Toffoli Gate Research Articles

Preparation of km-photon concatenated Greenberger–Horne–Zeilinger states for observing distinctive quantum effects at macroscopic scales

As a class of multipartite entangled states, the multipartite concatenated GHZ (C-GHZ) states remain superior stability under the influence of decoherence. We propose two scalable experimental realization of the multiphoton C-GHZ states based on the entanglers of multiphoton GHZ state. Given a $km$-photon GHZ state as an input state, if $m$ is odd, one can create a $km$-photon C-GHZ state. Also, generally, we design a scheme to prepare $km$-photon C-GHZ states from $km$ single-photon states by using $k$ entanglers of $m$-photon GHZ state and $k$ $m$-control Toffoli gates.

Efficient Experimental Estimation of Fidelity of Linear Optical Quantum Toffoli Gate

We propose an efficiently measurable lower bound on quantum process fidelity of N-qubit controlled-Z gates. This bound is determined by average output state fidelities for N partially conjugate product bases. A distinct advantage of our approach is that only fidelities with product states need to be measured while keeping the total number of measurements much smaller than what is necessary for full quantum process tomography. As an application, we use this method to experimentally estimate quantum process fidelity F of a three-qubit linear optical quantum Toffoli gate and we find that F≥0.83. We also demonstrate the entangling capability of the gate by preparing Greenberger-Horne-Zeilinger-type three-qubit entangled states from input product states.

Five two-qubit gates are necessary for implementing the Toffoli gate

In this paper, we settle the long-standing open problem of the minimum cost of two-qubit gates for simulating a Toffoli gate. More precisely, we show that five two-qubit gates are necessary. Before our work, it is known that five gates are sufficient and only numerical evidences have been gathered, indicating that the five-gate implementation is necessary. The idea introduced here can also be used to solve the problem of optimal simulation of three-qubit control phase introduced by Deutsch in 1989.

Scalable photonic quantum computing assisted by quantum-dot spin in double-sided optical microcavity

We investigate the possibility of achieving scalable photonic quantum computing by the giant optical circular birefringence induced by a quantum-dot spin in a double-sided optical microcavity as a result of cavity quantum electrodynamics. We construct a deterministic controlled-not gate on two photonic qubits by two single-photon input-output processes and the readout on an electron-medium spin confined in an optical resonant microcavity. This idea could be applied to multi-qubit gates on photonic qubits and we give the quantum circuit for a three-photon Toffoli gate. High fidelities and high efficiencies could be achieved when the side leakage to the cavity loss rate is low. It is worth pointing out that our devices work in both the strong and the weak coupling regimes.

Linear-depth quantum circuits forn-qubit Toffoli gates with no ancilla

We design a circuit structure with linear depth to implement an $n$-qubit Toffoli gate. The proposed construction uses a quadratic-size circuit consists of elementary 2-qubit controlled-rotation gates around the x axis and uses no ancilla qubit. Circuit depth remains linear in quantum technologies with finite-distance interactions between qubits. The suggested construction is related to the long-standing construction by Barenco et al. (Phys. Rev. A, 52: 3457-3467, 1995, arXiv:quant-ph/9503016), which uses a quadratic-size, quadratic-depth quantum circuit for an $n$-qubit Toffoli gate.

Composite Toffoli gate with two-round error detection

We introduce a fault-tolerant construction to implement a composite quantum operation of four overlapping Toffoli gates. The same construction can produce two independent Toffoli gates. This result lowers resource overheads in designs for quantum computers by more than an order of magnitude. The procedure uses Clifford operations and 64 copies of the non-Clifford gate $T=\mathrm{exp}[i\ensuremath{\pi}(I\ensuremath{-}{\ensuremath{\sigma}}^{z})/8]$. Quantum codes detect errors in the circuit. When the dominant source of error is $T$-gate failure with probability $p$, then the composite Toffoli circuit has a postselected failure rate of $3072{p}^{4}$ to lowest order.

Deterministic quantum logic gates and quantum cloning based on quantum dot-cavity coupled system

We propose two theoretical schemes for creating the CNOT gate and Toffoli gate deterministically by using the singly charged quantum dots inside the double-sided microcavities. Both of the two schemes only need one photon to act as an ancilla. A high fidelity can be achieved in the strong coupling regime. Finally, a quantum cloning machine is constructed theoretically to copy a quantum state near a given state by using the CNOT gate and the Toffoli gate, which simplifies the implementation of quantum cloning compared with previous proposals.

