of an individual Cs atom for AQS of a QKT with spin J = 15/2. As a baseline, we see close agreement between simulated and predicted dynamics in a mixed phase space over many tens of kicks. Prior work has shown the QKT dynamics reflects the separation between stable islands and sea of chaos in the classical QK, even in situations where the “fidelity” of the evolving QKT quantum state is poor. This suggests the former represents a “global” property that can be reliably simulated in the presence of errors, even when the microscopic behavior (the quantum state) cannot. We present data from experiments and numerical simulations in the presence of deliberately applied errors, showing that the frequency content of the perturbation plays a central role in the validity and robustness of AQS.
Phase-tuned entangled state generation between distant spin qubits
Clemens Matthiesen, UC Berkeley, Cambridge
(Session 13: Saturday from 4:45pm - 5:15pm)
Entanglement is the central resource in quantum information processing, sensing and communication. Distribution of entanglement through non-local interactions, using photon interference and detection, is an attractive feature of flexible computation architectures where spatially separate nodes are locally controlled and connected via photonic channels. I will present very recent work from the Atatüre group in Cambridge on the generation of controllable entangled states between two electron spins confined in optically active indium-gallium-arsenide (InGaAs) quantum dots (QD) situated metres apart. The combination of a minimal single-photon state-projection scheme and the strong coherent light-matter interaction in these systems enables a distant entanglement rate of 7.3 kHz, the highest reported to date. With full control over the single-photon interference, we demonstrate the creation of entangled states with arbitrary phase. In the outlook I will discuss some limiting features of this semiconductor system [1], and highlight alternative venues for electron spin qubits trapped in vacuum [2]. [1] R. Stockill et al., Nature Comms 7, 12745 (2016). [2] P. Peng, C. Matthiesen, H Häffner, arXiv:1611.00130 [quant-ph] (2016).
Faithful conversion of propagating quantum bits to mechanical motion
Karl Mayer, CU Boulder; NIST
(Session 3: Thursday from 2:30pm - 3:00pm)
Electromechanical devices are emerging as quantum information processing elements for superconducting circuits. By using a mechanical oscillator parametrically coupled to a microwave resonant circuit, these devices can store, amplify, and frequency-convert microwave fields. Experimental efforts to convert microwave fields to mechanical motion have so far been mostly limited to Gaussian states, such as coherent or squeezed states. We present experiments that demonstrate and characterize the conversion of non-Gaussian states, namely propagating microwave qubits prepared in mixed single-photon and superposition states. We perform state tomography to infer the density matrices for both the input states and the mechanical states after conversion, and compute the average fidelity for this conversion process to be in excess of 80%.
Scaling superconducting qubits: Toward a demonstration of quantum supremacy
Matthew Neeley, Google
(Session 3: Thursday from 1:45pm - 2:30pm)
Our group has proposed an experiment to demonstrate "Quantum Supremacy", using a quantum device to perform a well-defined computational task that cannot be done in reasonable time on even the largest classical supercomputers (arXiv:1608.00263). This will require a device with 49 qubits arranged in a 7-by-7 grid and one- and two-qubit gate error rates below about 0.1%. I will outline our experimental progress toward this goal and describe the challenges associated with scaling superconducting qubit devices to the level of several tens of qubits and beyond.
Quantum error correction of reference frame information
Sepehr Nezami, Stanford
(Session 9a: Friday from 5:45pm - 6:15pm)
The existence of quantum error correcting codes is one of the most counterintuitive and potentially technologically important discoveries of quantum information theory. But standard error correction refers to abstract quantum information, i.e. information that is independent from the physical incarnation of the systems used for storing the information. There are, however, other forms of information that are physical, one of the most ubiquitous being reference frame information. Here we analyze the problem of error correcting physical information. The basic question we seek to answer is whether such error correction is possible, and, if yes, the limitations to which it is subjected. The issue is highly nontrivial given the fact that the systems that need to be used for transmitting physical information must contain the physical quantity we are interested in, so all actions applying to them, including the encoding/decoding necessary for error correction, are subjected to limiting constraints.
