Capacitive single-electron transistor
View Patent ↗The invention is a sensitive measuring instrument, which is principally applied to quantum computation, especially to measurement of quantum bits consisting of superconducting micro and nano-structures. The state of a quantum bit is expressed as the voltage-time integral over a circuit component. Phase measurement is performed by measuring the capacitance of a single-electron transistor between the gate and ground.
1. A method of determining a phase of a qubit having states defined by different phases, comprising:
establishing the qubit in a superconducting single-electron transistor (SSET) having a gate, a ground and a capacitance;
measuring the capacitance of the SSET between the gate and ground; and
determining the phase from the measured gate-to-ground capacitance.
2. The method according to claim 1 , wherein the qubit is a charge-phase qubit.
3. The method of claim 1 , including:
forming the SSET from first and second Josephson junctions;
forming the qubit from third and fourth Josephson junctions;
connecting the first Josephson junction of the SSET to the third Josephson junction of the qubit with a first conductor; and
connecting the second Josephson junction of the SSET to the fourth Josephson junction of the qubit with a second conductor; and
grounding said first and second conductors with said grounding carried out through a capacitance.
4. The method of claim 1 , comprising connecting the SSET gate to a resonance circuit having a) an inductance connected to a control voltage, and b) a grounded capacitor.
5. The method of claim 4 , wherein the resonance circuit has a resonance frequency, and including:
determining the capacitance of the SSET by measuring the resonance frequency.
6. The method of claim 5 , including using a magnetic flux to initiate said measuring.
7. The method of claim 4 , including measuring the phase by registering a reflection of a voltage surge directed to an outer connection point of the resonance circuit.
8. A capacitive single-electron transistor (C-SET) detector circuit apparatus for measuring a phase of a qubit having states defined by different phases, comprising:
a superconducting single-electron transistor (SSET) adapted to support the qubit, the SSET having a ground, a source, a drain, a gate having an associated gate voltage and an associated gate-to-ground capacitance (C G ), and a source-drain phase difference (φ) that corresponds to said gate-to-ground capacitance and that corresponds to said qubit phase; and
a resonance circuit electrically connected to the gate of the SSET and adapted to have a resonance frequency that depends on said gate-to-ground capacitance so that measuring the resonance frequency yields the qubit's state.
9. The apparatus of claim 8 , wherein the resonance circuit includes:
a grounded shunt capacitor; and
an inductor connected to a control voltage.