Quantum switching networks are derived from conventional switching networks by replacing the classical switches by quantum switches. We give the quantum circuit design and routing of an n \times m network, called a quantum concentrator that can direct quantum bit packets in arbitrary quantum states from any of its k inputs to some of its k outputs, where 1 \le k \le m \le n. Our designs are based on sparse crossbars which are rectangular grids of 2 \times 2 crosspoints. Sparse crossbar concentrator structures with theoretically minimum crosspoint complexities for any values of n and m are well known but no self-routing algorithms have been reported for such concentrators. We transform two such families of optimal concentrators, called fat-slim and banded sparse crossbars into quantum networks and provide self-routing algorithms for these families of concentrators. In this process, we extend the notion of packet concentration to a quantum network and design self-routing quantum crosspoints from quantum gates. We address issues critical to quantum operation like reversibility and localized self-routing and give a rigorous proof that quantum fat-slim and banded sparse crossbar concentrators are self-routable. The self-routing algorithms described in the paper can be used for both quantum and classical sparse crossbar concentrators by the linearity property of all quantum systems.
Quantum switching networks are derived from conventional switching networks by replacing the classical switches by quantum switches. We give the quantum circuit design and routing of an n \times m network, called a quantum concentrator that can direct quantum bit packets in arbitrary quantum states from any of its k inputs to some of its k outputs, where 1 \le k \le m \le n. Our designs are based on sparse crossbars which are rectangular grids of 2 \times 2 crosspoints. Sparse crossbar concentrator structures with theoretically minimum crosspoint complexities for any values of n and m are well known but no self-routing algorithms have been reported for such concentrators. We transform two such families of optimal concentrators, called fat-slim and banded sparse crossbars into quantum networks and provide self-routing algorithms for these families of concentrators. In this process, we extend the notion of packet concentration to a quantum network and design self-routing quantum crosspoints from quantum gates. We address issues critical to quantum operation like reversibility and localized self-routing and give a rigorous proof that quantum fat-slim and banded sparse crossbar concentrators are self-routable. The self-routing algorithms described in the paper can be used for both quantum and classical sparse crossbar concentrators by the linearity property of all quantum systems.
Flexible distribution of data in the form of quantum bits or qubits among spatially separated entities is an essential component of envisioned scalable quantum computing architectures. Accordingly, we consider the problem of dynamically permuting groups of quantum bits, i.e., qubit packets, using networks of reconfigurable quantum switches. We demonstrate and then explore the equivalence between the quantum process of creation of packet superpositions and the process of randomly routing packets in the corresponding classical network. In particular, we consider an n × n Baseline network for which we explicitly relate the pairwise input-output routing probabilities in the classical random routing scenario to the probability amplitudes of the individual packet patterns superposed in the quantum output state. We then analyze the effect of using quantum random routing on a classically non-blocking configuration like the Benes network. We prove that for an n × n quantum Benes network, any input packet assignment with no output contention is probabilistically self-routable. In particular, we prove that with random routing on the first (log n-1) stages and bit controlled self-routing on the last log n stages of a quantum Benes network, the output packet pattern corresponding to routing with no blocking is always present in the output quantum state with a non-zero probability. We give a lower bound on the probability of observing such patterns on measurement at the output and identify a class of 2 n-1 permutation patterns for which this bound is equal to 1, i.e., for all the permutation patterns in this class the following is true: in every pattern in the quantum output assignment all the valid input packets are present at their correct output addresses. In the second part of this thesis we give the complete design of quantum sparse crossbar concentrators. Sparse crossbar concentrators are rectangular grids of simple 2 × 2 switches or crosspoints, with the switches arranged such that any k inputs can be connected to some k outputs. We give the design of the quantum crosspoints for such concentrators and devise a self-routing method to concentrate quantum packets. Our main result is a rigorous proof that certain crossbar structures, namely, the fat-slim and banded quantum crossbars allow, without blocking, the realization of all concentration patterns with self-routing. In the last part we consider the scenario in which quantum packets are queued at the inputs to an n × n quantum non-blocking switch. We assume that each packet is a superposition of m classical packets. Under the assumption of uniform traffic, i.e., any output is equally likely to be accessed by a packet at an input we find the minimum value of m such that the output quantum state contains at least one packet pattern in which no two packets contend for the same output. Our calculations show that for m=9 the probability of a non-contending output pattern occurring in the quantum output is greater than 0.99 for all n up to 64.
