Publications

@article{Frey2020,

title = {Simultaneous Spectral Estimation of Dephasing and Amplitude Noise on a Qubit Sensor via Optimally Band-Limited Control},

author = {Virginia Frey and Leigh M. Norris and Lorenza Viola and Michael J. Biercuk},

url = {https://link.aps.org/doi/10.1103/PhysRevApplied.14.024021},

doi = {10.1103/PhysRevApplied.14.024021},

year  = {2020},

date = {2020-08-01},

journal = {Phys. Rev. Appl.},

volume = {14},

issue = {2},

pages = {024021},

publisher = {American Physical Society},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{WOS:000559297800001,

title = {Simultaneous Spectral Estimation of Dephasing and Amplitude Noise on a 

 Qubit Sensor via Optimally Band-Limited Control},

author = {Virginia Frey and Leigh M. Norris and Lorenza Viola and Michael J. Biercuk},

doi = {10.1103/PhysRevApplied.14.024021},

issn = {2331-7019},

year  = {2020},

date = {2020-08-01},

journal = {PHYSICAL REVIEW APPLIED},

volume = {14},

number = {2},

abstract = {The fragility of quantum systems makes them ideally suited for sensing 

 applications at the nanoscale. However, interpreting the output signal 

 of a qubit-based sensor is generally complicated by background clutter 

 due to out-of-band spectral leakage, as well as ambiguity in signal 

 origin when the sensor is operated with noisy hardware. We present a 

 sensing protocol based on optimally band-limited ``Slepian functions'' 

 that can overcome these challenges, by providing narrowband sensing of 

 ambient dephasing noise, coupling additively to the sensor along the z 

 axis, while permitting isolation of the target noise spectrum from other 

 contributions coupling along a different axis. This is achieved by 

 introducing a finite-difference control modulation, which linearizes the 

 sensor's response and affords tunable band-limited ``windowing'' in 

 frequency. Building on these techniques, we experimentally demonstrate 

 two spectral estimation capabilities using a trapped-ion qubit sensor. 

 We first perform efficient experimental reconstruction of a ``mixed'' 

 dephasing spectrum, composed of a broadband 1/f -type spectrum with 

 discrete spurs. We then demonstrate the simultaneous reconstruction of 

 overlapping dephasing and control noise spectra from a single set of 

 measurements, in a setting where the two noise sources contribute 

 equally to the sensor's response. Our approach provides a direct means 

 to augment quantum-sensor performance in the presence of both complex 

 broadband noise environments and imperfect control signals, by optimally 

 complying with realistic time-bandwidth constraints.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Gupta.2020,

title = {Adaptive characterization of spatially inhomogeneous fields and errors in qubit registers},

author = {Riddhi Swaroop Gupta and Claire L Edmunds and Alistair R Milne and Cornelius Hempel and Michael J Biercuk},

doi = {10.1038/s41534-020-0286-0},

year  = {2020},

date = {2020-06-12},

journal = {npj Quantum Information},

volume = {6},

number = {1},

pages = {53},

abstract = {New quantum computing architectures consider integrating qubits as sensors to provide actionable information useful for calibration or decoherence mitigation on neighboring data qubits, but little work has addressed how such schemes may be efficiently implemented in order to maximize information utilization. Techniques from classical estimation and dynamic control, suitably adapted to the strictures of quantum measurement, provide an opportunity to extract augmented hardware performance through automation of low-level characterization and control. In this work, we present an adaptive learning framework, Noise Mapping for Quantum Architectures (NMQA), for scheduling of sensor–qubit measurements and efficient spatial noise mapping (prior to actuation) across device architectures. Via a two-layer particle filter, NMQA receives binary measurements and determines regions within the architecture that share common noise processes; an adaptive controller then schedules future measurements to reduce map uncertainty. Numerical analysis and experiments on an array of trapped ytterbium ions demonstrate that NMQA outperforms brute-force mapping by up to 20× (3×) in simulations (experiments), calculated as a reduction in the number of measurements required to map a spatially inhomogeneous magnetic field with a target error metric. As an early adaptation of robotic control to quantum devices, this work opens up exciting new avenues in quantum computer science.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Marciniak2020,

title = {High-power spectral beamsplitter for closely spaced frequencies},

author = {Ch. D Marciniak and A Rischka and R N Wolf and M J Biercuk},

doi = {10.1364/oe.390956},

year  = {2020},

date = {2020-04-01},

journal = {Optics Express},

volume = {28},

number = {8},

pages = {11372},

publisher = {The Optical Society},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kindem2020,

title = {Control and single-shot readout of an ion embedded in a nanophotonic cavity},

author = {Jonathan M. Kindem and Andrei Ruskuc and J. G. Bartholomew and Jake Rochman and Yan Qi Huan and Andrei Faraon},

url = {http://www.nature.com/articles/s41586-020-2160-9},

doi = {10.1038/s41586-020-2160-9},

issn = {0028-0836},

year  = {2020},

date = {2020-04-01},

journal = {Nature},

volume = {580},

number = {7802},

pages = {201–204},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Milne2020,

title = {Phase-Modulated Entangling Gates Robust to Static and Time-Varying Errors},

author = {Alistair R. Milne and Claire L. Edmunds and Cornelius Hempel and Federico Roy and Sandeep Mavadia and Michael J. Biercuk},

year  = {2020},

date = {2020-01-01},

journal = {Phys. Rev. Applied},

volume = {13},

number = {2},

pages = {024022},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Boele2020,

title = {Tailored nanodiamonds for hyperpolarized C 13 MRI},

author = {T Boele and D E J Waddington and T Gaebel and E Rej and A Hasija and L J Brown and D R McCamey and D J Reilly},

