W911NF22S0006 QC-S5 BAA Final.pdf

U.S. ARMY RESEARCH OFFICE

BROAD AGENCY ANNOUNCEMENT FOR

QUANTUM COMPUTING in the SOLID STATE with SPIN and SUPERCONDUCTING SYSTEMS (QC-S5)

W911NF-22-S-0006

Issued by:

U.S. Army Contracting Command-Aberdeen Proving Ground

Research Triangle Park Division

P.O. BOX 12211

Research Triangle Park, NC 27709-2211

I. OVERVIEW OF THE FUNDING OPPORTUNITY:

B. Additional Overview Information

II. DETAILED INFORMATION ABOUT THE FUNDING OPPORTUNITY

B. Federal Award Information

G. Agency Contacts:

I. OVERVIEW OF THE FUNDING OPPORTUNITY:

A. Required Overview Content

1. Federal Agency Name(s):

U.S. Army Research Office

Issuing Acquisition Office:

U.S. Army Contracting Command-Aberdeen Proving Ground, Research Triangle Park Division (ACC-APG RTP Division)

2. Funding Opportunity Title: Quantum Computing in the Solid State with Spin and Superconducting Systems (QC-S5)

3. Announcement Type Initial Announcement

4. Research Opportunity Number: W911NF-22-S-0006

5. Catalog of Federal Domestic Assistance (CFDA) Number:

12.431 – Basic Scientific Research

6. Response Dates:

White Papers: 4:00 PM Eastern Daylight Savings Time on: 15 July 2022 Proposals: 4:00 PM Eastern Daylight Savings Time on: 30 September 2022 See Section II. D. 4 for additional information.

B. Additional Overview Information

This Broad Agency Announcement (BAA) which sets forth research areas of interest to the Army Research Laboratory- Army Research Office (ARL-ARO) and the National Security Agency (NSA) is issued under paragraph 6.102(d)(2) of the Federal Acquisition Regulation (FAR), and 10 USC 2358 which provides for the competitive selection of basic research proposals. Proposals submitted in response to this BAA and selected for award are considered to be the result of full and open competition and in full compliance with the provision of Public Law 98-369, "The Competition in Contracting Act of 1984" and subsequent amendments.

The Department of Defense agencies involved in this program reserve the right to select for award; all, some, or none of the proposals submitted in response to this announcement. The participating DoD agencies will provide no funding for direct reimbursement of proposal development costs. Technical and cost proposals (or any other material) submitted in response to this BAA will not be returned. It is the policy of participating DoD agencies to treat all proposals as sensitive, competitive information and to disclose their contents only for the purposes of evaluation.

II. DETAILED INFORMATION ABOUT THE FUNDING OPPORTUNITY

A. Funding Opportunity Description

The U.S. Army Research Office (ARO), in collaboration with the Laboratory for Physical Sciences (LPS), is soliciting proposals for research in four research topic areas in the field of gate-based Quantum Computing (QC) in the Solid State with Spin and Superconducting qubit Systems (QC-S5).

The topic areas are as follows:

(A) Modular Quantum Gates (ModQ)

(B) Gates on Advanced qubits with Superior Performance (GASP)

(C) Fast control and readout schemes (FastCARS)

(D) Noise in solid-state spin and superconducting systems (NS5)

Responses to these topics must address the circuit gate-based model of quantum computation (QC) and must be suitable for universal control in multi-qubit architectures. Topics A, C, and D require the use of high fidelity, multi-qubit devices, such as gate-defined SiGe or MOS quantum dots or high fidelity multi-qubit superconducting qubit devices to achieve their objectives. Such qubits can be available in-house by the proposing team, via collaborations funded as part of this BAA and/or sourced from a suitable proven qubit foundry. High fidelity refers to the demonstrated ability to implement state-of-the-art low error universal quantum gates and low error readout. Using such qubits, these topics explore novel control techniques (C), noise (D), and information distribution schemes (A).

In contrast, topic B focuses on new spin and superconducting qubits which have a demonstrated superior performance metric when compared to standard leading gate-based qubits. Superior may be defined in relation to a particular performance metric, without sacrificing other important performance metrics (e.g., T1, T2, valley splitting, environmental requirements, etc.). The goal of topic B is to develop high fidelity multi-qubit gate schemes for such qubits.

