Efficient Multiqubit Entanglement via a Spin Bus

Abstract

We propose an experimentally feasible architecture with controllable long-range couplings built up from local exchange interactions. The scheme consists of a spin-bus, with strong, always-on interactions, coupled dynamically to external qubits of the Loss and DiVincenzo type. Long-range correlations are enabled by a spectral gap occurring in a finite-size chain. The bus can also form a hub for multiqubit entangling operations. We show how multiqubit gates may be used to efficiently generate W -states (an important entanglement resource). The spin-bus therefore provides a route for scalable solid-state quantum computation, using currently available experimental resources.PACS numbers: 03.67. Lx,03.67.Mn,73.21.La,03.67.Pp Spin qubits in quantum dots are considered leading candidates for quantum computation because of their long decoherence times and their affinity to scalable gating techniques [1,2]. A prominent form of interaction between the spins is the exchange coupling, which has long been considered crucial for quantum computing in quantum dots [1] and other spin-based qubits [3]. By utilizing coded qubits, the exchange coupling can even accomodate single qubit rotations [4,5]. Recent experiments lends support to the Loss & DiVincenzo proposal by demonstrating electrical control of the exchange coupling in spin qubits [6]. However, the range of the interaction is only tens of nanometers, leading to scaling and architectural constraints [7]. In bulk systems, an effective interaction, known as Ruderman-Kittel-KasuyaYosida or RKKY [8], arises due to the exchange coupling between localized moments and intermediary particles, such as electrons in a commonly shared electron gas [9] or virtual excitons [10]. The resulting, effective interactions are long-range. It is presently unknown whether a spin chain could play a similar intermediary role for electron spins in quantum dots, thereby enabling a more scalable quantum computing architecture. Here, we show how to engineer such long-range interactions between remote qubits connected to a spin-bus, and we identify appropriate bus gate operations. We also show how critical quantum resources, like an entangled many-body W -state, can be generated efficiently by means of a spin-bus. We find that long-range, many-body interactions can be achieved using established, electrically controlled gating methods.Architectures that use an intermediary bus to facilitate long-range interactions between remote qubits have been studied in various qubit schemes, with the CiracZoller proposal for trapped ions as a preeminent example [11]. In semiconductors, there have been bus proposals to transduce spin information into photon modes in resonant cavities [12] or transmission lines [13]. Spin chains have been proposed as a medium for long-range correlations [14] and dynamical modes [15]. However, the adiabatic spin-bus remains unexplored in the context of quantum computation.A spin-bus naturally combines long-range interactions with the connectivity needed for computations. Several ve...

Strong coupling of a spin qubit to a superconducting stripline cavity

“…Scaling up the spin-qubit architecture invariably requires on-chip quantum information transfer, whether via moving the electrons themselves [9][10][11] or via a "spin bus". 12 However, electron motion could lead to reduced spin coherence, 13 while the spin bus requires strong exchange couplings within a spin chain. An enticing alternative to move spin information is to transfer it coherently to a photon, which is mobile and decoherence-resistant.…”

Tunable Spin-Qubit Coupling Mediated by a Multielectron Quantum Dot

“…Approaches to implementing long-range interactions typically involve identifying a system that acts as a mediator of the interaction between the qubits, with proposed systems including optical cavities and microwave stripline resonators [21][22][23][24][25][26], floating metallic [27] and ferromagnetic [28] couplers, the collective modes of spin chains [29][30][31], superconducting systems [32,33], and multi-electron molecular cores [34]. Recently, long-range coupling of electrons located in the two outer quantum dots of a linear triple dot system has been demonstrated [35,36].…”

Quantum state transfer through a spin chain in a multiexcitation subspace

“…Entanglement properties in two-dimensional AFM models have been studied [8] and it has been shown that multiqubit entanglement can be generated efficiently via a quantum data bus consisting of spin chains with strong static AFM couplings [9,10]. The effects of fluctuating exchange couplings and magnetic fields on the fidelity of data bus transfer have also been investigated [11].…”