Analyzing the Toffoli gate in disordered circuit QED

We study the effects of imperfections on the fidelity of the Toffoli gate recently realized in a circuit~QED setup using quantum control methods. The noise is introduced in the interqubits interactions. The coupling constants are no longer fixed; instead, they fluctuate around average values obeying some given probability density functions characterizing the dynamical-imperfection case. We also consider the static-imperfection case in which the values of the coupling constants are not exactly known. We obtain a more robust gate by modifying the quantum optimization problem using a weighted average of the fidelity over an interval of coupling values as the objective functional.

Implementation of Toffoli gates with a single asymmetric HeisenbergXYinteraction

The Toffoli gate, which has practical applications in the implementation of practical quantum algorithms and quantum error correction, is usually constructed from concatenated two-qubit or qubit-qutrit gates. Here we show that the Toffoli gate for three qubits can be implemented through a single $\mathit{XY}$ interaction plus two single-qubit rotations. This method greatly simplifies the experimental implementation of the Toffoli gate and promises a much higher fidelity compared to those based on elementary gate decomposition. We present an example for the implementation of the required $\mathit{XY}$ interaction in cavity QED.

The Toffoli-Hadamard Gate System: an Algebraic Approach

Shi and Aharonov have shown that the Toffoli gate and the Hadamard gate give rise to an approximately universal set of quantum computational gates. The basic algebraic properties of this system have been studied in Dalla Chiara et al. (Foundations of Physics 39(6):559–572, 2009), where we have introduced the notion of Shi-Aharonov quantum computational structure. In this paper we propose an algebraic abstraction from the Hilbert-space quantum computational structures, by introducing the notion of Toffoli-Hadamard algebra. From an intuitive point of view, such abstract algebras represent a natural quantum generalization of both classical and fuzzy-like structures.

Quantum circuits ofT-depth one

We give a Clifford+T representation of the Toffoli gate of T-depth 1, using four ancillas. More generally, we describe a class of circuits whose T-depth can be reduced to 1 by using sufficiently many ancillas. We show that the cost of adding an additional control to any controlled gate is at most 8 additional T-gates, and T-depth 2. We also show that the circuit THT does not possess a T-depth 1 representation with an arbitrary number of ancillas initialized to 0.

Distilling one-qubit magic states into Toffoli states

For certain quantum architectures and algorithms, most of the required resources are consumed during the distillation of one-qubit magic states for use in performing Toffoli gates. I show that the overhead for magic-state distillation can be reduced by merging distillation with the implementation of Toffoli gates. The resulting routine distills 8 one-qubit magic states directly to a Toffoli state, which can be used without further magic to perform a Toffoli gate.

Teleportation-based Toffoli gate on cluster states via the Bell state analysis

An optical Toffoli gate is demonstrated via teleportations on the six-qubit entangling cluster state generated from single-qubit photons. It is implemented on the basis of entanglement swapping of the combined quantum system with three independent Bell state measurements. The output of this gate is then restored by suitable local operations and classical communications. We evaluate the implementing performance of the Toffoli gate fidelity for the operation process in different computational bases.

Optimization and Implementation of Reversible BCD Adder in Terms of Number of Lines

The Reversible logic plays an important role to obtain the minimal energy dissipation. It is the main requirement in the low power digital design. To implement a reversible function some additional lines are required. Sometimes it is more than the primary inputs which leads to increased size of the circuit. Hence the reducing the number of circuit lines is one of the major criterion in reversible logic. The general idea is to merge the garbage output lines with appropriate constant input lines. In this work, Toffoli gate implementation of BCD adder and press gate implementation of BCD adder is taken and an optimization algorithm has been applied to reduce the number of lines in the circuit. The kit used to implement the design is Vertex5 in which both the area and delay are analyzed.

Low-overhead constructions for the fault-tolerant Toffoli gate

We present two new constructions for the Toffoli gate which substantially reduce resource costs in fault-tolerant quantum computing. The first contribution is a Toffoli gate requiring Clifford operations plus only four $T = \exp(i\pi\sigma^z/8)$ gates, whereas conventional circuits require seven $T$ gates. An extension of this result is that adding $n$ control inputs to a controlled gate requires $4n$ $T$ gates, whereas the best prior result was $8n$. The second contribution is a quantum circuit for the Toffoli gate which can detect a single $\sigma^z$ error occurring with probability $p$ in any one of eight $T$ gates required to produce the Toffoli. By post-selecting circuits that did not detect an error, the posterior error probability is suppressed to lowest order from $4p$ (or $7p$, without the first contribution) to $28p^2$ for this enhanced construction. In fault-tolerant quantum computing, this construction can reduce the overhead for producing logical Toffoli gates by an order of magnitude.