url = {https://newapp.readcube.com/library/91063203-0e68-43c4-9bfb-057b95692169/item/79d3158e-ff93-4710-92a6-52420ae9a573},

doi = {10.1103/physrevb.101.155416},

issn = {2469-9950},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review B},

volume = {101},

issue = {15},

pages = {155416},

abstract = {Nanodiamond is poised to become an attractive material for hyperpolarized C13 magnetic resonance imaging if large nuclear polarizations can be achieved without the accompanying rapid spin-relaxation driven by paramagnetic species. Here we report enhanced and long-lived C13 polarization in synthetic nanodiamonds tailored by acid-cleaning and air-oxidation protocols. Our results separate the contributions of different paramagnetic species on the polarization behavior, identifying the importance of substitutional nitrogen defect centers in the nanodiamond core. These results are likely of use in the development of nanodiamond-based imaging agents with size distributions of relevance for examining biological processes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2020,

title = {A Cryo-CMOS Voltage Reference in 28nm FDSOI},

author = {Yuanyuan Yang and Kushal Das and Alireza Moini and David J. Reilly},

doi = {10.1109/LSSC.2020.3010234},

issn = {25739603},

year  = {2020},

date = {2020-01-01},

journal = {IEEE Solid-State Circuits Letters},

publisher = {Institute of Electrical and Electronics Engineers Inc.},

abstract = {The control interface of a large-scale quantum computer will likely require electronic sub-systems that operate in close proximity to the qubits, at deep cryogenic temperatures. In this paper, we report low-temperature performance of a custom cryo-CMOS voltage reference circuit fabricated in a 28nm fully depleted silicon on insulator (FDSOI) CMOS process, dissipating about 15μW. This MOS-only reference circuit is functional from room temperature down to liquid helium temperature (4K), showing a temperature coefficient of 0.6mV/K. The measured supply sensitivity of our reference circuit is better than -50dB at 4K temperature. Beyond the specific application as low-power reference, this circuit is an ideal test-vehicle for developing design approaches that mitigate the adverse effects of cryogenic temperatures on circuit performance.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Smith2020,

title = {Dispersive Readout of Majorana Qubits},

author = {Thomas B. Smith and Maja C. Cassidy and David J. Reilly and Stephen D. Bartlett and Arne L. Grimsmo},

doi = {10.1103/PRXQuantum.1.020313},

issn = {26913399},

year  = {2020},

date = {2020-01-01},

journal = {PRX Quantum},

volume = {1},

issue = {2},

publisher = {American Physical Society},

abstract = {We analyze a readout scheme for Majorana qubits based on dispersive coupling to a resonator. We consider two variants of Majorana qubits: the Majorana transmon and the Majorana box qubit. In both cases, the qubit-resonator interaction can produce sizeable dispersive shifts in the megahertz range for reasonable system parameters, allowing for submicrosecond readout with high fidelity. For Majorana transmons, the light-matter interaction used for readout manifestly conserves Majorana parity, which leads to a notion of quantum nondemolition (QND) readout that is stronger than for conventional charge qubits. In contrast, Majorana box qubits only recover an approximately QND readout mechanism in the dispersive limit where the resonator detuning is large. We also compare dispersive readout to longitudinal readout for the Majorana box qubit. We show that the latter gives faster and higher fidelity readout for reasonable parameters, while having the additional advantage of being manifestly QND, and so may prove to be a better readout mechanism for these systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Pauka2020,

title = {Repairing the surface of InAs-based topological heterostructures},

author = {S. J. Pauka and J. D. S. Witt and C. N. Allen and B. Harlech-Jones and A. Jouan and G. C. Gardner and S. Gronin and T. Wang and C. Thomas and M. J. Manfra and J. Gukelberger and J. Gamble and D. J. Reilly and M. C. Cassidy},

doi = {10.1063/5.0014361},

issn = {10897550},

year  = {2020},

date = {2020-01-01},

journal = {Journal of Applied Physics},

volume = {128},

issue = {11},

publisher = {American Institute of Physics Inc.},

abstract = {Candidate systems for topologically-protected qubits include two-dimensional electron gases (2DEGs) based on heterostructures exhibiting a strong spin–orbit interaction and superconductivity via the proximity effect. For InAs- or InSb-based materials, the need to form shallow quantum wells to create a hard-gapped p-wave superconducting state often subjects them to fabrication-induced damage, limiting their mobility. Here, we examine scattering mechanisms in processed InAs 2DEG quantum wells and demonstrate a means of increasing their mobility via repairing the semiconductor–dielectric interface. Passivation of charged impurity states with an argon–hydrogen plasma results in a significant increase in the measured mobility and reduction in its variance relative to untreated samples, up to 45 300 cm2/(V s) in a 10 nm deep quantum well.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Jarratt2020,

title = {Detection of the Quantum Capacitance of a Point Contact via Dispersive Gate Sensing},

author = {M. C. Jarratt and S. J. Waddy and A. Jouan and A. C. Mahoney and G. C. Gardner and S. Fallahi and M. J. Manfra and D. J. Reilly},

doi = {10.1103/PhysRevApplied.14.064021},

issn = {23317019},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review Applied},

volume = {14},

issue = {6},

publisher = {American Physical Society},

abstract = {The readout technique of dispersive gate sensing (DGS) uses an electrode embedded in a microwave resonator to detect the quantum capacitance of a mesoscopic device arising from single-electron tunneling. Here, we extend DGS from the detection of discrete tunnel events to the open regime, observing jumps in the capacitance of a quantum point contact (QPC) that arise when the one-dimensional sub-bands populate. We compare the signal from DGS to transport measurements for various QPC geometries, including measurements at finite bias. Unlike traditional charge sensing, which is limited by screening at high density, our results suggest that DGS can also probe the charge configuration of open quantum devices, where electrons are delocalized and multiple sub-bands are occupied.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Edmunds.2020,