Spectroscopy of quantum and non-Gaussian noise
Leigh Norris, Dartmouth
(Session 10: Saturday from 9:15am - 9:45am)
Precisely characterizing the decoherence effects arising from coupling to a noisy environment is essential for designing optimized error correction strategies and validation protocols for realistic quantum information processors. This challenge has prompted much of the recent interest in quantum noise spectroscopy, which seeks to estimate the spectral properties of noise affecting a target quantum system. Despite considerable theoretical and experimental advances, this effort has largely been confined to the case of classical, Gaussian phase noise on a single qubit. We overcome these limitations by introducing quantum noise spectroscopy protocols for both quantum and non-Gaussian phase noise. For realistic systems that include a pair of excitons coupled to a phonon bath and a qubit undergoing quadratic dephasing at an optimal point, we numerically demonstrate reconstruction of the asymmetric spectra unique to quantum environments and the polyspectra associated with higher order cumulants of non-Gaussian noise. In both cases, spectral reconstructions enable us to accurately predict the dynamics of qubits coupled to these noise sources. In addition to the practical value in characterizing a larger class of noise processes, this work highlights dynamical and spectral signatures unique to quantum and non-Gaussian noise sources.
Optimal digital dynamical decoupling for general decoherence via Walsh modulation
Haoyu Qi, Louisiana State
(Session 9a: Friday from 4:45pm - 5:15pm)
We provide a general framework for constructing digital dynamical decoupling sequences based on Walsh modulation, applicable to arbitrary qubit decoherence scenarios. Building on the equivalence between the Walsh formalism and the recently introduced concatenated-projection approach, we identify a family of optimal Walsh sequences which can be exponentially more efficient, in terms of the required total pulse number for fixed cancellation order, than known sequences based on concatenated design. Optimal sequences for a given cancellation order are highly non-unique, their performance depending sensitively on the control path. We provide an analytical upper bound to the achievable decoupling error, and argue how suitable path-optimized sequences within the optimal Walsh family can substantially outperform concatenated decoupling, while respecting realistic timing constraints. We validate these conclusions by numerically computing the average fidelity in a toy model capturing the essential feature of hyperfine-induced decoherence in a quantum dot.
Spin squeezing on nanophotonic waveguides
Xiaodong XQi, CQuIC, New Mexico
(Session 8: Friday from 2:45pm - 3:15pm)
Strong coupling between atoms and photons is a prerequisite for quantum information processing protocols ranging from quantum metrology to quantum communication and computation. This strong coupling effect can be achieved using nanophotonic waveguides whereby an ensemble of atoms are trapped in the evanescent field. In this talk, I will present our recent progress in the theoretical study of implementing spin squeezing using optical nanofibers (ONF) and square waveguides (SWG) with both birefringence and Faraday interactions as QND measurement. Various geometries of protocols will be discussed based on the analysis of optical depth per atom on ONF and SWG platforms. In calculating the spin squeezing parameter, we have established a set of stochastic master equations to describe the individual and collective spin dynamics. Our simulation shows that ~10 dB of spin squeezing can be reached with a few thousands of atoms on these nanophotonic waveguides. Using the fundamental TE and TM modes, the SWG could generate more spin squeezing compared to the ONF platform. Our result can be generalized to other nanophotonic platforms, for the implementation of non-Gaussian states, and to improve quantum sensing precision using spin squeezing techniques.