Following topics which fall outside this call and will not be considered for award:

1) Atomic and molecular systems (e.g., neutral atoms, trapped ions)

2) Optical photon-based QC

3) Systems without a path to universal 1 and 2 qubit gates

4) Immature qubits which have yet to demonstrate a superior performance metric in leading solid-state qubit approaches, without sacrificing performance in other metrics relevant to gate-based quantum computing

5) Quantum simulators or simulations

6) Quantum annealing, measurement-based QC or other non-gate-based QC approaches (exceptions may be granted for specific application areas, e.g. entanglement generation)

Multi-disciplinary teams are encouraged in response to each topic area. Examples of expertise which should be considered as part of each team includes theory, simulation, materials, fabrication and experimental.

Below each topic area is briefly introduced. Much more detail follows, with separate sections dedicated to each topic area.

A.1 Brief Topic Overviews

Topic A: The Modular Quantum Gates (ModQ) topic focuses on distributed quantum computing proof-of-concept demonstrations. For both Si-spin and superconducting qubit devices, the end goal of this topic is to demonstrate high fidelity and high speed two-qubit gates between separate quantum computing modules.

Topic B: The Gates on Advanced qubits with Superior Performance (GASP) topic aims to develop quantum gate protocols optimized for recently demonstrated, potentially superior, qubit approaches in both superconducting and leading semiconductor-spin systems. Superiority can be quantified in several ways such as longer lifetimes, less sensitivity to noise, ease of fabrication, relaxed environmental requirements etc. Superior performance on a particular metric must be achieved without sacrificing performance on other qubit metrics. The focus will be on high fidelity 1 & 2 qubit gates and readout for such promising new qubit designs.

Topic C: The Fast Control And Readout Schemes (FastCARS) topic is focused on novel qubit control and readout schemes. In particular, this topic addresses concepts for implementing closed loop control feedback for quantum operations, stabilization of quantum operation calibration against on-chip fluctuations through classical or autonomous mechanisms, quantization of control signal amplitude, or improved or novel readout schemes. Proposals must target a qubit system with a demonstrated universal gate set, well-understood two qubit gate operations, and with sufficient scale to understand the limitations imposed by multi-qubit operations.

Topic D: Noise in Solid-State Superconducting and Spin qubit Systems (NS5). The objective of the NS5 program is to improve qubit gate fidelities by finding techniques, materials and processes that substantially and consistently reduce noise in at least one of solid-state superconducting and spin systems. There are five subject areas of interest. The first subject area relates to reducing externally induced thermal noise that causes excess qubit excitation and decoherence. The second subject area is focused on reducing noise generated by materials causing excess qubit excitation and decoherence. The third subject area uses multiple qubit correlated noise to learn more about noise sources. The fourth subject is very similar to the third, but uses qubit sensors in combination with alternate environment sensors to learn more about noise sources. The final subject area concerns the injection of well characterized artificial noise and the correlation of that injected noise with qubit performance.

More detail on each of these topic areas is provided below.

Teams may submit to multiple topic areas, with the primary topic of focus identified and a full description of any leveraging across topics included.

A.2 Detailed Topic Descriptions

A.2.1 Topic A: Modular Quantum Gates (ModQ)

This topic aims to demonstrate high-fidelity gates between spatially separated solid-state qubit technologies in a distributed, or modular, network. Modular quantum computing is based on the concept of small, high-fidelity, reproducible nodes, linked together in a larger distributed quantum information network. Module-to-module interactions are typically slower and lower quality when compared with intra-module interactions. There are currently many different approaches aimed at realizing these links such as transduction to flying qubits, coupling to specialized communication qubits, measurement-based entanglement, phonon coupling, quantum state transfer, spin shuttling or waveguide coupling. Superconducting qubit technology is highly amenable to long-distance transduction schemes. Thus, there are many examples in the literature of entanglement generation over long distances, even between separate packages, albeit to-date with fidelities much lower than intra-chip gates. In contrast, spin qubits built from Si-heterostructures have interactions which are typically limited to very local qubit-qubit separations by the exchange interaction. There has been some recent pioneering work aimed at enabling long distance interactions via coupling to microwave photons, surface acoustic waves or spin shuttling.