title = {Dynamically corrected gates suppressing spatiotemporal error correlations as measured by randomized benchmarking},

author = {C. L. Edmunds and C. Hempel and R. J. Harris and V. Frey and T. M. Stace and M. J. Biercuk},

doi = {10.1103/physrevresearch.2.013156},

year  = {2020},

date = {2020-01-01},

journal = {Phys. Rev. Res.},

volume = {2},

number = {1},

pages = {013156},

abstract = {Quantum error correction provides a path to large-scale quantum computers, but is built on challenging assumptions about the characteristics of the underlying errors. In particular, the mathematical assumption of statistically independent errors in quantum logic operations is at odds with realistic environments where error sources may exhibit strong temporal and spatial correlations. We present experiments using trapped ions to demonstrate that the use of dynamically corrected gates (DCGs), generally considered for the reduction of error magnitudes, can also suppress error correlations in space and time throughout quantum circuits. We present a first-principles analysis of the manifestation of error correlations in randomized benchmarking and validate this model through experiments performed using engineered errors. We find that standard DCGs can reduce error correlations by ∼50× while increasing the magnitude of uncorrelated errors by a factor scaling linearly with the extended DCG duration compared to a primitive gate. We then demonstrate that the correlation characteristics of intrinsic errors in our system are modified by the use of DCGs, consistent with a picture in which DCGs whiten the effective error spectrum induced by external noise.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bentley.2020,

title = {Numeric Optimization for Configurable, Parallel, Error‐Robust Entangling Gates in Large Ion Registers},

author = {Christopher D B Bentley and Harrison Ball and Michael J Biercuk and Andre R R Carvalho and Michael R Hush and Harry J Slatyer},

doi = {10.1002/qute.202000044},

issn = {2511-9044},

year  = {2020},

date = {2020-01-01},

journal = {Advanced Quantum Technologies},

pages = {2000044},

abstract = {A class of entangling gates for trapped atomic ions is studied and the use of numeric optimization techniques to create a wide range of fast, error‐robust gate constructions is demonstrated. A numeric optimization framework is introduced targeting maximally‐ and partially‐entangling operations on ion pairs, multi‐ion registers, multi‐ion subsets of large registers, and parallel operations within a single register. Ions are assumed to be individually addressed, permitting optimization over amplitude‐ and phase‐modulated controls. Calculations and simulations demonstrate that the inclusion of modulation of the difference phase for the bichromatic drive used in the Mølmer–Sørensen gate permits approximately time‐optimal control across a range of gate configurations, and when suitably combined with analytic constraints can also provide robustness against key experimental sources of error. The impact of experimental constraints such as bounds on coupling rates or modulation band‐limits on achievable performance is further demonstrated. Using a custom optimization engine based on TensorFlow, for optimizations on ion registers up to 20 ions, time‐to‐solution of order tens of minutes using a local‐instance laptop is also demonstrated, allowing computational access to system‐scales relevant to near‐term trapped‐ion devices. Numeric optimization and control techniques are demonstrated to create a wide range of fast, robust, high‐fidelity gates. Complex drive controls, with both phase and amplitude modulation on the mediating laser field, are applied to enact multi‐body and parallel operations on chains of up to 20 ions. Control solutions incorporate real constraints on modulation hardware and robustness to laser noise sources.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Milne.2020,

title = {Phase-Modulated Entangling Gates Robust to Static and Time-Varying Errors},

author = {Alistair R Milne and Claire L Edmunds and Cornelius Hempel and Federico Roy and Sandeep Mavadia and Michael J Biercuk},

doi = {10.1103/physrevapplied.13.024022},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review Applied},

volume = {13},

number = {2},

pages = {024022},

abstract = {Entangling operations are among the most important primitive gates employed in quantum computing, and it is crucial to ensure high-fidelity implementations as systems are scaled up. We experimentally realize and characterize a simple scheme to minimize errors in entangling operations related to the residual excitation of mediating bosonic oscillator modes that both improves gate robustness and provides scaling benefits in larger systems. The technique employs discrete phase shifts in the control field driving the gate operation, determined either analytically or numerically, to ensure all modes are de-excited at arbitrary user-defined times. We demonstrate an average gate fidelity of 99.4(2)% across a wide range of parameters in a system of Yb171+ trapped ion qubits, and observe a reduction of gate error in the presence of common experimental error sources. Our approach provides a unified framework to achieve robustness against both static and time-varying laser amplitude and frequency detuning errors. We verify these capabilities through system-identification experiments revealing improvements in error susceptibility achieved in phase-modulated gates.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ball2020software,

title = {Software tools for quantum control: Improving quantum computer performance through noise and error suppression},

author = {Harrison Ball and Michael J Biercuk and Andre Carvalho and Jiayin Chen and Michael Hush and Leonardo De A Castro and Li Li and Per J Liebermann and Harry J Slatyer and Claire Edmunds and Virginia Frey and Cornelius Hempel and Alistair Milne},

