Alexander Yu. Vlasov
In this paper is shown an application of Clifford algebras to the construction of computationally universal sets of quantum gates for $n$-qubit systems. It is based on the well-known application of Lie algebras together with the especially simple commutation law for Clifford algebras, which states that all basic elements either commute or anticommute.
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A popular universal gate set for quantum computing with qubits is Clifford+T, as this can be readily implemented on many fault-tolerant architectures. For qutrits, there is an equivalent T gate, that, like its qubit analogue, makes Clifford+T approximately universal, is injectable by a magic state, and supports magic state distillation. However, it was claimed that a better gate set for qutrits might be Clifford+R, where R=diag(1,1,-1) is the metaplectic gate, as certain protocols and gates could more easily be implemented using the R gate than the T gate. In this paper we show that when we ha…
In this work, we introduce a new circuit optimization technique to reduce the number of T gates in Clifford+T circuits by treating T gates conjugated by Clifford gates as $\fracπ{4}$-rotations around Pauli operators. The tested benchmarks shows up to $71.43\%$ and an average $42.67\%$ reduction in T-count, both surpass the best performance reported. The worst case complexity of our algorithm is $O(nk^2)$ where $n$ is the number of qubits and $k$ is the number of T gates in the original Clifford+T circuit.
Fault-Tolerant Constant-Depth Clifford Gates on Toric Codes
针对2D环面码，提出并模拟了结合折叠横向门、Dehn扭转和单次逻辑Pauli测量的容错常深度逻辑门组，其能生成完整Clifford群并具备容错性与常深度特性。
The Clifford hierarchy is a set of gates that appears in the theory of fault-tolerant quantum computation, but its precise structure remains elusive. We give a complete characterization of the diagonal gates in the Clifford hierarchy for prime-dimensional qudits. They turn out to be $p^{m}$-th roots of unity raised to polynomial functions of the basis state to which they are applied, and we determine which level of the Clifford hierarchy a given gate sits in based on $m$ and the degree of the polynomial.
For a number of useful quantum circuits, qudit constructions have been found which reduce resource requirements compared to the best known or best possible qubit construction. However, many of the necessary qutrit gates in these constructions have never been explicitly and efficiently constructed in a fault-tolerant manner. We show how to exactly and unitarily construct any qutrit multiple-controlled Clifford+T unitary using just Clifford+T gates and without using ancillae. The T-count to do so is polynomial in the number of controls $k$, scaling as $O(k^{3.585})$. With our results we can cons…
With the Gottesman-Kitaev-Preskill (GKP) encoding, Clifford gates and error correction can be carried out using simple Gaussian operations. Still, non-Clifford gates, required for universality, require non-Gaussian elements. In their original proposal, GKP suggested a particularly simple method of using a single application of the cubic phase gate to perform the logical non-Clifford T-gate. Here we show that this cubic phase gate approach performs extraordinarily poorly, even for arbitrarily large amounts of squeezing in the GKP state. Thus, contrary to common belief, the cubic phase gate is n…
Controlled Gates in the Clifford Hierarchy
该研究针对量子比特Clifford层级中的受控门，推导并证明了其需满足的一组强约束必要条件，将结论扩展至特定酉直和构成的门，还给出了该条件具备充分性的相关证据。
Non-Clifford Gates are Required for Long-Term Memory