The goal of this topic is to explore such distributed quantum information schemes for both spin and superconducting technologies. Initially the focus will be on demonstrating 2-qubit gates between distant qubits, perhaps on the same chip, with spacing >> characteristic qubit sizes. By program end, the program target is to demonstrate high speed and high fidelity 2-qubit gates between distant modules.

ModQ is a four-year program and is broken-up into two phases, each 2 years in length.

1. Phase I: Proof-of-concept long-distance intra-module 2-qubit gates:

Phase I is focused on demonstrating the basic physics of the remote coupling scheme.

This demo can be implemented on the same chip, or within the same module, but the qubits involved in the gate must have a separation >> the typical qubit size-scale. The fidelity of the entanglement generation operation must be characterized, and the major sources of error tabulated. Finally, experimental results should inform plans for demonstrating 2-qubit gates between separate chips or modules in phase II.

Note, some long-distance gate schemes inherently involve coupling off-chip, e.g., microwave-to-optical transduction via mechanical resonators. For such cases, phase I should focus on demonstrating bi-directional transfer at the fidelities and rates required for the phase II demo.

Each team should propose quantitative milestones relevant to their approach for each year-end, with an emphasis on the end-of-phase milestones. These milestones should support the program goals.

While proposing teams are free to propose yearly goals relevant to their approach and maturity-level, guidance on yearly goals of phase I include:

a. Year 1 goals:

1. Proof-of-concept demonstration of intra-module remote entanglement scheme (low fidelity, low rates). Demonstration must be chosen to inform long-term approach to remote qubit entanglement. Two qubits are minimally required for this demonstration and/or a demonstration of high fidelity and high rate bi-directional transduction to flying qubits.

2. Characterization of dominant error sources in entangled state generation.

3. Begin planning inter-module remote entanglement generation scheme and detailing of expected challenges, limitations and additional technology development needed.

b. Year 2 goals:

1. Reduction of error sources and demonstration of high-fidelity intra-module remote entanglement schemes. Nominally, each group of separated qubits should have 2 or more qubits with a universal gate set and readout.

2. Demonstration of a basic quantum algorithm utilizing both groups of qubits and comparison with performance either expected from qubit characterization or simulated within a single module.

3. Deliver end-of-phase report: Refine plan detailing inter-module remote entanglement generation scheme and prediction of performance based on phase I proof-of-concept demonstrations, characterizations and simulations.

Teams focused on approaches that are already more advanced (e.g., already have a proof-of-concept demo of remote entanglement generation scheme) may skip year 1 and propose suitable new year 2 goals.

The algorithm proposed in year 2 should be chosen to gain insight into the effects of multiple ‘weak-spots’ in the qubit array, i.e., the goal is to understand if the module-to-module links are ‘bottle-necks’ or the dominant source of performance limitations in algorithm performance. The primary purpose of this algorithm task is to perform a rigorous test of the device. Hence, demonstration of an algorithm with practical application isn’t the focus, but algorithms designed to push the limits of the qubit array. For example, such algorithms may even consist of a random sequence of gates. The proposal should motivate the choice of algorithm and estimate the effects of the remote entanglement generation metrics on the algorithm output.

2. Phase II: Demonstration of high-fidelity inter-module 2-qubit gates:

Phase II is focused on taking the basic physics demos from phase I and implementing the remote-entanglement plan from end-of-year-2. This demo must be between two or more modules, with a universal set of 1 and 2 qubit gates and readout in each module (i.e., min of 2 qubits in each module). The fidelity of the entanglement generation operation must be characterized, and the major sources of error tabulated. Finally, entanglement generation between modules must be used either directly to perform 2-qubit gates between modules or used for teleported 2-qubit gates between qubits not involved in the inter-module communications.

Note, while some long-distance gate schemes inherently involve exiting to room temperature after transduction to flying qubits (e.g., microwave-to-optical transduction), only a single cryostat is required for the ModQ program. No cryostat-to-cryostat communications are required. Modules (or chips) maybe housed in the same cryogenic package, or within separate packages in the same cryostat.

Results from phase II demonstrations will be used to inform a scheme for arraying multiple modules, to be delivered in the final year of the program. While only two-module demonstrations are required in ModQ, three-module communication maybe a suitable goal for the more advanced approaches (e.g., those that skipped year 1 goals).

Each team should propose quantitative milestones relevant to their approach for each year-end, with an emphasis on the end-of-phase milestones. These milestones should support the program goals.