url = {https://arxiv.org/abs/2001.04060},

year  = {2020},

date = {2020-01-01},

journal = {arXiv},

volume = {quant-ph},

number = {2001.04060},

abstract = {Manipulating quantum computing hardware in the presence of imperfect devices and control systems is a central challenge in realizing useful quantum computers. Susceptibility to noise in particular limits the performance and algorithmic capabilities experienced by end users. Fortunately, in both the NISQ era and beyond, quantum control enables the efficient execution of quantum logic operations and quantum algorithms exhibiting robustness to errors, without the need for complex logical encoding. We introduce the first commercial-grade software tools for quantum control in quantum computing research from Q-CTRL, serving the needs of hardware Rtextbackslash&D teams, algorithm developers, and end users. We survey quantum control and its role in combating noise in near-term devices; our primary focus is on quantum firmware, the low-level software solutions designed to enhance the stability of quantum computational hardware at the physical layer. We explain the benefits of quantum firmware not only in error suppression, but also in simplifying compilation protocols and enhancing the efficiency of quantum error correction. We provide an overview of Q-CTRL's software tools for creating and deploying quantum control solutions at various layers of the quantum computing software stack. We describe our software architecture leveraging both distributed cloud computation and local custom integration into hardware systems, and explain how key functionality is integrable with other quantum programming languages. We present a detailed technical overview of product features including a control-optimization engine, filter functions for general systems, and noise characterization. Finally, we present a series of case studies demonstrating the utility of quantum control solutions derived from these tools in improving the performance of trapped-ion and superconducting quantum computer hardware.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tuckett.2020,

title = {Fault-Tolerant Thresholds for the Surface Code in Excess of 5% under Biased Noise},

author = {David K Tuckett and Stephen D Bartlett and Steven T Flammia and Benjamin J Brown},

doi = {10.1103/physrevlett.124.130501},

issn = {0031-9007},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review Letters},

volume = {124},

number = {13},

pages = {130501},

abstract = {Noise in quantum computing is countered with quantum error correction. Achieving optimal performance will require tailoring codes and decoding algorithms to account for features of realistic noise, such as the common situation where the noise is biased towards dephasing. Here we introduce an efficient high-threshold decoder for a noise-tailored surface code based on minimum-weight perfect matching. The decoder exploits the symmetries of its syndrome under the action of biased noise and generalizes to the fault-tolerant regime where measurements are unreliable. Using this decoder, we obtain fault-tolerant thresholds in excess of 6% for a phenomenological noise model in the limit where dephasing dominates. These gains persist even for modest noise biases: we find a threshold of ∼5% in an experimentally relevant regime where dephasing errors occur at a rate 100 times greater than bit-flip errors.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{10.1103/physrevx.10.031041b,

title = {Symmetry-Protected Self-Correcting Quantum Memories},

author = {Sam Roberts and Stephen D Bartlett},

doi = {10.1103/physrevx.10.031041},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review X},

volume = {10},

number = {3},

pages = {031041},

abstract = {A self-correcting quantum memory can store and protect quantum information for a time that increases without bound with the system size and without the need for active error correction. We demonstrate that symmetry can lead to self-correction in 3D spin-lattice models. In particular, we investigate codes given by 2D symmetry-enriched topological (SET) phases that appear naturally on the boundary of 3D symmetry-protected topological (SPT) phases. We find that while conventional on-site symmetries are not sufficient to allow for self-correction in commuting Hamiltonian models of this form, a generalized type of symmetry known as a 1-form symmetry is enough to guarantee self-correction. We illustrate this fact with the 3D “cluster-state” model from the theory of quantum computing. This model is a self-correcting memory, where information is encoded in a 2D SET-ordered phase on the boundary that is protected by the thermally stable SPT ordering of the bulk. We also investigate the gauge color code in this context. Finally, noting that a 1-form symmetry is a very strong constraint, we argue that topologically ordered systems can possess emergent 1-form symmetries, i.e., models where the symmetry appears naturally, without needing to be enforced externally.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{grimsmo2020quantum,

title = {Quantum computing with rotation-symmetric Bosonic codes},

author = {Arne L Grimsmo and Joshua Combes and Ben Q Baragiola},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review X},

volume = {10},

number = {1},

pages = {011058},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{milne2020quantum,

title = {Quantum oscillator noise spectroscopy via displaced Schrödinger cat states},

author = {Alistair R Milne and Cornelius Hempel and Li Li and Claire L Edmunds and Harry J Slatyer and Harrison Ball and Michael R Hush and Michael J Biercuk},

url = {https://arxiv.org/abs/2010.04375},

year  = {2020},

date = {2020-01-01},

journal = {arXiv preprint arXiv:2010.04375},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{smith2020dispersive,

title = {Dispersive readout of Majorana qubits},

author = {Thomas B Smith and Maja C Cassidy and David J Reilly and Stephen D Bartlett and Arne L Grimsmo},

year  = {2020},

date = {2020-01-01},

journal = {PRX Quantum},

volume = {1},

number = {2},

pages = {020313},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{yang2020cryo,

title = {A cryo-CMOS voltage reference in 28-nm FDSOI},

author = {Yuanyuan Yang and Kushal Das and Alireza Moini and David J Reilly},

year  = {2020},

date = {2020-01-01},

journal = {IEEE Solid-State Circuits Letters},

volume = {3},

pages = {186–189},

publisher = {IEEE},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{pauka2020characterizing,

title = {Characterizing quantum devices at scale with custom cryo-CMOS},

author = {SJ Pauka and K Das and JM Hornibrook and GC Gardner and MJ Manfra and MC Cassidy and DJ Reilly},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review Applied},

volume = {13},

number = {5},

pages = {054072},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{jarratt2020detection,

title = {Detection of the Quantum Capacitance of a Point Contact via Dispersive Gate Sensing},

author = {MC Jarratt and SJ Waddy and A Jouan and AC Mahoney and GC Gardner and S Fallahi and MJ Manfra and DJ Reilly},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review Applied},

volume = {14},

number = {6},

pages = {064021},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ball2019,

title = {Site-resolved imaging of beryllium ion crystals in a high-optical-access Penning trap with inbore optomechanics},