研究证明在去极化噪声下，仅用克利福德门无法长期存储信息，非克利福德门是必需的，这推翻了基于新量子比特供应的容错协议的可能性。
We explore the implementation of pseudo-random single-qubit rotations and multi-qubit pseudo-random circuits constructed only from Clifford gates and the T-gate, a phase rotation of pi/4. Such a gate set would be appropriate for computations performed in a fault tolerant setting. For single-qubit rotations the distribution of parameters found for unitaries constructed from Clifford plus T quickly approaches that of random rotations and require significantly fewer gates than the construction of arbitrary single-qubit rotations. For Clifford plus T pseudo-random circuits we find an exponential c…
A Classification of Transversal Clifford Gates for Qubit Stabilizer Codes
本文根据可横截实现的对角Clifford门集合对量子比特稳定子码进行分类，证明其门群必属于六个矩阵群族之一，并基于Rains的自同态矩阵代数理论给出了完整的对角Clifford对称性分类。
We show that qubit stabilizer states can be represented by non-negative quasi-probability distributions associated with a Wigner-Weyl-Moyal formalism where Clifford gates are positive state-independent maps. This is accomplished by generalizing the Wigner-Weyl-Moyal formalism to three generators instead of two---producing an exterior, or Grassmann, algebra---which results in Clifford group gates for qubits that act as a permutation on the finite Weyl phase space points naturally associated with stabilizer states. As a result, a non-negative probability distribution can be associated with each …
Semi-Cliffordness of the Clifford hierarchy for a single qudit in composite dimensions
本文证明单 qudit 的 Clifford 层级中每个门为半-Clifford 当且仅当维度无平方因子，并区分了四种门类型，指出在任意维度下第三级门均为广义半-Clifford。
Hardware efficient transpilation of quantum circuits to a quantum devices native gateset is essential for the execution of quantum algorithms on noisy quantum computers. Typical quantum devices utilize a gateset with a single two-qubit Clifford entangling gate per pair of coupled qubits, however, in some applications access to a non-Clifford two-qubit gate can result in more optimal circuit decompositions and also allows more flexibility in optimizing over noise. We demonstrate calibration of a low error non-Clifford Controlled-$\fracπ{2}$ phase (CS) gate on a cloud based IBM Quantum computing…
Fault-tolerant quantum computation is a basic problem in quantum computation, and teleportation is one of the main techniques in this theory. Using teleportation on stabilizer codes, the most well-known quantum codes, Pauli gates and Clifford operators can be applied fault-tolerantly. Indeed, this technique can be generalized for an extended set of gates, the so called ${\mathcal{C}}_k$ hierarchy gates, introduced by Gottesman and Chuang (Nature, 402, 390-392). ${\mathcal{C}}_k$ gates are a generalization of Clifford operators, but our knowledge of these sets is not as rich as our knowledge of…
Mutual transformations of arbitrary ternary qubit trees by Clifford gates
证明了节点数相同的三元量子比特树可通过自然定义的 Clifford 门序列相互变换，或转化为对应 Jordan-Wigner 变换的一维链标准表示。
Permutation gates in the third level of the Clifford hierarchy
本研究探讨了 Clifford 层级第三层中的置换门，完整刻画了半 Clifford 置换门，并证明任意第三层置换门均为阶梯形式 Toffoli 门的乘积，同时确定存在非半 Clifford 置换门的最小量子比特数为 7。
Topological subsystem color codes add to the advantages of topological codes an important feature: error tracking only involves measuring 2-local operators in a two dimensional setting. Unfortunately, known methods to compute with them were highly unpractical. We give a mechanism to implement all Clifford gates by code deformation in a planar setting. In particular, we use twist braiding and express its effects in terms of certain colored Majorana operators.
