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Reversible Computation

Reversible computation, quantum computation and computer architectures in between

Alexis De Vos, Michiel Boes, Stijn De Baerdemacker

Published Jan 1, 2010• 1 citation
OpenAlex

Abstract

Thanks to the cosine-sine decomposition of unitary matrices, an arbitrary quantum circuit, acting on w qubits, can be decomposed into 2^w-1 elementary quantum gates, called controlled V gates. Thanks to the Birkhoff decomposition of doubly stochastic matrices, an arbitrary (classical) reversible circuit, acting on w bits, can be decomposed into 2w-1 elementary gates, called controlled NOT gates. The question arises under which conditions these two synthesis methods are applicable for intermediate cases, i.e. computers based on some group, which simultaneously is a subgroup of the unitary group U(2^w) and a supergroup of the symmetric group S_{2^w}. It turns out that many groups either belong to a class that might have a cosine-sine-like decomposition but no Birkhoff-like decomposition and a second class that might have both decompositions. For an arbitrary group, in order to find out to which class it belongs, it suffices to evaluate a function Phi(m), deduced either from its order (in case of a finite group) or from its dimension (in case of a Lie group). Here m=2^w is the degree of the group.

Related papers

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Whereas quantum computing circuits follow the symmetries of the unitary Lie group, classical reversible computation circuits follow the symmetries of a finite group, i.e., the symmetric group. We confront the decomposition of an arbitrary classical reversible circuit with w bits and the decomposition of an arbitrary quantum circuit with w qubits. Both decompositions use the control gate as building block, i.e., a circuit transforming only one (qu)bit, the transformation being controlled by the other w−1 (qu)bits. We explain why the former circuit can be decomposed into 2w − 1 control gates, whereas the latter circuit needs 2w − 1 control gates. We investigate whether computer circuits, not based on the full unitary group but instead on a subgroup of the unitary group, may be decomposable either into 2w − 1 or into 2w − 1 control gates.

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Alexis De Vos, Stijn De Baerdemacker

Any matrix of the unitary group U(n) can be decomposed into matrices from two subgroups, denoted XU(n) and ZU(n). This leads to decompositions of an arbitrary quantum circuit into NEGATOR circuits and PHASOR circuits. The NEGATOR circuits are closely related to classical reversible computation.

•Jan 1, 2016•0 citations

The Decomposition of U(n) into XU(n) and ZU(n)

Alexis De Vos, Stijn De Baerdemacker

Any matrix of the unitary group U(n) can (up to a global phase) be decomposed into 2n-1 matrices from two subgroups, denoted XU(n) and ZU(n). This leads to the decomposition of an arbitrary quantum circuit into NEGATOR circuits and PHASOR circuits. The NEGATOR circuits are closely related to classical reversible computation.

IEEE International Symposium on Multiple-Valued Logic (ISMVL)•May 1, 2014•3 citations
Thanks to the cosine-sine decomposition of unitary matrices, an arbitrary quantum circuit, acting on w qubits, can be decomposed into 2^w-1 elementary quantum gates, called controlled V gates. Thanks to the Birkhoff decomposition of doubly stochastic matrices, an arbitrary (classical) reversible circuit, acting on w bits, can be decomposed into 2w-1 elementary gates, called controlled NOT gates. The question arises under which conditions these two synthesis methods are applicable for intermediate cases, i.e. computers based on some group, which simultaneously is a subgroup of the unitary group U(2^w) and a supergroup of the symmetric group S_{2^w}. It turns out that many groups either belong to a class that might have a cosine-sine-like decomposition but no Birkhoff-like decomposition and a second class that might have both decompositions. For an arbitrary group, in order to find out to which class it belongs, it suffices to evaluate a function Phi(m), deduced either from its order (in case of a finite group) or from its dimension (in case of a Lie group). Here m=2^w is the degree of the group.
Whereas quantum computing circuits follow the symmetries of the unitary Lie group, classical reversible computation circuits follow the symmetries of a finite group, i.e., the symmetric group. We confront the decomposition of an arbitrary classical reversible circuit with w bits and the decomposition of an arbitrary quantum circuit with w qubits. Both decompositions use the control gate as building block, i.e., a circuit transforming only one (qu)bit, the transformation being controlled by the other w−1 (qu)bits. We explain why the former circuit can be decomposed into 2w − 1 control gates, whereas the latter circuit needs 2w − 1 control gates. We investigate whether computer circuits, not based on the full unitary group but instead on a subgroup of the unitary group, may be decomposable either into 2w − 1 or into 2w − 1 control gates.
Classical reversible computers on w bits are isomorphic to the (finite) symmetric group S 2 w ; quantum computers on w qubits are isomorphic to the (Lie) unitary group U(2 w ). We investigate and classify groups X which represent computers intermediate between classical reversible computers and quantum computers. Such intermediate groups X may exist in three flavours: finite groups of order larger than (2 w )!, infinite but discrete groups, and Lie groups of dimension smaller than (2 w ). For our purpose, we start from 1-qubit transformations, represented by 2 × 2 unitary matrices. We call this group the creator. Its members are called gates and act on one qubit. Controlled gates are quantum circuits acting on w qubits, such that the 1-qubit transformation (applied to a particular qubit) depends on the state of the w − 1 other qubits. The controlled gates generate the group X of 2 w × 2 w matrices, called the creation. We discuss all creators of order up to 8. Additionally a creator of order 16 and one of order 192 are discussed.
Classical reversible circuits, acting on www~bits, are represented by permutation matrices of size 2w×2w2^w \times 2^w2w×2w. Those matrices form the group P(2w2^w2w), isomorphic to the symmetric group {\bf S}2w_{2^w}2w​. The permutation group P(nnn), isomorphic to {\bf S}n_nn​, contains cycles with length~ppp, ranging from~1 to L(n)L(n)L(n), where L(n)L(n)L(n) is the so-called Landau function. By Lagrange interpolation between the ppp~matrices of the cycle, we step from a finite cyclic group of order~ppp to a 1-dimensional Lie group, subgroup of the unitary group U(nnn). As U(2w2^w2w) is the group of all possible quantum circuits, acting on www~qubits, such interpolation is a natural way to step from classical computation to quantum computation.
As two basic building blocks for any quantum circuit, we consider the 1-qubit PHASOR circuit ϕ( θ ) and the 1-qubit NEGATOR circuit N ( θ ). Both are roots of the IDENTITY circuit. Indeed: both ϕ(0) and N (0) equal the 2 × 2 unit matrix. Additionally, the NEGATOR is a root of the classical NOT gate. Quantum circuits (acting on w qubits) consisting of controlled PHASORs are represented by matrices from ZU(2 w ); quantum circuits consisting of controlled NEGATORs are represented by matrices from XU(2 w ). Here, ZU( n ) and XU( n ) are subgroups of the unitary group U( n ): the group XU( n ) consists of all n × n unitary matrices with all 2 n line sums (i.e. all n row sums and all n column sums) equal to 1 and the group ZU( n ) consists of all n × n unitary diagonal matrices with first entry equal to 1. Any U( n ) matrix can be decomposed into four parts: U = exp( iα ) Z 1 XZ 2 , where both Z 1 and Z 2 are ZU( n ) matrices and X is an XU( n ) matrix. We give an algorithm to find the decomposition. For n = 2 w it leads to a four-block synthesis of an arbitrary quantum computer.