Specific guidance on goals of phase-II is:

c. Year 3 goals:

1. Proof-of-concept demonstration of inter-module remote entanglement scheme (low fidelity, low rates).

2. Characterization of dominant error sources in entangled state generation

3. Initial designs/plans for 3-10 modules and identification of any gate modifications required or any supporting technology developments required.

d. Year 4 goals:

1. Reduction of error sources and demonstration of high-fidelity remote entanglement schemes (inter-module). Each module must have 2 or more qubits with a universal gate-set.

2. Demonstration of a basic inter-module algorithm (from phase I) and comparison with performance from phase I proof-of-concept demonstrations.

3. Deliver end-of-phase report: Refine plan detailing multi-module (3-10 modules) remote entanglement generation scheme and prediction of performance based on proof-of-concept demonstrations

Teams focused on approaches that are already more advanced (e.g., already have a proof-of-concept demo of remote entanglement generation scheme) may propose more advanced year 4 goals such as implementation of a distributed network of 3 or more modules.

White papers and proposals should provide quantitative metrics for gate fidelities, gate speeds for each year and should detail algorithms, supporting technology development, or other demos to be demonstrated.

Possible approaches to meet program goals include (but not limited to):

1. Resonant microwave photon coupling

2. Phonon-mediated transduction, e.g., surface acoustic wave

3. Measurement-induced entanglement

4. Waveguide coupling

5. Hybrid schemes with entanglement generation via specialized communication qubit, e.g., hole spin or charge qubit

6. Continuously driven entanglement generation schemes

7. Quantum-state-transfer

8. Spin shuttling

9. Transduction to flying qubit via quantum intermediator, e.g., mechanical resonators, atomic or atomic-like element

10. Transduction to flying qubit via resonant electro-optics

Approaches outside the scope of this topic and will not be considered for award:

1. Qubit technologies without prior demonstrations of multi-qubit “modules” with high fidelity 1-qubit gates, 2-qubit gates and readout.

2. Schemes with only classical communication between modules

3. Modules only focused on I/O

4. Long-distance communication schemes limited to intra-module implementation

A.2.2 Topic B: Gates on Advanced qubits with Superior Performance (GASP)

The Gates on Advanced qubits with Superior Performance (GASP) topic aims to advance new promising and potentially superior qubit approaches beyond basic demonstrations, and focus on high fidelity gate operations. Well-studied, standard qubit approaches are now the mainstay of modern commercial and academic quantum computing efforts. For example, transmon qubits are the workhorse for many online cloud quantum computing services. Thus, such systems are becoming highly optimized and reproducible as part of these offerings. However, these qubits can only be pushed so far. Again, for example, transmon qubits can be limited by dielectric losses due to surfaces or two-level systems. Similarly, electron-spin encodings in SiGe or MOS devices can be limited by charge noise or uncontrolled valley-splitting.

Alternatively, new qubit designs are currently being explored by many research groups which promise to overcome these limitations, and thus potentially offer superior performance. For example, hole spins in Ge quantum wells can offer qubits without valley states, a low effective mass enabling simplified fabrication and low disorder for reproducible fabrication. Such superior properties can be potentially attained without sacrificing other important metrics such as nuclear spin noise, spurious energy levels, and detrimental charge noise levels. Up to 4-hole-spin qubit devices have been demonstrated to date. Similarly, bosonic encodings in superconducting cavities are actively being explored as alternatives to transmon qubits. Such encodings can have inherent error correction capabilities and can benefit from high quality factor resonant modes.

These advantages can be attained without sacrificing other important properties such as immunity to charge noise and high-fidelity readout.

Gate fidelity is the key metric of the GASP program. Superior performance demonstrations such as reduced valley splitting or enhanced qubit coherence are insufficient without corresponding demonstrations of state-of-the-art gate and readout fidelities. For example, transmon 1-qubit gate fidelities regularly perform beyond 99.9%, while 2-qubit performance is fast approaching 99.9%.

Spin qubits in silicon also have single qubit fidelities demonstrations beyond 99.9% and recent 2-qubit demos are already exceeding 99%. The overarching goal of GASP is to demonstrate gate fidelities reaching and/or exceeding these state-of-the-art results in a 4-qubit device architecture, but using advanced qubit approaches.

