swap gate (Sec.5.3). The exchange interaction is readily available in many spin-based qubits systems, such as quantum dots [18]. Following the same ap-proach, we also find a similar result for qubits coupled via a XY-type interaction, which happens to be the interaction between superconducting qubits [112] as well as between optically coupled spin qubits [113], or for qubits coupled via magnetic dipole-dipole interaction (Sec.5.4). Our proposal works with two input pairs of spin-1/2 qubits and only requires a single two-qubit interaction. In comparison with existing protocols, we achieve an advantage by allowing fordifferenttwo-qubit manipulations locally in the bilateral operation. For a single purification step, the derived protocol requires no extra single-spin operations, making it much faster and less susceptible to gate errors.
The only needed operation, the
√
swap gate, has been implemented experimentally, with a gate time below 0.2 ns [114] (see also [48]), making the implementation of our proposal within reach of current technology. Furthermore, the required single-shot measurement of an electron spin state has also been successfully performed [115,116].
5.2 Spin Qubits in Lateral Quantum Dots
The spin of an electron is a two-level system and presents a natural platform to implement a qubit. Single or few electrons can be trapped and confined in all three spatial dimensions utilizing quantum dots (QDs). If the extent of the confinement is comparable to the de Broglie wavelength of the electron (tens of nanometers in a typical semiconductor structure), the orbital energy levels are quantized; thus, QDs are also namedartificial atoms. Many types of QDs exist, with variations, e.g., in size and material composition. In this section, we concentrate on the description of electron spins in lateral QDs [19], which have been extensively studied in the context of quantum information processing [18,20,24].
5.2 Spin Qubits in Lateral Quantum Dots
QD i QD j
2DEG
Si
Sj
Gates antum dots
e e
(a) (b)
V
x
Figure 5.1–Spin qubits in lateral quantum dots.(a) Part of a quantum register composed of two electrons (e) with spinsSiandSj, confined in gate-defined quantum dots. The electrons stem from a two-dimensional electron gas (2DEG) that is created in a semiconductor heterostructure beneath the gates and that confines electrons in the drawing plane. (b) Typical interaction potential V between two electrons in adjacent quantum dots (QDs)iandj[118] along one lateral direction (here thexaxis).
Lateral QDs are formed from a two-dimensional electron gas (2DEG), in which electrons are confined in one spatial direction and still able to move in the remaining two. A 2DEG can be formed in a semiconductor heterostructure, such as AlGaAs/GaAs, where due to a band-gap mismatch of the two materials, the electrons are confined to the GaAs layer in this case. In the following, we concentrate on AlGaAs/GaAs heterostructures, unless otherwise stated. Confinement in the lateral direction, i.e. within the plane of the 2DEG, can be achieved by metallic surface gates above the electron layer. The electric field of the electrodes generates an approximately parabolic potential that localizes the electrons within 100 nm [117]. A schematic depiction is given in Fig.5.1. Capacitive coupling to a source and drain reservoir allows to change the charge state of the QD. In this fashion, a lateral QD can be filled with exactly one electron that can be coherently manipulated for quantum information purposes.
Various ideas emerged of how to encode a qubit in the spin state of one or several electrons confined in electrically-defined QDs. In the original idea [18], also known as theLoss-DiVincenzo proposal, the spin eigenstates of a single electron in a QD define the computational basis,
|0i ≡ | ↑ i ≡
mS = +1 2
, (5.1)
|1i ≡ | ↓ i ≡
mS =−1 2
, (5.2)
also referred to asspin up(| ↑ i) and spin down(| ↓ i). Here, we denote the electron spin byS and theSzeigenstates by|mS =±1/2i. The growth direction of the heterostructure defines the spin quantization axisz, i.e. the direction normal to the 2DEG plane. Other proposals suggested two-electron spin states [119,120] and three-electron spin states [121] of multiple quantum dots
51
Chapter 5: Entanglement Purification with the Exchange Interaction
as computational basis. And also the charge degree of freedom can be exploited as qubit [122].
Substantial progress has been made in the fields of multi-electron spin qubits and charge qubits;
however, we were interested in entanglement purification schemes for single spins and therefore focus on single-spin qubits in the following. We present how individual spins can be initialized, manipulated and measured, and how two qubits can be coupled.
An external magnetic fieldBis coupled to the electron spin according to the Zeeman Hamiltonian
Hz =дµBB·S, (5.3)
whereдis the effectiveд-factor andµB is the Bohr magneton. The spin states| ↑ iand| ↓ ican be split by a constant magnetic field componentBzalong thezaxis by the Zeeman energyдµBBz. Due to the energy separation, the states| ↑ iand| ↓ iacquire different phases during the time evolution generated byHz, corresponding to rotations on the Bloch sphere around thezaxis (cf. Fig.3.1).