author = {H. Ball and Ch. D. Marciniak and R. N. Wolf and A. T. -H. Hung and K. Pyka and M. J. Biercuk},

doi = {10.1063/1.5049506},

year  = {2019},

date = {2019-05-01},

journal = {Rev. Sci. Instrum.},

volume = {90},

number = {5},

pages = {053103},

publisher = {AIP Publishing},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Waddington2019,

title = {Phase-Encoded Hyperpolarized Nanodiamond for Magnetic Resonance Imaging},

author = {David E J Waddington and Thomas Boele and Ewa Rej and Dane R McCamey and Nicholas J C King and Torsten Gaebel and David J Reilly},

url = {https://newapp.readcube.com/library/91063203-0e68-43c4-9bfb-057b95692169/item/7A4E3FB4-F55B-8AAC-608C-0F471CDEE189},

doi = {10.1038/s41598-019-42373-w},

issn = {2045-2322},

year  = {2019},

date = {2019-01-01},

journal = {Scientific Reports},

volume = {9},

issue = {1},

pages = {5950},

abstract = {Surface-functionalized nanomaterials are of interest as theranostic agents that detect disease and track biological processes using hyperpolarized magnetic resonance imaging (MRI). Candidate materials are sparse however, requiring spinful nuclei with long spin-lattice relaxation (T1) and spin-dephasing times (T2), together with a reservoir of electrons to impart hyperpolarization. Here, we demonstrate the versatility of the nanodiamond material system for hyperpolarized 13C MRI, making use of its intrinsic paramagnetic defect centers, hours-long nuclear T1 times, and T2 times suitable for spatially resolving millimeter-scale structures. Combining these properties, we enable a new imaging modality, unique to nanoparticles, that exploits the phase-contrast between spins encoded with a hyperpolarization that is aligned, or anti-aligned with the external magnetic field. The use of phase-encoded hyperpolarization allows nanodiamonds to be tagged and distinguished in an MRI based on their spin-orientation alone, and could permit the action of specific bio-functionalized complexes to be directly compared and imaged.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sharma2019,

title = {Enhancement of nuclear spin coherence times by driving dynamic nuclear polarization at defect centers in solids},

author = {Girish Sharma and Torsten Gaebel and Ewa Rej and David J. Reilly and Sophia E. Economou and Edwin Barnes},

doi = {10.1103/PhysRevB.99.205423},

issn = {24699969},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review B},

volume = {99},

issue = {20},

publisher = {American Physical Society},

abstract = {The hyperpolarization of nuclear spins can enable powerful imaging and sensing techniques provided the hyperpolarization is sufficiently long lived. Recent experiments on nanodiamond C13 nuclear spins demonstrate that relaxation times can be extended by three orders of magnitude by building up dynamic nuclear polarization (DNP) through the driving of electron-nuclear flip-flop processes at defect centers. This finding raises the question of whether the nuclear spin coherence times are also impacted by this hyperpolarization process. Here, we theoretically examine the effect of DNP on the nuclear spin coherence times as a function of the hyperpolarization drive time. We do this by developing a microscopic theory of DNP in a nuclear spin ensemble coupled to microwave-driven defect centers in solids and subject to spin diffusion mediated by internuclear dipolar interactions. We find that, similarly to relaxation times, the nuclear spin coherence times can be increased substantially by a few orders of magnitude depending on the driving time. Our theoretical model and results will be useful for current and future experiments on enhancing nuclear spin coherence times via DNP.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Croot2019,

title = {Gate-Sensing Charge Pockets in the Semiconductor-Qubit Environment},

author = {X. G. Croot and S. J. Pauka and M. C. Jarratt and H. Lu and A. C. Gossard and J. D. Watson and G. C. Gardner and S. Fallahi and M. J. Manfra and D. J. Reilly},

doi = {10.1103/PhysRevApplied.11.064027},

issn = {23317019},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review Applied},

volume = {11},

issue = {6},

publisher = {American Physical Society},

abstract = {Dispersive gate sensing (DGS) uses radio-frequency (rf) reflectometry to locally probe the quantum capacitance of a gate electrode. Applying DGS to heterostructure-based qubit devices, we report the repeated observation of anomalous signals that we attribute to pockets of charge in the potential landscape likely under, and surrounding, the surface gates that define quantum-dot qubits. Interestingly, these charge pockets appear to evade detection with conventional charge sensors but manifest strongly in the response of the gate sensor. Configuring a quantum point contact (QPC) as a highly localized heat source, we show how these charge pockets likely form close to the end of the gate electrodes, in close proximity to gate-defined qubits. The presence of uncontrolled charge may lead to offsets in gate voltage and further contribute to charge noise that produces decoherence in semiconductor qubits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bosco2019,

title = {Transmission Lines and Metamaterials Based on Quantum Hall Plasmonics},

author = {S. Bosco and D. P. Divincenzo and D. J. Reilly},

doi = {10.1103/PhysRevApplied.12.014030},

issn = {23317019},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review Applied},

volume = {12},

issue = {1},

publisher = {American Physical Society},

abstract = {The characteristic impedance of a microwave transmission line is typically constrained to a value Z0=50ω, in part because of the low impedance of free space and the limited range of permittivity and permeability realizable with conventional materials. Here we suggest the possibility of constructing high-impedance transmission lines by exploiting the plasmonic response of edge states associated with the quantum Hall effect in gated devices. We analyze various implementations of quantum Hall transmission lines based on distributed networks and lumped-element circuits, including a detailed account of parasitic capacitance and Coulomb drag effects, which can modify device performance. We additionally conceive of a metamaterial structure comprising arrays of quantum Hall droplets and analyze its unusual properties. The realization of such structures holds promise for efficiently wiring-up quantum circuits on chip, as well as engineering strong coupling between semiconductor qubits and microwave photons.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yang.2019,

