We present a complete classification of quantum stabilizer gates in terms of the functions they generate assuming the ability to swap qubits and use ancillary workspace. Because we view these stabilizer circuits as subroutines of some general quantum computation, we insist that any ancilla qubits used during the computation must not change in an input-dependent manner. This is the first attempt at a quantum extension of the classification of reversible classical gates introduced by Aaronson et al., another part of an ambitious program to classify all quantum gate sets.
The classification uses, at its center, a reinterpretation of the tableau representation of stabilizer gates to give circuit decompositions, from which elementary generators can easily be extracted. There are a total of 57 different stabilizer classes generated in this way, 30 of which arise from the single-qubit subgroups of the Clifford group. At a high level, the remaining classes are arranged according to the bases they preserve. For instance, the CNOT gate preserves the X and Z bases because it maps X-basis elements to X-basis elements and Z-basis elements to Z-basis elements. The remaining classes are characterized by more subtle tableau invariants; for instance, the T_4 and phase gate generate a proper subclass of Z-preserving gates.
We present a complete classification of quantum stabilizer gates in terms of the functions they generate assuming the ability to swap qubits and use ancillary workspace. Because we view these stabilizer circuits as subroutines of some general quantum computation, we insist that any ancilla qubits used during the computation must not change in an input-dependent manner. This is the first attempt at a quantum extension of the classification of reversible classical gates introduced by Aaronson et al., another part of an ambitious program to classify all quantum gate sets.
The classification uses, at its center, a reinterpretation of the tableau representation of stabilizer gates to give circuit decompositions, from which elementary generators can easily be extracted. There are a total of 57 different stabilizer classes generated in this way, 30 of which arise from the single-qubit subgroups of the Clifford group. At a high level, the remaining classes are arranged according to the bases they preserve. For instance, the CNOT gate preserves the X and Z bases because it maps X-basis elements to X-basis elements and Z-basis elements to Z-basis elements. The remaining classes are characterized by more subtle tableau invariants; for instance, the T_4 and phase gate generate a proper subclass of Z-preserving gates.
We examine the following problem: given a collection of Clifford gates, describe the set of unitaries generated by circuits composed of those gates. Specifically, we allow the standard circuit operations of composition and tensor product, as well as ancillary workspace qubits as long as they start and end in states uncorrelated with the input, which rule out common "magic state injection" techniques that make Clifford circuits universal. We show that there are exactly 57 classes of Clifford unitaries and present a full classification characterizing the gate sets which generate them. This is the first attempt at a quantum extension of the classification of reversible classical gates introduced by Aaronson et al., another part of an ambitious program to classify all quantum gate sets. The classification uses, at its center, a reinterpretation of the tableau representation of Clifford gates to give circuit decompositions, from which elementary generators can easily be extracted. The 57 different classes are generated in this way, 30 of which arise from the single-qubit subgroups of the Clifford group. At a high level, the remaining classes are arranged according to the bases they preserve. For instance, the CNOT gate preserves the X and Z bases because it maps X-basis elements to X-basis elements and Z-basis elements to Z-basis elements. The remaining classes are characterized by more subtle tableau invariants; for instance, the T_4 and phase gate generate a proper subclass of Z-preserving gates.
We present a complete classification of all possible sets of classical reversible gates acting on bits, in terms of which reversible transformations they generate, assuming swaps and ancilla bits are available for free. Our classification can be seen as the reversible-computing analogue of Post's lattice, a central result in mathematical logic from the 1940s. It is a step toward the ambitious goal of classifying all possible quantum gate sets acting on qubits. Our theorem implies a linear-time algorithm (which we have implemented), that takes as input the truth tables of reversible gates G and H, and that decides whether G generates H. Previously, this problem was not even known to be decidable. The theorem also implies that any n-bit reversible circuit can be "compressed" to an equivalent circuit, over the same gates, that uses at most 2^n*poly(n) gates and O(1) ancilla bits; these are the first upper bounds on these quantities known, and are close to optimal. Finally, the theorem implies that every non-degenerate reversible gate can implement either every reversible transformation, or every affine transformation, when restricted to an "encoded subspace." Briefly, the theorem says that every set of reversible gates generates either all reversible transformations on n-bit strings (as the Toffoli gate does); no transformations; all transformations that preserve Hamming weight (as the Fredkin gate does); all transformations that preserve Hamming weight mod k for some k; all affine transformations (as the Controlled-NOT gate does); all affine transformations that preserve Hamming weight mod 2 or mod 4, inner products mod 2, or a combination thereof; or a previous class augmented by a NOT or NOTNOT gate. Ruling out the possibility of additional classes, not in the list, requires some arguments about polynomials, lattices, and Diophantine equations.
