Selective writing and read-out of a register of static qubits

Abstract

We propose a setup comprising an arbitrarily large array of static qubits (SQs), which interact with a flying qubit (FQ). The SQs work as a quantum register, which can be written or read out by means of the FQ through quantum state transfer (QST). The entire system, including the FQ's motional degrees of freedom, behaves quantum mechanically. We demonstrate a strategy allowing for selective QST between the FQ and a single SQ chosen from the register. This is achieved through a perfect mirror located beyond the SQs and suitable modulation of the inter-SQ distances.A prominent paradigm in quantum information processing (QIP) [1] is to employ flying qubits (FQs) and static qubits (SQs) as the carriers and registers of quantum information, respectively [2]. Key to such an idea is the ability to write and read out the information content of a SQ by means of a FQ. By this, here we mean that an efficient quantum state transfer (QST) between these two types of qubits must be possible on demand. In this picture, control over memory allocation appears to be a desirable if not indispensable requirement. For instance, one can envisage the situation where only one or a few SQs are available, e.g. because the remaining ones are encoding some information to save. On the other hand, one may need to carry away only the information saved in certain specific SQs. Alternatively, only a restricted area of the register of SQs may be interfaced with some external processing network where one would like to eventually convey information or from which output data are to be received. In such cases, the ability of selecting the exact location where the information content of the FQ should be uploaded or downloaded is demanded. Ideally, according to the schematics in figure 1, one would like the FQ to reach the specific target SQ, then fully transfer its quantum state to this and eventually fly away. Evidently, this picture is implicitly based on the assumption that, firstly, the motional degrees of freedom (MDOFs) of the FQ are in fact fully classical and, secondly, these can be accurately controlled. Despite its simplicity, although interesting research along these lines is being carried out mostly through the so-called surface acoustic waves (see e.g. [3] and references therein), such an approach calls for a very high level of control.If set within a fully quantum framework, the most natural situation to envisage is the one where the FQ, besides bearing an internal spin, moves in a quantum mechanical way and hence propagates as a wave-like object. Such a circumstance substantially complicates the dynamics in that, besides the complex spin-spin interactions, intricate wave-like effects such as multiple reflections between the many SQs occur as well. This appears to be an adverse environment to accomplish selective QST: while ideally one would like to focus the FQ's wave packet right on the target SQ, the former is expected to spread throughout the SQs' register. Thus, not only is it non-trivial what strategy would enable selective QST but ...

Manipulating quantum information with spin torque

“…The use of “flying spins” to manipulate static qubits has been discussed in the past 11 12 13 14 15 16 17 18 19 20 21 and it has been noted that the reflection matrix [ R ] in equation (2) represents a unitary transformation suitable for quantum operations if the barriers at in Fig. 1(b,c) are large enough to reflect the incident electrons completely 20 21 , which we employ in this proposal. What is new about the present proposal’s method is the use of sequential interactions with a large number of itinerant electrons, each interaction involving a process of entanglement and reflection, equation (2) , followed by a collapse, without post-selection, of the quantum state, equation (3) , resulting in a deterministic, approximately unitary operation.…”

“…In this paper we will first show that (1) all standard single qubit operations can be effected without any magnetic field through interactions of the form with the itinerant or “flying” non-equilibrium spin population while (2) two qubit operations can be implemented through separate interactions of the form and with the flying spin population . The latter process has been discussed earlier by several authors 11 12 13 14 15 16 17 18 19 20 21 and we draw on this work, but there is a key distinction with the present work as explained in the next section.…”

Recurrence in Resonant Transmission of One-Dimensional Array of Delta Potentials

“…It has been used to study the conduction properties of crystals through the Kronig-Penney model [22] and Anderson localization in a disordered impurity array [23,24,25,26]. In a solid state quantum information scenario, δ-function potential barriers are used to depict the instantaneous interaction between a flying spin and a fixed magnetic impurity [27,28,29,30,31,32], to implement teleportation [33] and quantum memory [34].…”