title = {Silicon qubit fidelities approaching incoherent noise limits via pulse engineering},

author = {C H Yang and K W Chan and R Harper and W Huang and T Evans and J C C Hwang and B Hensen and A Laucht and T Tanttu and F E Hudson and S T Flammia and K M Itoh and A Morello and S D Bartlett and A S Dzurak},

url = {https://www.nature.com/articles/s41928-019-0234-1.epdf?shared_access_token=dwgc0pOWyz17aQfkQBd4wtRgN0jAjWel9jnR3ZoTv0O-UqCO5w04UHpr1iaXzevFLT0JVyPpg9mA3x2230uknqWY4orZ4AuvqFwyDcjiIUIlYS9H_CiaXwr7TrJtnh4_b0MJCLVSEYws_x-aTqCHRw%3D%3D},

doi = {10.1038/s41928-019-0234-1},

year  = {2019},

date = {2019-01-01},

journal = {Nature Electronics},

volume = {2},

number = {4},

pages = {151–158},

abstract = {Spin qubits created from gate-defined silicon metal–oxide–semiconductor quantum dots are a promising architecture for quantum computation. The high single qubit fidelities possible in these systems, combined with quantum error correcting codes, could potentially offer a route to fault-tolerant quantum computing. To achieve fault tolerance, however, gate error rates must be reduced to below a certain threshold and, in general, correlated errors must be removed. Here we show that pulse engineering techniques can be used to reduce the average Clifford gate error rates for silicon quantum dot spin qubits down to 0.043%. This represents a factor of three improvement over state-of-the-art silicon quantum dot devices and extends the randomized benchmarking coherence time to 9.4 ms. By including tomographically complete measurements in our randomized benchmarking, we infer a higher-order feature of the noise called the unitarity, which measures the coherence of noise. This, in turn, allows us to theoretically predict that average gate error rates as low as 0.026% may be achievable with further pulse improvements. These spin qubit fidelities are ultimately limited by incoherent noise, which we attribute to charge noise from the silicon device structure or the environment. Pulse engineering techniques can be used to reduce the average Clifford gate error rates for silicon quantum dot spin qubits down to 0.043%, a factor of three improvement over state-of-the-art silicon devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tuckett.2019,

title = {Tailoring Surface Codes for Highly Biased Noise},

author = {David K Tuckett and Andrew S Darmawan and Christopher T Chubb and Sergey Bravyi and Stephen D Bartlett and Steven T Flammia},

doi = {10.1103/physrevx.9.041031},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review X},

volume = {9},

number = {4},

pages = {041031},

abstract = {The surface code, with a simple modification, exhibits ultrahigh error-correction thresholds when the noise is biased toward dephasing. Here, we identify features of the surface code responsible for these ultrahigh thresholds. We provide strong evidence that the threshold error rate of the surface code tracks the hashing bound exactly for all biases and show how to exploit these features to achieve significant improvement in the logical failure rate. First, we consider the infinite bias limit, meaning pure dephasing. We prove that the error threshold of the modified surface code for pure dephasing noise is 50%, i.e., that all qubits are fully dephased, and this threshold can be achieved by a polynomial time-decoding algorithm. We demonstrate that the subthreshold behavior of the code depends critically on the precise shape and boundary conditions of the code. That is, for rectangular surface codes with standard rough or smooth open boundaries, it is controlled by the parameter g=gcd(j,k), where j and k are dimensions of the surface code lattice. We demonstrate a significant improvement in the logical failure rate with pure dephasing for coprime codes that have g=1 and closely related rotated codes, which have a modified boundary. The effect is dramatic: The same logical failure rate achievable with a square surface code and n physical qubits can be obtained with a coprime or rotated surface code using only O(n) physical qubits. Finally, we use approximate maximum-likelihood decoding to demonstrate that this improvement persists for a general Pauli noise biased toward dephasing. In particular, comparing with a square surface code, we observe a significant improvement in the logical failure rate against biased noise using a rotated surface code with approximately half the number of physical qubits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{west2019gate,

title = {Gate-based single-shot readout of spins in silicon},

author = {Anderson West and Bas Hensen and Alexis Jouan and Tuomo Tanttu and Chih-Hwan Yang and Alessandro Rossi and M Fernando Gonzalez-Zalba and Fay Hudson and Andrea Morello and David J Reilly and others},

year  = {2019},

date = {2019-01-01},

journal = {Nature nanotechnology},

volume = {14},

number = {5},

pages = {437–441},

publisher = {Nature Publishing Group UK London},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{waddington2019phase,

title = {Phase-encoded hyperpolarized nanodiamond for magnetic resonance imaging},

author = {David EJ Waddington and Thomas Boele and Ewa Rej and Dane R McCamey and Nicholas JC King and Torsten Gaebel and David J Reilly},

year  = {2019},

date = {2019-01-01},

journal = {Scientific reports},

volume = {9},

number = {1},

pages = {5950},

publisher = {Nature Publishing Group UK London},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{yang2019cryo,

title = {Cryo-CMOS band-gap reference circuits for quantum computing},

author = {Yuanyuan Yang and Kushal Das and Alireza Moini and David J Reilly},

year  = {2019},

date = {2019-01-01},

journal = {arXiv preprint arXiv:1910.01217},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{bosco2019transmission,

title = {Transmission lines and metamaterials based on quantum hall plasmonics},

author = {Stefano Bosco and David P DiVincenzo and David J Reilly},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review Applied},

volume = {12},

number = {1},

pages = {014030},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{sharma2019enhancement,

title = {Enhancement of nuclear spin coherence times by driving dynamic nuclear polarization at defect centers in solids},