Climbing the Clifford Hierarchy
本文研究了平方根可攀登到第三层级的克莱因门，揭示了克莱因层级的结构与性质。
Quantum Accreditation with Non-Clifford Two-qubit Gates
本文提出了一种适用于包含非Clifford双量子比特门的量子电路的量子认证协议家族，通过上界限制无误差与有误差计算概率分布的总变差距离，并增强了协议对小扰动的鲁棒性，同时将Pauli旋风推广到非Pauli单量子比特基。
Randomized benchmarking is a widely used experimental technique to characterize the average error of quantum operations. Benchmarking procedures that scale to enable characterization of $n$-qubit circuits rely on efficient procedures for manipulating those circuits and, as such, have been limited to subgroups of the Clifford group. However, universal quantum computers require additional, non-Clifford gates to approximate arbitrary unitary transformations. We define a scalable randomized benchmarking procedure over $n$-qubit unitary matrices that correspond to protected non-Clifford gates for a…
Benchmarking non-Clifford gates using only Pauli twirling group
本文提出了一种基于Pauli转移特征的基准测试方法，用于高效评估非Clifford门的保真度，通过局部Pauli操作估计量子通道的Pauli转移矩阵元素。
Non-Clifford Fusion: T-Gate Optimization for Quantum Simulation
提出NCF编译框架，通过分组共轭泡利字符串来同时合成量子门，平均减少T门数量57.4%、T门深度49.1%和克利福德门数量49.0%，优化哈密顿模拟。
We generalize an efficient exact synthesis algorithm for single-qubit unitaries over the Clifford+T gate set which was presented by Kliuchnikov, Maslov and Mosca. Their algorithm takes as input an exactly synthesizable single-qubit unitary--one which can be expressed without error as a product of Clifford and T gates--and outputs a sequence of gates which implements it. The algorithm is optimal in the sense that the length of the sequence, measured by the number of T gates, is smallest possible. In this paper, for each positive even integer $n$ we consider the "Clifford-cyclotomic" gate set co…
Optimal number of parametrized rotations and Hadamard gates in parametrized Clifford circuits with non-repeated parameters
本研究提出一种低复杂度高效算法，可最优缩减参数不重复的参数化Clifford电路中的参数化旋转门数量，同时证明既有Hadamard门优化流程对该类电路的最优性，算法在少内部Hadamard门的电路中效率突出。
Efficient encoding of the 2D toric code logical state using local Clifford gates
提出一种仅用局域 $CX$ 和 $H$ 门、深度 $2L+1$ 的电路高效编码二维环面码逻辑态的算法。
We developed a general framework for synthesizing target gates by using a finite set of basic gates, which is a crucial step in quantum compilation. When approximating a gate in SU($n$), a naive brute-force search requires a computational complexity of $O(1/\varepsilon^{(n^2 - 1)})$ to achieve an approximation with error $\varepsilon$. In contrast, by using our method, the complexity can be reduced to $O(-n^2 \log\varepsilon/\varepsilon^{((n^2 - 1)/2)})$. This method requires almost no assumptions and can be applied to a variety of gate sets, including Clifford+$T$ and Clifford+$V$. Further, w…
The design of a four-dimensional toric code is explored with the goal of finding a lattice capable of implementing a logical $\mathsf{CCCZ}$ gate transversally. The established lattice is the octaplex tessellation, which is a regular tessellation of four-dimensional Euclidean space whose underlying 4-cell is the octaplex, or hyper-diamond. This differs from the conventional 4D toric code lattice, based on the hypercubic tessellation, which is symmetric with respect to logical $X$ and $Z$ and only allows for the implementation of a transversal Clifford gate. This work further develops the estab…
Teleportation is a crucial element in fault-tolerant quantum computation and a complete understanding of its capacity is very important for the practical implementation of optimal fault-tolerant architectures. It is known that stabilizer codes support a natural set of gates that can be more easily implemented by teleportation than any other gates. These gates belong to the so called $\mathcal{C}_k$ hierarchy introduced by Gottesman and Chuang (Nature \textbf{402}, 390). Moreover, a subset of $\mathcal{C}_k$ gates, called semi-Clifford operations, can be implemented by an even simpler architect…
Surface and color codes are two forms of topological quantum error correction in two spatial dimensions with complementary properties. Surface codes have lower-depth error detection circuits and well-developed decoders to interpret and correct errors, while color codes have transversal Clifford gates and better code efficiency in the number of physical qubits needed to achieve a given code distance. A formal equivalence exists between color codes and folded surface codes, but it does not guarantee the transferability of any of these favorable properties. However, the equivalence does imply the…