Factoring using 2n+2 qubits with Toffoli based modular multiplication
Martin Roetteler, Microsoft
(Session 9c: Friday from 5:45pm - 6:15pm)
We describe an implementation of Shor's quantum algorithm to factor n-bit integers using only 2n+2 qubits. In contrast to previous space-optimized implementations, ours features a purely Toffoli based modular multiplication circuit. The circuit depth and the overall gate count are in O(n^3) and O(n^3 log(n)), respectively. We thus achieve the same space and time costs as Takahashi et al., while using a purely classical modular multiplication circuit. As a consequence, our approach evades most of the cost overheads originating from rotation synthesis and enables testing and localization of faults in both, the logical level circuit and an actual quantum hardware implementation. We implemented and simulated a Toffoli network for the entire controlled modular multiplication piece of Shor's algorithm in LIQUi|>, for real-world bit-sizes of up to 8,192. Asymptotically, our new (in-place) constant-adder, which is used to construct the modular multiplication circuit, uses only dirty ancilla qubits and features a circuit size and depth in O(n log(n)) and O(n), respectively. Our resource estimates determine also the constants for the scaling of the circuit size.
Experimental demonstration of robust phase estimation near the Heisenberg limit
Kenneth Rudinger, Sandia
(Session 11: Saturday from 11:30am - 12:00pm)
High-fidelity gate operations are one of many requirements for full-scale quantum computation. A variety of benchmarking and tomographic protocols have been developed to aid in the characterization and improvement of these operations. In this talk, we will discuss robust phase estimation (RPE), a particular protocol that can be used to learn the phases of quantum operations to very high accuracy. Unlike many other phase estimation protocols, RPE requires no ancillae nor near-perfect state preparation or measurement. We demonstrate the first published experimental implementation of RPE on a single-qubit system (a trapped Yb\(^+\) ion), and use it to learn the phases of X and Y rotations to within \(\sim10^{-4}\) radians. This accuracy requires only 352 experimental samples per phase, and exhibits Heisenberg-like scaling. We also explore how this accuracy appears to outperform the original theoretical bounds on RPE. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04 94AL85000.
Investigating the quantum approximate optimization algorithm's advantage over classical algorithms
Ciaran Ryan-Anderson, CQuIC New Mexico, Sandia
(Session 9c: Friday from 4:15pm - 4:45pm)
The Quantum Approximate Optimization Algorithm (QAOA) is designed to find approximate solutions to combinatorial optimization problems. The approximation quality of QAOA is a function of the parameters to the algorithm, one of which corresponds to the depth of a quantum circuit realizing QAOA. Recently, in [1], it has been shown that even when QAOA is used in its lowest-depth form, it can produce distributions that are hard to sample from classically. This indicates that QAOA can demonstrate some level of ``Quantum Supremacy," at least for the task of sampling from a distribution. However, QAOA is foremost an optimization algorithm, and QAOA's complexity as an optimization algorithm is largely open. In this work we investigate QAOA's advantage over classical algorithms from an optimization perspective. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] E. Farhi and A. W. Harrow, Quantum supremacy through the quantum approximate optimization algorithm, (2016), arXiv:1602.07674.
On chip nonlinear quantum devices
Linda Sansoni, Paderborn
(Session 7: Friday from 10:15am -11:00am)
In the last years the challenge of showing quantum supremacy has greatly attracted the interest of the scientific community. In this context the adoption of integrated photonic platforms has shown a great potential to finally confirm the advantage of using quantum resources compared to classical ones. Integrated photonics is indeed an optimal candidate for the experimental implementation of highly complex and compact quantum circuits. Despite the enormous development in this field, one of the major issues still remains a reliable and efficient generation of quantum states of light. Integrated waveguide sources have suitable features for this purpose as high brightness and stability. Nevertheless requirements as generation of light on different spatial modes or the possibility to operate the devices outside the lab, are still a major challenge. Here we present how we address these challenges by exploiting new waveguide designs in lithium niobate substrates and the adoption of fiber-hybrid technology. Our devices range from multichannel sources of entangled states to a fully plug and play source of heralded single photons. With these achievements we bring the quantum technology to a next level of development and a step closer to the adoption of a fully integrated platform for quantum information applications.