Proposal submissions to the GASP program may pursue a qubit approach that has been demoed in the literature, but not yet demoed by the proposing team. However, the team must show basic T1, T2*, and T2 performance within the first year of the program, coupled with a demonstration of the desired superior property, e.g., insensitivity to valley splitting, longer lifetimes, reduced sensitivity to noise, reduced environmental requirements, etc. Proposals must use published or unpublished results to clearly support and articulate promised performance levels beyond standard qubit approaches. This proof may rely on theoretical analyses; backed up by compelling experimental evidence.

GASP is a four-year program and is broken-up into two phases, each 2 years in length:

1. Phase I: High fidelity 1-qubit gates and readout:

Phase I is focused on demonstrating basic qubit performance and high fidelity 1-qubit gates, including readout. Limitations on gate performance shall be carefully characterized using an appropriate characterization method and predictions for 2-qubit gates derived.

The characterization methods to be implemented should be described clearly in the proposal. The landscape for various types of 2-qubit gates shall be theoretically explored and the leading candidates chosen for exploration during phase II.

Each team should propose quantitative milestones relevant to their approach for each year-end, with an emphasis on the end-of-phase milestones. These milestones should support the program goals.

Guidance for goals of phase I are:

Year 1 goals:

1. Coherence times (T1, T2*, T2) basic demo coupled with a demonstration of the superior property of the chosen qubit approach

2. Initial demonstration of universal 1-qubit gates and readout

3. Careful characterization of limiting noise sources

4. Theoretically compare various approaches to 2-qubit gate implementations

5. Develop designs for a device with up to 4 coupled qubits

Year 2 goals:

1. Coherence time (T1, T2*, T2) enhancement via elimination of dominating noise sources and/or design or control enhancements

2. Demonstration of 1-qubit gates and readout gate fidelities approaching or surpassing traditional qubit approaches and demonstration of reproducibility on 3 or more devices

3. Re-design if necessary

4. Fabricate initial 2-qubit gate phase II devices with up to 4 coupled qubits

5. Deliver end-of-phase report: Update of 2-qubit gate performance predictions based on 1-qubit characterizations and simulations

Teams focused on approaches that are already more advanced (e.g., already have basic gate schemes) may propose more ambitious yearly goals.

2. Phase II: Demonstrations of high-fidelity 2-qubit gates:

Phase II is focused on maintaining 1-qubit gate and readout performance in at least a 4-qubit device while approaching or exceeding state-of-the-art 2-qubit gate performance.

Year 3 will focus on initial 2-qubit demonstrations to inform optimum design for year 4.

Ideally multiple 2-qubit gate protocols shall be explored and compared for optimum performance levels.

Results from phase II demonstrations will be used to inform a scheme for arraying to a 10-qubit device and results will inform predictions on performance levels.

Guidance for goals of phase II is:

Year 3 goals:

1. Demo initial 2-qubit gate types in device with up to 4 coupled qubits

2. Characterize dominant error sources in gate implementation using appropriate tomographic techniques

3. Re-design if necessary and begin design and simulation of 10-qubit processor

Year 4 goals:

1. Reduce error sources and demonstrate high fidelity 2-qubit gates. Devices must be capable of high fidelity 1-qubit gates, 2-qubit gates and readout. Goal is to reach or surpass gate and readout fidelities of traditional qubit approaches while simultaneously maintaining the qubits superior performance metric

2. Understand issues such as crosstalk and correlated noise

3. Compare performance of gates, noise limitations and control complexity to traditional qubit approaches

4. Deliver end-of-phase report: Finalize designs and simulations of a 10-qubit processor using this new approach or a modified plan based on experimental feedback. Include discussions of any identified research gaps and future possible novel research directions

Teams focused on approaches that are already more advanced (e.g., already have single qubit gates on single qubit devices) may propose more advanced year 4 goals such as the demonstration of basic quantum algorithms on a multi-qubit device.

Each team should propose quantitative milestones relevant to their approach for each year-end, with an emphasis on the end-of-phase milestones. These milestones should support the program goals.

White papers and proposals should provide quantitative metrics for gate fidelities, gate speeds for each year and should detail algorithms or other demos to be demonstrated.