Transitions between the two spin states can be driven by an ac magnetic fieldBx(t)=B⊥cos(ωt) perpendicular to the quantization axis, known aselectron spin resonance(ESR). In the resonant case, i.e. forдµBBz = ω, the probability of a spin flip oscillates in time between 0 and 1. The probability is less than 1 in the case of nonresonant driving. Using ESR, coherent oscillations between the states| ↑ iand| ↓ iwith a Rabi period of 108 ns have been reported, reaching a gate fidelity of 73 % [123]. Although electric fields do not couple directly to the electron spin, they can still be used to drive spin transitions. Due to spin-orbit interaction, an oscillating electric field generated through the metallic gates creates an effective magnetic field that oscillates with the same frequency, which in turn drives Rabi oscillations between the two spin states. This so-called electric-dipole-induced spin resonance(EDSR) therefore allows spin rotations about thexaxis of the Bloch sphere by purely electrical means with Rabi periods of about 220 ns [124]. The fastest gate time could be reached by utilizing yet another mechanism to create an effective magnetic field from an electric field, i.e. by the application of a slanted magnetic field, achieving a spin-flip time of 20 ns [125].
Because of the small magnetic moment associated with the electron spin, the information about the spin state needs to be transferred to a different measurable quantity in order to read out the qubit. One idea is to convert the information into a charge signal that depends on the spin state of the electron. An example of such a spin-to-charge conversion process is the spin-dependent tunneling of the QD electron into a proximate ancilla dot. The presence or absence of an electron in the ancilla can be detected by a quantum point contact, because the electric field of the electron changes its conductance [18,126]. This technique enabled single-shot readout of the electron spin state with average fidelities of up to 86 % [115,116,127]. Qubit initialization can work in principle in the same fashion, i.e. the measurement projects the spin into a definite state, and has been demonstrated for silicon-based spin qubits [128].
In order to perform quantum computation, two-qubit operations have to be implemented.
5.2 Spin Qubits in Lateral Quantum Dots
Therefore, two such gate-controlled qubits must be coupled. If two QDs are so close to each other that the electronic wave functions overlap, tunneling events between the two dots become possible. This gives rise to an exchange interaction of the two electrons described by the Heisenberg Hamiltonian [18,118]
Hij(t)= J(t)Si·Sj = 1
4J(t)σi·σj, (5.4)
whereSi =σi/2 is the spin operator of the electron in QDiandσi is the vector of Pauli matrices.
The spin state of the two electrons is either a spin singlet |Si, or one of the spin triplet states|T+i,
|T0iand|T−i, that are given by
The effective exchange couplingJ(t)splits the spin triplets from the spin singlet due to a virtual electron exchange between the two dots, which is only possible for the singlet state. Therefore, the energy of the singlet state is lowered by the exchange interaction. One can also see this by rewriting the HamiltonianHij(t)for the exchange interaction between the electrons in QDiand j[Eq. (5.4)] in the form
Hij(t)= J(t) 1
41−PΨ−
, (5.9)
wherePΨ−= |Ψ−ihΨ−|is the projector onto the singlet subspace. A quantitative description of the exchange couplingJ(t)has been derived in Ref.118. The coupling can be controlled via the gate voltages, thus effectively changing the tunnel barrier between the two dots. In this fashion, specific two-qubit operations necessary for quantum information processing can be implemented. As we describe below, arbitrary powers of the two-qubit swap gate [Eq.3.11] are generated depending on the strength and temporal shape of the exchange interactionJ(t). The time evolution of the two-electron spin state coupled viaHij(t)is given by1,2
Uij(α)=e−iR0tdt0Hi j(t0) =e−iα/4
Chapter 5: Entanglement Purification with the Exchange Interaction
The matrix representation is in the product basis{| ↑↑ i,| ↑↓ i,| ↓↑ i,| ↓↓ i}and we set the initial timet0=0. We parametrize the time evolution by the so-calledpulse areaα =R t
0
dt0J(t0). Here, we see that the temporal shape of the exchange coupling determines the two-qubit evolution.
Exchange interaction is commonly applied for some finite timet, which we will refer to as an exchange pulse. If we, e.g., assume a constant exchange couplingJ, the pulse area is simply given byα = Jt. For an exchange pulse withα =π, the swap operation is implemented,
up to an irrelevant global phase factor of exp(−iπ/4). If an exchange pulse with a pulse area of α = π/2 is applied, two spins can be entangled via Heisenberg exchange interaction, and the corresponding quantum gate is the
which is a universal two-qubit gate [18]. In a register composed of several gate-controlled qubits, an elementary quantum gate between spins in dotsiandj, e.g., is implemented by turning on the exchange interaction between these dots for a specific time and leaving all other couplings turned off. Thereby, only the state of the intended qubit pair is changed. However, the cnot gate required for several quantum information protocols [3] is not directly generated by the Heisenberg exchange interaction. As shown in Sec. 3.2, additional single-qubit rotations are required to construct a cnot gate between qubitiandjfrom
√
Since single-spin rotations on the order of 100 ns are much slower than exchange-based two-qubit operations (see below), such an implementation is challenging. It is this fact, which motivates the development of protocols that can be directly implemented by the Heisenberg exchange interaction, such as the entanglement purification scheme derived in Sec.5.3.
The coupling of two electron spins via Heisenberg exchange interaction has been successfully demonstrated in AlGaAs/GaAs [48,114,129,130] and Si/SiGe QDs [131]. In the seminal work of Ref.114, J. R. Pettaet al.demonstrated electrical control of the exchange interaction between two QDs and the implementation of a
√
swap gate within 180 ps. Further developments included the demonstration of an exchange-based two-qubit coupling together with independent single-qubit