We use our Clifford algebra technique, that is nilpotents and projectors which are binomials of the Clifford algebra objects $γ^a$ with the property $\{γ^a,γ^b\}_+ = 2 η^{ab}$, for representing quantum gates and quantum algorithms needed in quantum computers in an elegant way. We identify $n$-qubits with spinor representations of the group SO(1,3) for a system of $n$ spinors. Representations are expressed in terms of products of projectors and nilpotents. An algorithm for extracting a particular information out of a general superposition of $2^n$ qubit states is presented. It reproduces for a …
In a continuous-variable optical system, the Gottesman-Kitaev-Preskill (GKP) qubit is a promising candidate for fault-tolerant quantum computation. To implement non-Clifford operations on GKP qubits, non-Gaussian operations are required. In this context, the implementation of a cubic phase gate by combining nonlinear feedforward with ancillary states has been widely researched. Recently, however, it is pointed out that the cubic phase gate is not the most suitable for non-Clifford operations on GKP qubits. In this work, we show that we can achieve linear optical implementation of non-Clifford …
Quantum gates in experiment are inherently prone to errors that need to be characterized before they can be corrected. Full characterization via quantum process tomography is impractical and often unnecessary. For most practical purposes, it is enough to estimate more general quantities such as the average fidelity. Here we use a unitary 2-design and twirling protocol for efficiently estimating the average fidelity of Clifford gates, to certify a 7-qubit entangling gate in a nuclear magnetic resonance quantum processor. Compared with more than $10^8$ experiments required by full process tomogr…
Clifford synthesis via generalized S and CZ gates
本文展示通过最多2n个多量子比特联合测量实现任意n量子比特克莱因群单元ary，且测量可分成两个互不干扰的集合，为实现克莱因群单元ary提供了灵活的空间时间权衡。
Universal Fault Tolerance with Non-Transversal Clifford Gates
我们提出一个通用框架，通过标志小工具实现任意克利福德电路的容错，允许横向T门与非横向克利福德门结合，以O(d^2 log(nd^2 log n))辅助比特实现距离d容错，并在[[15,1,3]]码上演示了非横向Hadamard门。
Characterization of permutation gates in the third level of the Clifford hierarchy
本研究表征了Clifford层级第三层的置换门，证明其可表示为阶梯形式Toffoli门的乘积（左右乘Clifford置换），给出了充要条件，并构造出逆元不在对应高层的非半Clifford置换门族。
Theory of quantum error mitigation for non-Clifford gates
本文提出针对非Clifford门的量子误差缓解方法，通过随机Pauli门和测量处理实现通道转换，以应对非Clifford门的复杂噪声特性。
We show that the class QMA does not change even if we restrict Arthur's computing ability to only Clifford gate operations (plus classical XOR gate). The idea is to use the fact that the preparation of certain single-qubit states, so called magic states, plus any Clifford gate operations are universal for quantum computing. If Merlin is honest, he sends the witness plus magic states to Arthur. If Merlin is malicious, he might send other states to Arthur, but Arthur can verify the correctness of magic states by himself. We also generalize the result to QIP[3]: we show that the class QIP[3] does…
Fault-tolerant, error-corrected quantum computation is commonly acknowledged to be crucial to the realisation of large-scale quantum algorithms that could lead to extremely impactful scientific or commercial results. Achieving a universal set of quantum gate operations in a fault-tolerant error-corrected framework suffers from a `conservation of unpleasantness'. In general, no matter what error correction technique is employed, there is always one element of a universal gate set that carries a significant resource overhead - either in physical qubits, computational time, or both. Specifically,…
Fold-Transversal Clifford Gates for Quantum Codes
该研究将折叠概念从表面码推广到CSS码，提出仅用横向门和量子比特置换实现LDPC量子码逻辑操作的通用方法，并在Bring's code上验证，限制其4比特逻辑子空间可得到完整Clifford群。
Optimal Hadamard gate count for Clifford$+T$ synthesis of Pauli rotations sequences
本文针对Clifford+T综合中Hadamard门减少问题，提出以最少Hadamard门综合π/4 Pauli旋转序列的算法，并进一步最优最小化首末T门之间的Hadamard门数量，从而有效降低T门计数。
Quantum Phase Estimation is one of the most useful quantum computing algorithms for quantum chemistry and as such, significant effort has been devoted to designing efficient implementations. In this article, we introduce TFermion, a library designed to estimate the T-gate cost of such algorithms, for an arbitrary molecule. As examples of usage, we estimate the T-gate cost of a few simple molecules and compare the same Taylorization algorithms using Gaussian and plane-wave basis.