Possible approaches to meet program goals include (but are not limited to):

1. Bosonic encodings in resonant cavities

2. Hole-spin encodings

3. High spin nuclear dopants in Si

4. Fluxonium qubits, or other protected qubit designs

5. Hybrid electron/donor/hole-spin devices

6. Driven, autonomous and/or dissipative systems

7. Hybrid spin-superconducting devices

1. Gate optimization using standard well understood qubit designs, e.g., transmon qubits, 1- 3 electron spin encodings in SiGe or MOS

2. Qubits which are still exploratory in nature, i.e., qubits with no demonstrations to support claims of improved/superior performance (e.g., qubits based on 2D materials, Majorana Fermion)

3. Qubit approaches without a clear path to scaling to multi-qubit devices with universal gate sets

A.2.3 Topic C: Fast Control And Readout Schemes (FastCARS)

Systems with high accuracy and repeatability often incorporate precision control feedback loops, either open loop using periodic re-calibration of processes, or closed loop. In this context closed loop refers to online continuously updated control processes requiring no user interaction, while open loop refers to using stored, user estimated, or periodically updated control parameters.

Coherent single-quanta systems such as qubits pose challenging constraints that stymie many classical closed-loop feedback methods, leading to open loop schemes [Khaneja2005, Teske2019, Harper2020]. Nevertheless, innovative approaches to feedback exist which exploit either the semi-classical nature of the control fields themselves [Nakajima2021] or which engineer truly quantum mechanical and coherent feedback loops using auxiliary quantum systems or coherent probe fields [Kerckhoff2010, Kapit2016, Gross2021].

All approaches to control signal feedback require fast and accurate readout of very small analog signals such as single RF photons, single electron movements, or single flux quanta. The very small signal levels and challenging amplification chain requirements make state readout one of the most expensive quantum operations, leading many calibration and benchmarking protocols to specifically minimize the number of measurements [Knill2008, Nielsen2020] or to use machine learning approaches to reduce measurement overhead [Seif2018]. Readout times which are a sizeable fraction of the inhomogeneous coherence time of idle qubits pose a significant challenge to state stabilization through parity checking. Finally, many existing readout schemes require considerable chip and refrigerator cold plate area, and have significant wiring and power loads, limiting their implementation on multi-qubit devices.

The FastCARS topic, calls for proposals to address concepts for implementing closed loop control feedback for quantum operations, stabilization of quantum operation calibration against on-chip fluctuations through classical or autonomous mechanisms, quantization of control signal amplitude, or improved or novel readout schemes. Proposals may either be purely theoretical or contain both experimental and theoretical work. Proposals must target a qubit system with a demonstrated universal gate set, well-understood two qubit gate operations, and with sufficient scale to understand the limitations imposed by multi-qubit operations. Proposals must place the proposed work in its proper context by citing related experimental and theoretical results.

Proposals need not address all research areas in this topic, though we welcome those which incorporate multiple areas.

FastCARS Research Area 1: Feedback for calibration, drift control, and stabilization

As qubit and quantum control experiments increase in scale, complexity, and time, sources of drift in either qubit parameters or control signals can dominate observed errors in benchmarking, tomography, or even the outcomes of long sequences of quantum gates. Hence, such systems would benefit from efficient and autonomous routines for drift or error control. Drift may arise from intrinsic mechanisms such as changes in the microscopic charge or magnetic environment experienced by the qubits, or extrinsic mechanisms such as temperature fluctuations or uncompensated changes in control signals.

Multiple approaches exist to address lower frequency qubit error terms, normally by implementing some form of feedback either to control signals or other qubit tuning parameters such as flux bias lines or gate electrode voltages, corrected or insensitive quantum gates, and optimal control. Ancillary quantum systems near the qubit could experience a similar local environment or control signals, and form part of a classical or quantum coherent feedback loop.

Error terms arising from white noise or high frequency mechanisms may need reduction through improvements in materials, fabrication processes, or protected encodings, which are outside the scope of this research area.

For this research area, we seek novel proposals to stabilize quantum operations by implementing forms of closed-loop feedback and stabilization, either through classical feedback loops or adding autonomous quantum feedback loops. The ideal feedback/stabilization technique does not measure or project the quantum state of the qubit or cause additional decoherence directly through measurement backaction, which may imply the use of an auxiliary quantum system or sensor coupled to or nearby the qubit. We particularly focus on three related areas:

1. In-situ calibration or compensation of control signals at a point very near the qubits

2. Sensing the local qubit environment and compensating for fluctuations by adjusting control fields

3. Autonomous quantum feedback, directly implementing corrections to errors such as leakage through the action of auxiliary quantum systems or coherent probe fields without classical feedback loops

Proposal structure:

Proposals are sought for four years of work addressing the goals of this research area.