We start by studying the subgroup structures underlying stabilizer circuits. Then we apply our results to provide two normal forms for stabilizer circuits. These forms are computed by induction using simple conjugation rules in the Clifford group and our algorithms do not rely on a special decomposition in the symplectic group. The first normal form has shape CX-CZ-P-Z-X-H-CZ-P-H, where CX (resp. CZ) denotes a layer of CNOT (resp. controlled-Z) gates, P a layer of phase gates, X (resp. Z) a layer of Pauli-X (resp. Pauli-Z) gates. Then we replace most of the controlled-Z gates by CNOT gates to obtain a second normal form of type P-CX-CZ-CX-Z-X-H-CZ-CX-P-H. In this second form, both controlled-Z layers have depth 1 and together contain therefore at most n controlled-Z gates. We also consider normal forms for stabilizer states and graph states. Finally we carry out a few tests on classical and quantum computers in order to show experimentally the utility of these normal forms to reduce the gate count of a stabilizer circuit.
The first theme of this book-quantum error correction-has now been pre sented. We carry on in this chapter with a discussion of fault-tolerant quantum computing (FTQC), which is our second theme. The discussion will take place in the context of quantum stabilizer codes and is based on Refs. [1-6]. See also Refs. [7,8]. Section 5.1 introduces the idea of fault-tolerance and makes ba sic definitions, while Section 5.2 presents a procedure for doing fault-tolerant quantum error correction. The theory of FTQC is then taken up in the fol lowing five sections. Section 5.3 examines the action of unitary operators on a quantum stabilizer code. It introduces two important groups: (i) the normalizer of the stabilizer S whose elements are encoded unitary operations; and (ii) the Clifford group which is generated by the Hadamard, phase, and CNOT gates. The discussion focuses on unitary operations that belong to the inter section of these two groups. By combining measurements on ancilla qubits (Section 5.4) with a four-qubit unitary operation introduced in Section 5.5, it will be possible to fault-tolerantly implement these gates on encoded data for stabilizer codes that encode a single qubit. Section 5.6 presents an extension of this approach that works for stabilizer codes encoding multiple qubits, and Section 5.7 completes the theoretical development by showing how a faulttolerant encoded Toffoli gate can be constructed for any quantum stabilizer code. By this point a theoretical framework is in place that can construct a set of fault-tolerant encoded quantum gates that is capable of universal quan tum computation using any quantum stabilizer code. The final two sections illustrate the theory by applying it to the [5,1,3] and [4,2,2] stabilizer codes. A brief summary of universal sets of quantum gates is given in Appendix C.
In this thesis, two models of reversible computing are classified, and the relation of reversible computing to quantum computing is explored. First, a finite, complete set of identities is given for the symmetric monoidal category generated by the computational ancillary bits along with the controlled-not gate. In doing so, it is proven that this category is equivalent to the category of partial isomorphisms between non-empty finitely-generated commutative torsors of characteristic 2. Next, a finite, complete set of identities is given for the symmetric monoidal category generated by the computational ancillary bits along with the Toffoli gate. In doing so, it is proven that this category is equivalent to the category of partial isomorphisms between finite powers of the two element set. The relation between reversible and quantum computing is also explored. In particular, the category with the controlled-not gate as a generator is extended to be complete for the real stabilizer fragment of quantum mechanics. This is performed by translating the identities to and from the angle-free fragment of the ZX-calculus, and showing that these translations are inverse to each other.
Finite group extensions offer a natural language to quantum computing. In a nutshell, one roughly describes the action of a quantum computer as consisting of two finite groups of gates: error gates from the general Pauli group and stabilizing gates within an extension group . In this communication we explore the nice adequacy between group theoretical concepts such as commutators, normal subgroups, groups of automorphisms, short exact sequences, wreath products etc. and the coherent quantum computational primitives. The structure of the single-qubit and two-qubit Clifford groups is analyzed in detail. As a byproduct, we discover that M 20 , the smallest perfect group for which the commutator subgroup departs from the set of commutators, underlies quantum coherence of the two-qubit system. We recover similar results by looking at the automorphisms of a complete set of mutually unbiased bases.