Attainability of the quantum information bound in pure state models
Fabricio Toscano, Instituto de Fisica, Universidade Federal do Rio de Janeiro (UFRJ), Brasil
(Session 9b: Friday from 5:15pm - 5:45pm)
The attainability of the quantum Cramer-Rao bound [QCR], the ultimate limit in the precision of the estimation of a physical parameter, requires the saturation of the quantum information bound [QIB]. This occurs when the Fisher information associated to a given measurement on the quantum state of a system which encodes the information about the parameter coincides with the quantum Fisher information associated to that quantum state. Braunstein and Caves [PRL 72, 3439 (1994)] have shown that the QIB can always be achieved via a projective measurement in the eigenvectors basis of an observable called symmetric logarithmic derivative. However, such projective measurement depends, in general, on the value of the parameter to be estimated. Requiring, therefore, the previous knowledge of the quantity one is trying to estimate. For this reason, it is important to investigate under which situation it is possible to saturate the QCR without previous information about the parameter to be estimated. Here, we show the complete solution to the problem of which are all the initial pure states and the projective measurements that allow the global saturation of the QIB, without the knowledge of the true value of the parameter, when the information about the parameter is encoded in the system by a unitary process.
Detecting non-Markovian effects in quantum computing architectures
Andrzej Veitia, Oregon
(Session 9b: Friday from 4:15pm - 4:45pm)
We present a family of tests for detection of non-Markovian effects in quantum gate sequences. A central feature of our method is its insensitivity to state preparation and measurement errors (SPAM). Although our method is not scalable, we will discuss its application to few-qubit systems as a means of detecting spatial correlations in a quantum computing architectures
Chained Bell inequality experiment with high-efficiency measurements
Yong Wan, NIST
(Session 7: Friday from 11:30am - 12:00pm)
Recent Bell test experiments that have demonstrated violation of Bell inequalities, have successfully falsified theories of local realism [1-3]. A chained Bell inequality experiment [4], a generalization of the standard Clauser-Horne-Shimony-Holt experiment, utilizes 2N different pairs of measurement settings. The correlations observed in such an experiment can be modeled as a mixture of a local-realistic distribution and a “non-local” distribution that maximally violates the inequality. Using a chained Bell inequality, one can set an upper limit on the fraction of the mixture that satisfies local realism [5-7]. Here, we describe a chained Bell inequality experiment on trapped ions. An entangled pair of trapped Be+ ions is generated using a Mølmer-Sørensen gate [8]. The individual measurement settings are randomized and applied to the ions via single qubit operations. The ions are measured individually with high efficiency. We quantify the local-realistic fraction to be below 0.327 at the 95% confidence level without the fair-sampling or independent-and-identical-distributions assumptions. This work was supported by IARPA, ONR, and the NIST Quantum Information Program. [1] B. Hensen et al., Nature 526, 682 (2015). [2] L. K. Shalm et al, Phys. Rev. Let. 115 250402 (2015). [3] M. Giustina et al., Phys. Rev. Let. 115 250401 (2015). [4] P. M. Pearle, Phys. Rev. D 2, 1418 (1970). [5] A. C. Elitzur, S. Popescu, and D. Rohrlich, Phys. Lett. A 162, 25 (1992). [6] J. Barrett, A. Kent, and S. Pironio, Phys. Rev.Lett. 97, 170409 (2006). [7] P. Bierhorst, J. Phys. A: Math. Theor. 49 215301 (2016). [8] J. P. Gaebler et al., Phys. Rev. Lett. 117, 060505 (2016).