author = {Girish Sharma and Torsten Gaebel and Ewa Rej and David J Reilly and Sophia E Economou and Edwin Barnes},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review B},

volume = {99},

number = {20},

pages = {205423},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{pauka2019cryogenic,

title = {A cryogenic interface for controlling many qubits},

author = {SJ Pauka and K Das and R Kalra and A Moini and Y Yang and M Trainer and A Bousquet and C Cantaloube and N Dick and GC Gardner and others},

year  = {2019},

date = {2019-01-01},

journal = {arXiv preprint arXiv:1912.01299},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{croot2019gate,

title = {Gate-sensing charge pockets in the semiconductor-qubit environment},

author = {XG Croot and SJ Pauka and MC Jarratt and H Lu and AC Gossard and JD Watson and GC Gardner and S Fallahi and MJ Manfra and DJ Reilly},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review Applied},

volume = {11},

number = {6},

pages = {064027},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Gupta2018,

title = {Machine Learning for Predictive Estimation of Qubit Dynamics Subject to Dephasing},

author = {Riddhi Swaroop Gupta and Michael J. Biercuk},

url = {https://link.aps.org/doi/10.1103/PhysRevApplied.9.064042},

doi = {10.1103/PhysRevApplied.9.064042},

year  = {2018},

date = {2018-06-01},

journal = {Phys. Rev. Appl.},

volume = {9},

issue = {6},

pages = {064042},

publisher = {American Physical Society},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Croot2018,

title = {Device Architecture for Coupling Spin Qubits via an Intermediate Quantum State},

author = {X. G. Croot and S. J. Pauka and J. D. Watson and G. C. Gardner and S. Fallahi and M. J. Manfra and D. J. Reilly},

doi = {10.1103/PhysRevApplied.10.044058},

issn = {23317019},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Applied},

volume = {10},

issue = {4},

publisher = {American Physical Society},

abstract = {We demonstrate a scalable device architecture that facilitates indirect exchange between singlet-triplet spin qubits, mediated by an intermediate quantum state. The device comprises five quantum dots, which can be independently loaded and unloaded via tunneling to adjacent reservoirs, avoiding charge latch-up common in linear dot arrays. In a step toward realizing two-qubit entanglement based on indirect exchange, the architecture permits precise control over tunnel rates between the singlet-triplet qubits and the intermediate state. We show that by our separating qubits by approximately 1 μm, the residual capacitive coupling between them is reduced to approximately 7 μeV.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hempel.2018,

title = {Quantum Chemistry Calculations on a Trapped-Ion Quantum Simulator},

author = {Cornelius Hempel and Christine Maier and Jonathan Romero and Jarrod McClean and Thomas Monz and Heng Shen and Petar Jurcevic and Ben P Lanyon and Peter Love and Ryan Babbush and Alán Aspuru-Guzik and Rainer Blatt and Christian F Roos},

url = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.031022},

doi = {10.1103/physrevx.8.031022},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review X},

volume = {8},

number = {3},

pages = {031022},

abstract = {Quantum-classical hybrid algorithms are emerging as promising candidates for near-term practical applications of quantum information processors in a wide variety of fields ranging from chemistry to physics and materials science. We report on the experimental implementation of such an algorithm to solve a quantum chemistry problem, using a digital quantum simulator based on trapped ions. Specifically, we implement the variational quantum eigensolver algorithm to calculate the molecular ground-state energies of two simple molecules and experimentally demonstrate and compare different encoding methods using up to four qubits. Furthermore, we discuss the impact of measurement noise as well as mitigation strategies and indicate the potential for adaptive implementations focused on reaching chemical accuracy, which may serve as a cross-platform benchmark for multiqubit quantum simulators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Gupta.2018,

title = {Machine Learning for Predictive Estimation of Qubit Dynamics Subject to Dephasing},

author = {Riddhi Swaroop Gupta and Michael J Biercuk},

url = {https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.9.064042?utm_source=email&utm_medium=email&utm_campaign=prapplied-alert},

doi = {10.1103/physrevapplied.9.064042},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Applied},

volume = {9},

number = {6},

pages = {064042},

abstract = {Decoherence remains a major challenge in quantum computing hardware, and a variety of physical-layer controls provide opportunities to mitigate the impact of this phenomenon through feedback and feed-forward control. In this work, we compare a variety of machine-learning algorithms derived from diverse fields for the task of state estimation (retrodiction) and forward prediction of future qubit-state evolution for a single qubit subject to classical, non-Markovian dephasing. Our approaches involve the construction of a dynamical model capturing qubit dynamics via autoregressive or Fourier-type protocols using only a historical record of projective measurements. A detailed comparison of achievable prediction horizons, model robustness, and measurement-noise-filtering capabilities for Kalman filters (KFs) and Gaussian process regression (GPR) algorithms is provided. We demonstrate superior performance from the autoregressive KF relative to Fourier-based KF approaches and focus on the role of filter optimization in achieving suitable performance. Finally, we examine several realizations of GPR using different kernels and discover that these approaches are generally not suitable for forward prediction. We highlight the linkages between predictive performance and kernel structure, and we identify ways in which forward predictions are susceptible to numerical artifacts.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Mavadia.2018,

title = {Experimental quantum verification in the presence of temporally correlated noise},

author = {S Mavadia and C L Edmunds and C Hempel and H Ball and F Roy and T M Stace and M J Biercuk},