Universal quantum gates for hybrid systems assisted by quantum dots inside double-sided optical microcavities

We present some deterministic schemes to construct universal quantum gates, that is, controlled- NOT, three-qubit Toffoli, and Fredkin gates, between flying photon qubits and stationary electron-spin qubits assisted by quantum dots inside double-sided optical microcavities. The control qubit of our gates is encoded on the polarization of the moving single photon and the target qubits are encoded on the confined electron spins in quantum dots inside optical microcavities. Our schemes for these universal quantum gates on a hybrid system have some advantages. First, all the gates are accomplished with the success probability of 100% in principle. Second, our schemes require no additional qubits. Third, the control qubits of the gates are easily manipulated and the target qubits are perfect for storage and processing. Fourth, the gates do not require that the transmission for the uncoupled cavity is balanceable to the reflectance for the coupled cavity, in order to get a high fidelity. Fifth, the devices for the three universal gates work in both the weak coupling and the strong coupling regimes, and they are feasible in experiment.

A Meet-in-the-Middle Algorithm for Fast Synthesis of Depth-Optimal Quantum Circuits

We present an algorithm for computing depth-optimal decompositions of logical operations, leveraging a meet-in-the-middle technique to provide a significant speed-up over simple brute force algorithms. As an illustration of our method we implemented this algorithm and found factorizations of the commonly used quantum logical operations into elementary gates in the Clifford+T set. In particular, we report a decomposition of the Toffoli gate over the set of Clifford and T gates. Our decomposition achieves a total T-depth of 3, thereby providing a 40% reduction over the previously best known decomposition for the Toffoli gate. Due to the size of the search space the algorithm is only practical for small parameters, such as the number of qubits, and the number of gates in an optimal implementation.

Logic Synthesis of Ternary Quantum Circuits with Minimal Qutrits

This paper investigates a universal approach of synthesizing arbitrary ternary logic circuits in quantum computation based on the truth table technology. It takes into account of the relationship of classical logic and quan-tum logic circuits. By adding inputs with constant value and garbage outputs, the classical non-reversible logic can be transformed into reversible logic. Combined with group theory, it provides an algorithm using the ternary Swap gate, ternary NOT gate and ternary Toffoli gate library. Simultaneously, the main result shows that the numbers of qutrits we use are minimal compared to other methods. We also illustrate with two examples to test our approach.

Efficient Algorithms for Optimal 4-Bit Reversible Logic System Synthesis

Owing to the exponential nature of the memory and run-time complexity, many methods can only synthesize 3-bit reversible circuits and cannot synthesize 4-bit reversible circuits well. We mainly absorb the ideas of our 3-bit synthesis algorithms based on hash table and present the efficient algorithms which can construct almost all optimal 4-bit reversible logic circuits with many types of gates and at mini-length cost based on constructing the shortest coding and the specific topological compression; thus, the lossless compression ratio of the space ofn-bit circuits reaches near2×n!. This paper presents the first work to create all 3120218828 optimal 4-bit reversible circuits with up to 8 gates for the CNT (Controlled-NOT gate, NOT gate, and Toffoli gate) library, and it can quickly achieve 16 steps through specific cascading created circuits.

Mediated gates between spin qubits

In a typical quantum circuit, nonlocal quantum gates are applied to nonproximal qubits. If the underlying physical interactions are short-range (e.g., exchange interactions between spins), intermediate swap operations must be introduced, thus increasing the circuit depth. Here we develop a class of "mediated" gates for spin qubits, which act on nonproximal spins via intermediate ancilla qubits. At the end of the operation, the ancillae return to their initial states. We show how these mediated gates can be used (1) to generate arbitrary quantum states and (2) to construct arbitrary quantum gates. We provide some explicit examples of circuits that generate common states [e.g., Bell, Greenberger-Horne-Zeilinger (GHZ), W, and cluster states] and gates (e.g.,square-root-SWAP, SWAP, CNOT, and Toffoli gates). We show that the depths of these circuits are often shorter than those of conventional swap-based circuits. We also provide an explicit experimental proposal for implementing a mediated gate in a triple-quantum-dot system.