Experimental time-optimal universal control of spin qubits in solids
Xiaoting Wang, Louisiana State
(Session 2: Thursday from 11:15am - 11:45am)
Quantum control of systems plays an important role in modern science and technology. The ultimate goal of quantum control is to achieve high-fidelity universal control in a time-optimal way. Although high-fidelity universal control has been reported in various quantum systems, experimental implementation of time-optimal universal control remains elusive. Here, we report the experimental realization of time-optimal universal control of spin qubits in diamond. By generalizing a recent method for solving quantum brachistochrone equations [X. Wang et al., Phys. Rev. Lett. 114, 170501 (2015)], we obtained accurate minimum-time protocols for multiple qubits with fixed qubit interactions and a constrained control field. Single- and two-qubit time-optimal gates are experimentally implemented with fidelities of 99% obtained via quantum process tomography. Our work provides a time-optimal route to achieve accurate quantum control and unlocks new capabilities for the emerging field of time-optimal control in general quantum systems.
Elucidating reaction mechanisms on quantum computers
Nathan Wiebe, Microsoft
(Session 4: Thursday from 4:30pm - 5:00pm)
It is well known that quantum simulation promises exponential speedups for finding full configuration interaction (FCI) solutions for quantum chemistry over the best known classical algorithms. But when will this be useful? How large or a quantum computer will we need to achieve this? Here we provide estimates that show that a reasonable sized quantum computer can be used to help understand how biological nitrogen fixation works, which is a problem that requires an FCI solution. This understanding could lead to a new generation of energy efficient methods for making fertilizer that would be significant industrially. Our work considers the overheads of fault tolerance and circuit synthesis and also introduces fundamentally new circuits for simulating chemical dynamics with lower depth and introduces new methods for parallelizing phase estimation over independent quantum computers. These latter contributions help address the biggest drawback of non-variational quantum eigensolvers: their inability to be parallelized.
The surface code with a twist
Theodore Yoder, MIT
(Session 9a: Friday from 3:45pm - 4:15pm)
The surface code is one of the most successful approaches to topological quantum error-correction. It boasts the smallest known syndrome extraction circuits and correspondingly largest thresholds. Defect-based logical encodings of a new variety called twists have made it possible to implement the full Clifford group without state distillation. Here we investigate a patch-based encoding involving a modified twist. In our modified formulation, the resulting codes, called triangle codes for the shape of their planar layout, have only weight-four checks and relatively simple syndrome extraction circuits that maintain a high, near surface-code-level threshold. They also use 25% fewer physical qubits per logical qubit than the surface code. Moreover, benefiting from the twist, we can implement all Clifford gates by lattice surgery without the need for state distillation. By a surgical transformation to the surface code, we also develop a scheme of doing the same gates on surface code patches in an atypical planar layout, though with less qubit efficiency than the triangle code. Finally, we remark that logical qubits encoded in triangle codes are naturally amenable to logical tomography, and the smallest triangle code can demonstrate high-pseudothreshold fault-tolerance to depolarizing noise using just 13 physical qubits.
A nanophotonic platform integrating quantum memories and single qubits based on rare-earth ions
Tian Zhong, IQIM, Caltech
(Session 13: Saturday from 4:15pm - 4:45pm)
The integration of rare-earth ions in an on-chip photonic platform would enable quantum repeaters and scalable quantum networks. Here we demonstrate a nanophotonic platform consisting of yttrium vanadate (YVO) photonic crystal nanobeam resonators coupled to a spectrally dilute ensemble of Nd ions. The cavity acts as a memory when prepared with spectral hole burning, meanwhile it permits addressing of single ions. For quantum memory, atomic frequency comb (AFC) protocol was implemented in a Nd:YVO nanocavity cooled to 475 mk. We measure an efficiency at 2% at a storage time of ~100 ns with an efficient WSi superconducting nanowire detector (SNSPD). The small mode volume of the cavity results in a peak atomic spectral density of <10 ions per homogeneous linewidth, suitable for probing single ions when detuned. The high-cooperativity coupling of a single ion yields a strong signature (20%) in the cavity reflection spectrum. We estimate a signal-to-noise ratio exceeding 10 for addressing a single Nd ion. This, combines with the AFC memory, constitutes a promising platform for preparation, storage and detection of rare-earth qubits on the same chip.