Proposals must describe the approach to adding feedback, the types of fluctuations or drift to be compensated for, estimates of the bandwidth that may be practically achieved, and any limitations this approach places on single and two qubit gates. Key metrics for understanding the successful implementation feedback should be proposed. Proposers must include yearly milestones with sufficient detail to understand how the work relates to the goals of this research area, and be specific enough to assess the likelihood of success.

FastCARS Research Area 2: Readout

Fast, accurate, and scalable readout of qubit states is one of the key requirements of any future quantum computer. Readout must be faster than the dynamical processes being studied directly.

Furthermore, fast high-fidelity readout is required for standard quantum error correction codes and any readout approach must be scalable to handle large qubit arrays and the resulting crosstalk complexity.

For superconducting qubits, the research community has largely coalesced around dispersive readout with a cavity, in conjunction with a quantum limited amplifier. For semiconducting qubits, it was recently demonstrated that readout could be done in the same manner [Borjans2021]. This is in addition to two other well-established semiconductor qubit readout methods [Elzerman2004, Barthel2010], both of which involve the readout of charge in a quantum dot. It is presently unclear which readout method for semiconductor qubits will scale favorably and remain suitable for closed loop feedback. In addition to the methods mentioned above, there are several other readout approaches currently under investigation, including optical readout [Gehl2017, Lecocq2021] and quantum flux parametrons [Grover2020]. There are several approaches to improving readout in superconducting and semiconducting platforms.

Examples include improvements of the amplification chain, engineering of the single-electron transistor or cavity, machine-learning assisted readout and multiplexing. Improvement in readout that may arise from the qubit fabrication process or material considerations is beyond the scope of this research area.

For this research area, we seek novel proposals on improving the readout of qubits in superconducting and semiconducting quantum computing platforms. We particularly seek proposals which focus on the following areas:

1. In-situ improvement of readout methods and amplification chain

2. Reducing or accounting for measurement backaction

3. Novel approaches to readout. Examples include, but are not limited to, optical readout

The ideal proposal addresses speed, fidelity and scalability of readout in tandem and has minimal backaction.

Proposal structure:

Proposals are sought for four years of work addressing the goals of this research area. Proposals must describe the readout method and why it is expected to out-perform current readout methods. This must include realistically achievable estimates for readout metrics, i.e. signal-to-noise ratio, bandwidth, and fidelity, and a discussion on the scalability of the proposed method to ten qubits. We welcome proposals that address simultaneous readout of qubits. Proposals involving dispersive readout in semiconducting systems should incorporate best practices from readout of superconducting qubits when appropriate. Proposers must include yearly milestones with sufficient detail to understand how the work relates to the goals of this research area, and be specific enough to assess the likelihood of success.

FastCARS Research Area 3: Qubit Control

In superconducting and semiconducting quantum computing platforms, typically coaxial cables deliver classical (i.e., mean field) signals from signal generators that are kept at room temperature. At a scale dependent on qubit and control signal type, this method becomes impracticable due to the size of the coaxial cables and the heat load the coaxial cables place on the cryostat. Electrical non-idealities due to factors such as thermalization impact control of the amplitude and phase of these classical signals, which introduces additional control errors on top of intrinsic qubit relaxation or decoherence. Taken together, these issues place potential roadblocks on the development of quantum computers. Investigations into possible solutions are still in the beginning stages, and the practical limits of each technique to reduce qubit control error are unclear.

To overcome these challenges, this research area seeks novel proposals on improving qubit control in superconducting and semiconducting quantum computing platforms in the following two areas:

1. Delivery of qubit control signals via optical elements, with or without conversion to electrical signals in-situ

2. Control signals at the quantum limit, which improve the fidelity of control signals though quantization (e.g. counted control quanta such as photons, flux quanta, electrons, etc.)

Optical signal delivery has the potential to enable multiplexing of high bandwidth signals with minimal thermal leakage and enable single point cold grounding schemes.