url = {https://doi.org/10.1038/s41534-017-0052-0},

doi = {10.1038/s41534-017-0052-0},

year  = {2018},

date = {2018-01-01},

journal = {npj Quantum Information},

volume = {4},

number = {1},

pages = {7},

abstract = {Growth in the capabilities of quantum information hardware mandates access to techniques for performance verification that function under realistic laboratory conditions. Here we experimentally characterise the impact of common temporally correlated noise processes on both randomised benchmarking (RB) and gate-set tomography (GST). Our analysis highlights the role of sequence structure in enhancing or suppressing the sensitivity of quantum verification protocols to either slowly or rapidly varying noise, which we treat in the limiting cases of quasi-DC miscalibration and white noise power spectra. We perform experiments with a single trapped 171Yb+ ion-qubit and inject engineered noise ∝σtextasciicircumz to probe protocol performance. Experiments on RB validate predictions that measured fidelities over sequences are described by a gamma distribution varying between approximately Gaussian, and a broad, highly skewed distribution for rapidly and slowly varying noise, respectively. Similarly we find a strong gate set dependence of default experimental GST procedures in the presence of correlated errors, leading to significant deviations between estimated and calculated diamond distances in the presence of correlated σtextasciicircumz errors. Numerical simulations demonstrate that expansion of the gate set to include negative rotations can suppress these discrepancies and increase reported diamond distances by orders of magnitude for the same error processes. Similar effects do not occur for correlated σtextasciicircumx or σtextasciicircumy errors or depolarising noise processes, highlighting the impact of the critical interplay of selected gate set and the gauge optimisation process on the meaning of the reported diamond norm in correlated noise environments. Experiments reveal that the presence of correlated noise may compromise the interpretation of techniques for the validation of quantum hardware. A team led by Michael Biercuk at Australia’s University of Sydney and National Measurement Institute, carried out experiments on a single trapped 171Yb+ ion to test the reliability of widespread techniques for characterisation, validation and verification of quantum hardware. Although error processes are often assumed to be statistically independent, in practice slowly varying external fields may introduce temporal correlations in noise. The experiments revealed that the outcome of randomised benchmarking and gate-set tomography differ substantially in presence of correlated noise, and reveal an unexpected sequence-dependent behaviour. These results demonstrate that the reliability of standard performance benchmarking techniques is strongly influenced by the statistical properties of the noise affecting the hardware, complicating direct comparisons between experiments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tuckett.2018,

title = {Ultrahigh Error Threshold for Surface Codes with Biased Noise},

author = {David K Tuckett and Stephen D Bartlett and Steven T Flammia},

url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.050505?utm_source=email&utm_medium=email&utm_campaign=prl-alert},

doi = {10.1103/physrevlett.120.050505},

issn = {0031-9007},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Letters},

volume = {120},

number = {5},

pages = {050505},

abstract = {We show that a simple modification of the surface code can exhibit an enormous gain in the error correction threshold for a noise model in which Pauli Z errors occur more frequently than X or Y errors. Such biased noise, where dephasing dominates, is ubiquitous in many quantum architectures. In the limit of pure dephasing noise we find a threshold of 43.7(1)% using a tensor network decoder proposed by Bravyi, Suchara, and Vargo. The threshold remains surprisingly large in the regime of realistic noise bias ratios, for example 28.2(2)% at a bias of 10. The performance is, in fact, at or near the hashing bound for all values of the bias. The modified surface code still uses only weight-4 stabilizers on a square lattice, but merely requires measuring products of Y instead of Z around the faces, as this doubles the number of useful syndrome bits associated with the dominant Z errors. Our results demonstrate that large efficiency gains can be found by appropriately tailoring codes and decoders to realistic noise models, even under the locality constraints of topological codes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{morello2018would,

title = {What would you do with 1000 qubits},

author = {Andrea Morello and David Reilly},

year  = {2018},

date = {2018-01-01},

journal = {Quantum Science and Technology},

volume = {3},

number = {3},

pages = {030201},

publisher = {IOP Publishing},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{croot2018device,

title = {Device architecture for coupling spin qubits via an intermediate quantum state},

author = {XG Croot and SJ Pauka and JD Watson and GC Gardner and S Fallahi and MJ Manfra and DJ Reilly},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Applied},

volume = {10},

number = {4},

pages = {044058},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Waddington2017,

title = {Nanodiamond-enhanced MRI via in situ hyperpolarization},

author = {David E J Waddington and Mathieu Sarracanie and Huiliang Zhang and Najat Salameh and David R Glenn and Ewa Rej and Torsten Gaebel and Thomas Boele and Ronald L Walsworth and David J Reilly and Matthew S Rosen},

url = {https://newapp.readcube.com/library/91063203-0e68-43c4-9bfb-057b95692169/item/044ba919-c287-4c85-b053-7b7d17fd20e2},

doi = {10.1038/ncomms15118},

issn = {2041-1723},

year  = {2017},

date = {2017-01-01},

journal = {Nature Communications},

volume = {8},

pages = {ncomms15118},

abstract = {Nanodiamonds are of interest as nontoxic substrates for targeted drug delivery and as highly biostable fluorescent markers for cellular tracking. Beyond optical techniques, however, options for noninvasive imaging of nanodiamonds in vivo are severely limited. Here, we demonstrate that the Overhauser effect, a proton–electron polarization transfer technique, can enable high-contrast magnetic resonance imaging (MRI) of nanodiamonds in water at room temperature and ultra-low magnetic field. The technique transfers spin polarization from paramagnetic impurities at nanodiamond surfaces to 1H spins in the surrounding water solution, creating MRI contrast on-demand. We examine the conditions required for maximum enhancement as well as the ultimate sensitivity of the technique. The ability to perform continuous in situ hyperpolarization via the Overhauser mechanism, in combination with the excellent in vivo stability of nanodiamond, raises the possibility of performing noninvasive in vivo tracking of nanodiamond over indefinitely long periods of time.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}