 Publication Date:
 NSFPAR ID:
 10191000
 Journal Name:
 IEEE Micro
 Page Range or eLocationID:
 1 to 1
 ISSN:
 02721732
 Sponsoring Org:
 National Science Foundation
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Quantum computation is traditionally expressed in terms of quantum bits, or qubits. In this work, we instead consider threelevel qutrits. Past work with qutrits has demonstrated only constant factor improvements, owing to the log2(3) binarytoternary compression factor. We present a novel technique using qutrits to achieve a logarithmic depth (runtime) decomposition of the Generalized Toffoli gate using no ancillaa significant improvement over linear depth for the best qubitonly equivalent. Our circuit construction also features a 70x improvement in twoqudit gate count over the qubitonly equivalent decomposition. This results in circuit cost reductions for important algorithms like quantum neurons and Grover search. We develop an opensource circuit simulator for qutrits, along with realistic nearterm noise models which account for the cost of operating qutrits. Simulation results for these noise models indicate over 90% mean reliability (fidelity) for our circuit construction, versus under 30% for the qubitonly baseline. These results suggest that qutrits offer a promising path towards scaling quantum computation.

Compiling highlevel quantum programs to machines that are size constrained (i.e. limited number of quantum bits) and time constrained (i.e. limited number of quantum operations) is challenging. In this paper, we present SQUARE (Strategic QUantum Ancilla REuse), a compilation infrastructure that tackles allocation and reclamation of scratch qubits (called ancilla) in modular quantum programs. At its core, SQUARE strategically performs uncomputation to create opportunities for qubit reuse. Current Noisy IntermediateScale Quantum (NISQ) computers and forwardlooking FaultTolerant (FT) quantum computers have fundamentally different constraints such as data locality, instruction parallelism, and communication overhead. Our heuristicbased ancillareuse algorithm balances these considerations and fits computations into resourceconstrained NISQ or FT quantum machines, throttling parallelism when necessary. To precisely capture the workload of a program, we propose an improved metric, the "active quantum volume," and use this metric to evaluate the effectiveness of our algorithm. Our results show that SQUARE improves the average success rate of NISQ applications by 1.47X. Surprisingly, the additional gates for uncomputation create ancilla with better locality, and result in substantially fewer swap gates and less gate noise overall. SQUARE also achieves an average reduction of 1.5X (and up to 9.6X) in active quantum volume for FT machines.

Highly entangled “graph” states of photons have applications in universal quantum computing and in quantum communications. In the latter context, they have been proposed as the key ingredient in the establishment of longdistance entanglement across quantum repeater networks. Recently, a general deterministic approach to generate repeater graph states from quantum emitters was given. However, a detailed protocol for the generation of such states from realistic systems is still needed in order to guide experiments. Here, we provide such explicit protocols for the generation of repeater graph states from two types of quantum emitters: NV centers in diamond and selfassembled quantum dots. A crucial element of our designs is an efficient controlledZ gate between the emitter and a nuclear spin, used as an ancilla qubit. Additionally, a fast protocol for using pairs of exchangecoupled quantum dots to produce repeater graph states is described. Our focus is on nearterm experiments feasible with existing experimental capabilities.

Running quantum programs is fraught with challenges on on today’s noisy intermediate scale quantum (NISQ) devices. Many of these challenges originate from the error characteristics that stem from rapid decoherence and noise during measurement, qubit connections, crosstalk, the qubits themselves, and transformations of qubit state via gates. Not only are qubits not “created equal”, but their noise level also changes over time. IBM is said to calibrate their quantum systems once per day and reports noise levels (errors) at the time of such calibration. This information is subsequently used to map circuits to higher quality qubits and connections up to the next calibration point. This work provides evidence that there is room for improvement over this daily calibration cycle. It contributes a technique to measure noise levels (errors) related to qubits immediately before executing one or more sensitive circuits and shows that justintime noise measurements can benefit late physical qubit mappings. With this justintime recalibrated transpilation, the fidelity of results is improved over IBM’s default mappings, which only uses their daily calibrations. The framework assess two major sources of noise, namely readout errors (measurement errors) and twoqubit gate/connection errors. Experiments indicate that the accuracy of circuit results improves by 3304%more »

Abstract The quantum simulation of quantum chemistry is a promising application of quantum computers. However, for
N molecular orbitals, the gate complexity of performing Hamiltonian and unitary Coupled Cluster Trotter steps makes simulation based on such primitives challenging. We substantially reduce the gate complexity of such primitives through a twostep lowrank factorization of the Hamiltonian and cluster operator, accompanied by truncation of small terms. Using truncations that incur errors below chemical accuracy allow one to perform Trotter steps of the arbitrary basis electronic structure Hamiltonian with$${\mathcal{O}}({N}^{4})$$ $O\left({N}^{4}\right)$ gate complexity in small simulations, which reduces to$${\mathcal{O}}({N}^{3})$$ $O\left({N}^{3}\right)$ gate complexity in the asymptotic regime; and unitary Coupled Cluster Trotter steps with$${\mathcal{O}}({N}^{2})$$ $O\left({N}^{2}\right)$ gate complexity as a function of increasing basis size for a given molecule. In the case of the Hamiltonian Trotter step, these circuits have$${\mathcal{O}}({N}^{3})$$ $O\left({N}^{3}\right)$ depth on a linearly connected array, an improvement over the$${\mathcal{O}}({N}^{2})$$ $O\left({N}^{2}\right)$ scaling assuming no truncation. As a practical example, we show that a chemically accurate Hamiltonian Trotter step for a 50 qubit molecular simulation can be carried out in the molecular orbital basis with as few as 4000 layers of parallel nearestneighbor twoqubit gates, consisting of fewer than 10^{5}nonClifford rotations. We also apply our algorithm to iron–sulfur clusters relevant for elucidating the mode of action of metalloenzymes.$${\mathcal{O}}({N}^{3})$$ $O\left({N}^{3}\right)$