Quantum switching networks are analogs of classical switching networks in which classical switches are replaced by quantum switches. These networks are used to switch quantum data among a set of quantum sources and receivers. They can also be used to efficiently switch classical data, and help overcome some limitations of classical switching networks by utilizing the unique properties of quantum information systems, such as superposition and parallelism. In this thesis, we design several such networks which can be broadly put in the following three categories: 1. Quantum unicast networks: We give the design of quantum Baseline network (QBN) which is a self-routing and unicast quantum packet switch that uses the Baseline topology. The classical version of the network blocks packets internally even when there are no output contentions and each input packet is addressed to a different output. The QBN uses the principles of quantum superposition and parallelism to overcome such blocking. Also, for assignments that have multiple input packets addressed to an output, this network creates a quantum superposition of all these packets on that output, ensuring that all packets have non-zero probabilities of being observed on that output. 2. Quantum concentrators: We introduce a new network called quantum concentrator, which is a key component of our quantum multicasting network design. This concentrator is also an n × n quantum switching network, to be denoted by n-QC, and which, for any m, 1 ≤ m ≤ n, routes arbitrary quantum states on any m of its inputs to its top m outputs. This network uses O(n log n) quantum gates, and has a gate level depth of O(log 2 n). We also give several variations of this network, the main ones being order-preserving and priority quantum concentrators. 3. Quantum multicast networks: We first design a quantum multicasting network, called a generalized quantum connector (GQC) which can be used to multicast quantum information from n input sources to n outputs. Since general quantum states cannot be copied due to the no-cloning theorem, this network actually multicasts superposed classical information packets, contained in a finite number of qubits at each input. Copying needed for such multicasting is obtained by Wootters and Zurek's quantum copying machines or controlled-not gates. This n-input and n-output (n × n) network, to be denoted by n-GQC, is recursively constructed using n/2-GQCs and uses O(n log 2 n) quantum gates. The time complexity of this network in terms of gate level depth is O(log 3 n). We also give two variations of this network which improve its behavior when routing multicast assignments that have multiple input packets contending for same outputs.
We present the design of a self-routing quantum packet switch which improves on a previous design given by the authors [1]. Like the earlier design, this switch too routes packets represented by quantum bits (qubits) but it reduces the routing overhead per packet from O(log N) qubits to O(1) qubits by eliminating the need for an extra "dummy" inputoutput pair and their associated "dummy" packets at each 2 2 internal switch. The quantum self-routing switch creates a superposition of all the maximum size non-blocking subsets of input packets at its outputs, which cannot be achieved by any classical self-routing switching network that is internally blocking. In addition to the network design, we give a method to characterize the output quantum state of the switch using the concepts of frames and balanced matrices.
In this paper, we present a quantum multicasting network, called a generalized quantum connector (n-GQC), which can be used to multicast quantum information from n inputs to n outputs. This network is recursively constructed using n/2-GQCs and consists of O(n log2n) quantum gates. The key component of the n-GQC is another network, called an n-quantum concentrator (n-QC). This concentrator is also an n × n quantum network, and can route arbitrary quantum states on any m of its inputs to its top m outputs, for any m, 1 ¿ m ¿ n. Its quantum gate complexity is O(n log n). The quantum gate-level depths of n-QC and n-GQC are O(log2n) and O(log3n), respectively. Both n-QC and n-GQC are based on the classical self-routing concentrators and generalized connection networks given by Lee and Oruc [1]. While these networks work for multicasting classical packets, they cannot be used to multicast quantum packets as they employ balancer switches with both forward and backward propagation of packets. We introduce a quantum balancer switch that works using a forward propagation of packets only, thereby facilitating the n-QC and n-GQC designs presented in the paper.
The distribution of quantum entanglement appears to be an important component of applications of quantum communications and networks. The ability to centralize the sourcing of entanglement in a quantum network can provide for improved efficiency and enable a variety of network structures. A necessary feature of an entanglement-sourcing network node comprising several sources of entangled photons is the ability to reconfigurably route the generated pairs of photons to network neighbors depending on the desired entanglement sharing of the network users at a given time. One approach to such routing is the use of a photonic switching network. The requirements for an entanglement distribution switching network are less restrictive than for typical conventional applications, leading to design freedom that can be leveraged to optimize additional criteria. In this paper, we present a mathematical framework defining the requirements of an entanglement-distribution switching network. We then consider the design of such a switching network using a number of 2 × 2 crossbar switches, addressing the interconnection of these switches and efficient routing algorithms. In particular, we define a worst-case loss metric and consider 6 × 6, 8 × 8, and 10 × 10 network designs that optimize both this metric and the number of crossbar switches composing the network. We pay particular attention to the 10 × 10 network, detailing novel results proving the optimality of the proposed design. These optimized network designs have great potential for use in practical quantum networks, thus advancing the concept of quantum networks toward reality.
One of the most important challenges in information networks is to gather data from a larger set of nodes to a smaller set of nodes. This can be done via the use of a concentrator architecture in the connection topology. This paper is a proof-of-concept that demonstrates a quantum-based controller in large interconnection networks can asymptotically perform this task faster. We specifically present quantum algorithms for routing concentration assignments on full capacity fat-and-slim concentrators, bounded fat-and-slim concentrators, and regular fat-and-slim concentrators. Classically, the concentration assignment takes O(n) time on all these concentrators, where n is the number of inputs. Powered by Grover's quantum search algorithm, our algorithms take O(nclnc) time, where c is the capacity of the concentrator. Thus, our quantum algorithms are asymptotically faster than their classical counterparts, when cln2c=o(n). In general, c=nμ, satisfies cln2c=o(n), implying a time complexity of O(n0.5(1+μ)lnn), for any μ,0<μ<1.