Randomized benchmarking for qudit Clifford gates
本文提出针对量子比特门的酉门随机基准测试（URB）方法，利用酉2设计，并证明其不同于多比特对称子空间的URB，通过伪代码便于应用。
Asymptotically Good Quantum Codes with Addressable and Transversal Non-Clifford Gates
本文构造了首族渐近好量子码，支持横向可寻址的非克利福德CCZ门，通过深度为一的物理电路实现任意三个逻辑量子比特上的逻辑门，改进了先前具有逆多对数参数的构造。
We study two-qubit circuits over the Clifford+CS gate set, which consists of the Clifford gates together with the controlled-phase gate CS=diag(1,1,1,i). The Clifford+CS gate set is universal for quantum computation and its elements can be implemented fault-tolerantly in most error-correcting schemes through magic state distillation. Since non-Clifford gates are typically more expensive to perform in a fault-tolerant manner, it is often desirable to construct circuits that use few CS gates. In the present paper, we introduce an efficient and optimal synthesis algorithm for two-qubit Clifford+C…
On Classical Simulation of Quantum Circuits Composed of Clifford Gates
本文以初学者友好的方式逐步解析 Gottesman-Knill 定理，利用矩阵和张量积等概念及叠加纠缠示例，直观阐释了仅含 Clifford 门的量子电路的经典模拟原理。
The Gottesman-Knill theorem asserts that a quantum circuit composed of Clifford gates can be efficiently simulated on a classical computer. Here we revisit this theorem and extend it to quantum circuits composed of Clifford and T gates, where T is the single-qubit 45-degree phase shift. We assume that the circuit outputs a bit string x obtained by measuring some subset of w qubits. Two simulation tasks are considered: (1) computing the probability of a given output x, and (2) sampling x from the output probability distribution. It is shown that these tasks can be solved on a classical computer…
For universal quantum computation, a major challenge to overcome for practical implementation is the large amount of resources required for fault-tolerant quantum information processing. An important aspect is implementing arbitrary unitary operators built from logical gates within the quantum error correction code. A synthesis algorithm can be used to approximate any unitary gate up to arbitrary precision by assembling sequences of logical gates chosen from a small set of universal gates that are fault-tolerantly performable while encoded in a quantum error-correction code. However, current p…
A simple asymptotically optimal Clifford circuit compilation algorithm
提出一种简单的渐近最优 Clifford 电路编译算法，将任意 n 量子比特 Clifford 算子分解为仅含 CNOT/CPHASE 门的三个子电路，电路规模为 O(n^2/log n)，推导仅依赖辛表示和矩阵操作。
Fast equivalence checking of quantum circuits of Clifford gates
提出一种基于数学结果的确定性算法，可在O(n·m)时间内检查Clifford门量子电路的等价性，结合Stim模拟器在万级量子比特上实现比现有方法快数个数量级的验证。
CALA-$n$: A Quantum Library for Realizing Cost-Effective 2-, 3-, 4-, and 5-bit Gates on IBM Quantum Computers using Bloch Sphere Approach, Clifford+T Gates, and Layouts
本文提出基于布洛赫球和Clifford+T门的量子布局感知方法，构造2至5量子比特的高成本效益n位门，并证明其转译后量子成本始终低于标准n位门，可用于构建IBM量子计算机量子库。
Achieving the limits of automorphism gates
本文发展了一般理论，刻画稳定子码上自同构门可达到的最大逻辑群，证明其由可寻址S和CX门生成，并给出达到该群所需的物理量子比特数下界，揭示了代价分界线在对角门与CX型门之间。
Scalable Spider Nests (...Or How to Graphically Grok Transversal Non-Clifford Gates)
作为图形化参悟系列第二篇，本文用可扩展ZX-演算简洁表述蜘蛛巢恒等式并证明其可由单条新规则归纳导出，结合CSS码ZX图景刻画了任意CSS码中Clifford层级第三级的全部横向对角门。
