This paper presents a study on valley filters and valley valves in graphene. The authors show that a quantum point contact (QPC) with zigzag edges can produce a highly nonequilibrium valley population in a carbon monolayer. The valley polarization is better than 95% if the Fermi level lies above the Dirac point both inside and outside of the constriction. The polarity of the valley filter can be inverted by local application of a gate voltage to the point contact region. Two valley filters in series may function as an electrostatically controlled "valley valve", representing a zero-magnetic-field counterpart to the familiar spin valve. The study uses a tight-binding model calculation to show that a nonequilibrium valley polarization can be realized in a sheet of graphene, upon injection of current through a ballistic point contact with zigzag edges. The polarity can be inverted by local application of a gate voltage to the point contact region. The potential of graphene for carbon electronics rests on the possibilities offered by its unusual band structure to create devices that have no analogue in silicon-based electronics. Conduction and valence bands in graphene form conically shaped valleys, touching at a point called the Dirac point. There are two inequivalent Dirac points in the Brillouin zone, related by time-reversal symmetry. Electrons and holes in each valley are massless, with a large energy-independent velocity. The authors demonstrate that the valley polarization present inside the QPC is not destroyed by intervalley scattering at the transition from a narrow to a wide region. The operation of the valley filter is demonstrated in Fig. 4. The top panel shows the conductance, while the bottom panel shows the valley polarization — both as a function of the electrochemical potential in the narrow region. For positive μ₀ the current flows entirely within the conduction band, and we obtain plateaus of quantized conductance at odd multiples of 2e²/h. Smoothing of the potential step improves the flatness of the plateaus. The plateaus in the conductance at G = (2n + 1) × 2e²/h correspond to plateaus in the valley polarization at P = 1/(2n + 1). On the lowest n = 0 plateau, and for 0 < μ₀ ≲ Δ, the polarization is more than 95%. For negative μ₀ the current makes a transition from the conduction band in the wide regions to the valence band in the narrow region. This interband transition has previously been studied in an unbounded system. In the QPC studied here we find that the interband transition destroys the conductance quantization — except on the first plateau, which remains quite flat in the entire interval -3Δ/2 < μ₀ < 3Δ/2. The resonances at negative μ₀ are due to quasi-bound states in the valence band. The polarity of the valley filter is invertedThis paper presents a study on valley filters and valley valves in graphene. The authors show that a quantum point contact (QPC) with zigzag edges can produce a highly nonequilibrium valley population in a carbon monolayer. The valley polarization is better than 95% if the Fermi level lies above the Dirac point both inside and outside of the constriction. The polarity of the valley filter can be inverted by local application of a gate voltage to the point contact region. Two valley filters in series may function as an electrostatically controlled "valley valve", representing a zero-magnetic-field counterpart to the familiar spin valve. The study uses a tight-binding model calculation to show that a nonequilibrium valley polarization can be realized in a sheet of graphene, upon injection of current through a ballistic point contact with zigzag edges. The polarity can be inverted by local application of a gate voltage to the point contact region. The potential of graphene for carbon electronics rests on the possibilities offered by its unusual band structure to create devices that have no analogue in silicon-based electronics. Conduction and valence bands in graphene form conically shaped valleys, touching at a point called the Dirac point. There are two inequivalent Dirac points in the Brillouin zone, related by time-reversal symmetry. Electrons and holes in each valley are massless, with a large energy-independent velocity. The authors demonstrate that the valley polarization present inside the QPC is not destroyed by intervalley scattering at the transition from a narrow to a wide region. The operation of the valley filter is demonstrated in Fig. 4. The top panel shows the conductance, while the bottom panel shows the valley polarization — both as a function of the electrochemical potential in the narrow region. For positive μ₀ the current flows entirely within the conduction band, and we obtain plateaus of quantized conductance at odd multiples of 2e²/h. Smoothing of the potential step improves the flatness of the plateaus. The plateaus in the conductance at G = (2n + 1) × 2e²/h correspond to plateaus in the valley polarization at P = 1/(2n + 1). On the lowest n = 0 plateau, and for 0 < μ₀ ≲ Δ, the polarization is more than 95%. For negative μ₀ the current makes a transition from the conduction band in the wide regions to the valence band in the narrow region. This interband transition has previously been studied in an unbounded system. In the QPC studied here we find that the interband transition destroys the conductance quantization — except on the first plateau, which remains quite flat in the entire interval -3Δ/2 < μ₀ < 3Δ/2. The resonances at negative μ₀ are due to quasi-bound states in the valence band. The polarity of the valley filter is inverted