A proof-of-concept experiment illustrating this approach was recently achieved [Lecocq2021]. Challenges to implementing optical signal delivery include efficient signal conversion to voltage or RF pulses, stable optical alignment inside cryostats, proper functioning of active optical elements such as diodes or interferometers at cryogenic temperatures, and strict power limits. Proposals are sought which address these and other challenges, particularly those proposals which include coherent control of qubits using control signals delivered optically. Proposals for optical readout (either directly or using driven optical modulators) should focus on research area 2.

Control signals at the quantum limit may improve qubit control error by suppressing small fluctuations in signal amplitude, though this may lead to additional constraints on control signal duration and timing. Classical implementations of this concept already exist, such as magnetic flux logic and single electron transistor logic. Challenges to this approach include very small signal levels (and hence slower operations more prone to other decoherence mechanisms), generating and counting signal quanta, and reducing control signal dissipation mechanisms not relevant for large signal levels. Generating these control signals likely must take place inside the cryostat or even on chip with the qubits. Proposals are sought which address these and other challenges, particularly those proposals which include coherent control of qubits using such a mechanism.

Proposal structure:

Proposals may either be purely theoretical or contain both experimental and theoretical work.

Proposals must describe the control method and why it is expected to out-perform current methods. This includes realistically achievable estimates for footprint and power use, improvements in control fidelity, and a discussion on possible noise introduced by the new control method. Proposals should be implementable with commonly available tools and equipment. Proposals must place the proposed work in its proper context by citing related experimental and theoretical results. Experimental implementations should be proposed to be implemented on a relatively well-understood qubit system. Proposals are sought for three years of work, with the option of extending successful proposals for an additional year. Proposers must include yearly milestones with sufficient detail to understand how the work relates to the goals of this research area, and be specific enough to assess the likelihood of success.

1. Open-loop control improvements such as compensated gates, dynamically corrected gates, or optimal control

2. Quantum error correction or state stabilization through parity checking

3. Improvements in coaxial cables

4. Dynamically corrected gates

5. Replacing coaxial cables with superconducting ribbons

6. Cryogenic electronics such as AWGs, though smaller active or passive primarily analog circuits may be included in proposals.

7. Improving automated inversion kernels for pulse predistortion

Citations:

[Elzerman2004] Elzerman, J.M. et al., Single-shot read-out of an individual electron spin in a quantum dot, Nature 430, 431 (2004).

[Khaneja2005] Khaneja, Navin et al., Optimal control of coupled spin dynamics: design of NMR pulse sequences by gradient ascent algorithms, Jour. Mag. Res. 172, 296 (2005).

[Knill2008] Knill, Emmanuel et al., Randomized benchmarking of quantum gates, Phys.

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A.2.4 Topic D: Noise in Solid-State Superconducting and Spin Systems (NS5)

The Objective of the Noise in Solid-State Superconducting and Spin Systems (NS5) program is to improve qubit gate fidelities by finding techniques, materials and processes that substantially and consistently reduce noise in at least one of the Solid-State Superconducting and Spin Systems. To achieve the above objective requires that the noise sources are understood and characterized. Superconducting and spin qubits are reaching new levels of performance due to a combination of novel device design, material optimization and noise reduction. As a result, superconducting qubits are now regularly exceeding 100 micro-seconds lifetimes and coherence times while spin qubits in Si-based heterostructures regularly reach 99.9% 1-qubit gate fidelities.

At this level of performance, it is time to re-examine noise sources and the effects of noise sources on qubit performance to shed light on issues which may have been previously hidden by poor qubit performance levels. Further, although there is still likely much more noise immunity to be gained by qubit design, progress can also be made with all designs by reducing the underlying noise. Thus, tackling the noise head-on may be highly beneficial.

Solid-state superconducting and spin qubit systems are highly sensitive to external perturbations.

Hence careful filtering, shielding and device design needs to be considered when developing high performance qubit experiments. Furthermore, comparisons between both systems often reveal common noise issues such as charge noise and noise on control lines. The goal of this topic is to characterize deleterious noise sources, develop methods for reducing these noise levels and demonstrating resulting enhanced qubit performance levels. Scope includes both time and space variation of the qubit environment.

This topic has 5 specific research areas: excess thermal noise, noise due to materials making up the qubit devices, correlated noise, local sensors, and precisely controlled artificial noise sources.

Teams proposing to these topics should include sufficient expertise for both device and noise theory and modelling as well as design, fab and test.

In each research area, 4-year proposals are sought and each proposal must provide…