Recently, there has been an emergence of useful applications for noisy intermediate-scale quantum (NISQ) devices notably, though not exclusively, in the fields of quantum machine learning and variational quantum algorithms. In such applications, circuits of various depths and composed of different sets of gates are run on NISQ devices. Therefore, it is crucial to find practical ways to capture the general performance of circuits on these devices. Motivated by this pressing need, we modified the standard Clifford randomized benchmarking (RB) and interleaved RB schemes targeting them to hardware…
Clifford gates play a role in the optimisation of Clifford+T circuits. Reducing the count and the depth of Clifford gates, as well as the optimal scheduling of T gates, influence the hardware and the time costs of executing quantum circuits. This work focuses on circuits protected by the surface quantum error-correcting code. The result of compiling a quantum circuit for the surface code is called a topological assembly. We use queuing theory to model a part of the compiled assemblies, evaluate the models, and make the empiric observation that at least for certain Clifford+T circuits (e.g. add…
The Clifford group is the set of gates generated by the controlled not gates, the Hadamard gate and the P={{1,0},{0,i}} gate. We will say that a n-qubit state is a Clifford state if it can be prepared using Clifford gates. In this paper we study the set of all 4-qubit Clifford states. We prove that there are 293760 states and their entanglement entropy must be either 0, 2/3, 1, 4/3 and 5/3. We also show that any pair of these states can be connected using local gates and at most 3 CNOT gates. We achieve this by splitting the 293760 states into 18 groups where each pair of states in a group can…
Reduced constant-cost implementations of Clifford operations using global interactions
本文提出利用全局纠缠门以常数代价实现任意长度 Clifford 及 CNOT 序列，无需辅助比特，并给出高效编译算法。
Optimal constant-cost implementations of Clifford operations using global interactions
本文提出一种利用全局纠缠门实现任意长度 Clifford 运算序列的常数代价（最多四次）方法，无需辅助比特，达到理论最优门数，且所需驱动功率更低，并给出高效编译算法。
Quantum error correction and fault-tolerance have provided the possibility for large scale quantum computations without a detrimental loss of quantum information. A very natural class of gates for fault-tolerant quantum computation is the Clifford gate set and as such their usefulness for universal quantum computation is of great interest. Clifford group gates augmented by magic state preparation give the possibility of simulating universal quantum computation. However, experimentally one cannot expect to perfectly prepare magic states. Nonetheless, it has been shown that by repeatedly applyin…
Constant-rate quantum codes with low-weight stabilizers and full logical Clifford actions via transversal and fold-transversal gates
本文构造了一族恒定速率、稳定子权重亚对数增长的量子码，仅用横向和折叠横向门即可实现完整逻辑Clifford群，为低空间开销容错量子计算提供路径。
When visualised as an operation on the Bloch sphere, the qubit "pi-over-eight" gate corresponds to one-eighth of a complete rotation about the vertical axis. This simple gate often plays an important role in quantum information theory, typically in situations for which Pauli and Clifford gates are insufficient. Most notably, when it supplements the set of Clifford gates then universal quantum computation can be achieved. The "pi-over-eight" gate is the simplest example of an operation from the third level of the Clifford hierarchy (i.e., it maps Pauli operations to Clifford operations under co…
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